Organic Evolution in terms of the Implicate and Explicate Orders.

Part VIII :

The evolutionary diversification in the Order Diptera.

B. The Infraorder Bibionomorpha (Diptera) (Sequel).

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After having -- in the previous document -- dealt with the superfamilies Bolitophilidea and Cecidomyiidea, we will now discuss the superfamily Bibionidea [still] of the infraorder Bibionomorpha.

Superfamily Bibionidea

Figure 1 :  Bibionidea.
A -- Bibio  marci L.  Bibionidae, male. General view. Body-length 10 mm.
B -- Penthetria  holosericea MEIGEN.  Penthetriidae, general view of larva, top view. Length 12 mm.
( From ROHDENDORF, 1964,  A, after LINDNER, 1928,  B, after HENNIG, 1948-1952)

The superfamily Bibionidea, which is a bit younger group of diptera as compared to the fungivoroids, has its up-to-now earliest representatives in lower Jurassic faunae. Today -- that is, in the recent era -- the Bibionidea include three families with an overall number of species of about 500, of wich more than three quarters belong to the largest family Bibionidae (about 390 species). 100 species belong to the family Penthetriidae [more or less coinciding with Pleciidae], while the family Hesperinidae has only two species  [ROHDENDORF, 1964] [According to KRIVOSJEINA, 1969, there are six species, all belonging to the genus  Hesperinus WALK.]. This family is a clearly expressed relict group.
The chief features of these insects include (1) the development of a sharp sexual dimorphism (refinement of the antennae and eyes of the males), (2) the comparatively large body-size, (3) the soil-dwelling way of life of the phytophagous larvae having gnawing mouthparts, (4) large costalized wings (but hard, slow flight!) and (5) strong prehensile legs.

Figure 1a :  Bibionidea.
A -- Bibio  marci L., March-fly,  Bibionidae. Feeding from a flower.
(After OLDROYD, 1964)

The features of individual development are known only for the two main families of the Bibionidea and do not include one or another newly developed trait, apart from the armament of the body of the larvae, most distinct in the Penthetriidae (see Figure 1B ).
We notice that up until today [1964] the features of larval feeding and the whole process of metabolism in the larvae are still unknown [See, however, Part V in which we have included the many data given by KRIVOSJEINA, 1969 ].  Next we give a few more drawings of larvae of some Bibionidea.

Figure 2 :  Lateral view of larvae of Bibionidea [Bibionomorpha].
Top -- Bibio  pomonae FABR. [Bibionomorpha-Bibionidae].
Bottom -- Plecia  nigra LUND [Bibionomorpha-Pleciidae].
For the larva of  Penthetria  see Figure 1 B, above .
(After KRIVOSJEINA, 1969)

The nature of reproduction in the Bibionidea is also little known. There only exists information about their laying the eggs in the soil, in which act the armoured fore-legs apparently play a role. The winged form of the representatives of the family Bibionidae feeds from flowers. Those of the family Penthetriidae aparently do not feed. We must stress the insufficiency of our knowledge of the features of the living-conditions of the Bibionidea. All what is known to us about the way of life and organization of these insects, does not allow to draw whatever determined conclusions concerning the character of the conflicts which have determined the origin of the group as a whole. Apparently, the primary determining traits, which were responsible for the branching-off of the first Bibionidea from ancient Fungivoridea (which happened, probably, in triassic times), were the increase of the absolute size of the body, which took place alongside the development of prehensile legs and a moderate costalization of the wings. The conditions which contributed to the increase of body-size are not clear. Perhaps the plentifulness of easily assimilatable food was the cause of this process. This chief pathway of the historical development of the Bibionidea led to the appearance of the jurassic groups Eopleciidae  ( Figure 3 ),  Mesopleciidae  ( Figure 4 ),  and Paraxymyiidae  ( Figure 5 ),  and further to the caenozoic Hesperinidae and Penthetriidae.

Figure 3 :  Wing of  Eoplecia  primitiva HANDL.
Upper Liassic of Germany  [ The Liassic is :  lower Jurassic].
Length of wing 3.7 mm.
Interpretation of wing-venation according to HANDLIRSCH.
(From ROHDENDORF, 1946, after HANDLIRSCH, 1925)

Figure 4 :  Mesoplecia  jurassica ROHD.  Jurassic of Karatau [southern Kazachstan], neighborhood of the village Michailovka.  Specimen No. 2231.
Length of body 7.5 mm.  Length of wing 7.0 mm.
(After ROHDENDORF, 1938)

Figure 5 :  Wing of  Paraxymyia  quadriradialis ROHD..
Jurassic of Karatau [southern Kazachstan]. The village Halkino.  Specimen PIN No. 2472/584 [as given in ROHDENDORF's figure 50 of the wing, and PIN no. 2452/584, as given in ROHDENDORF's figure 49 of the whole insect].
Length of insect with inflated abdomen 3.75 mm.  Actual length about 3.25 mm.
Length of wing 2.25 mm. Male.
(After ROHDENDORF, 1946)

Soon after the branching-off of the original forms of the superfamily, the origin of the still very small Protobibionidae with sharply costalized wings took place, which, in turn, later led to the formation of the caenozoic Bibionidae. Not excluded, however, is the possibility that these early costalized bibionoids were not phylogenetically connected with the Bibionidae, that is, that they were just a parallel similar group that originated significantly earlier. The essence of conflicts which determined the origin of the costalized Bibionidae remains unclear. Regarding this we have insufficient knowledge which does not guarantee an all-sided clarification of the living-conditions of these insects. The armoured and very strong prehensile legs with well-developed appendages of the pretarsus, the sharply expressed sexual dimorphism with respect to the structure of the head and the color of the body, the lifting costalized wings, and, finally, the 'simple' larvae, all point to the peculiar nature of the conflict. But to uncover the latter is difficult.

Next we reproduce an account about the family Bibionidae as given by OLDROYD, 1964 :
The family Bibionidae, though related to the families Mycetophilidae and Sciaridae [we can say, they are Bibionidea that are related to the Fungivoridea], has become more thoroughly terrestrial, both as adults and as larvae. The adult flies are bigger and heavier, and some have a distinctly armoured appearance, complete with formidable spurs and spines on the legs and head (For a general view of the insect see Figure 1(A)  and  Figure 1a ).  The adults are predominantly flower-lovers, and hoverers in the sun, both habits that are shared by a great many flies. They are called 'March-flies' in North America, and  Bibio  marci  has been known in Europe as 'St Mark's fly', both names associating the Bibionids with spring, and a return to sunshine and flowers.  Dilophus  febrilis  and other small Bibionidae swarm over early flowering shrubs.
Male Bibionids have an enlargement of the upper facets of the eyes, possibly being associated with the habit of dancing in swarms. Bibionids certainly do this.
The larvae of Bibionids are among the most terrestrial, and probably the most primitive, of any flies. They have strong chewing mouthparts, and eat most kinds of vegetable food. In the soil they are often gregarious, and may be found as a mass of grubs in a pocket in the soil. They also breed in dung, and in caves.
As is appropriate to their terrestrial habits, Bibionid larvae have a very complete set of open spiracles, two thoracic pairs and eight on the abdomen.

The families Pleciidae, Hesperinidae, Pachyneuridae, and Axymyiidae

Within the complex of bibionoid and related flies (midges) the systematic status and position of the genera  Plecia, Penthetria, Pachyneura, Hesperinus, Axymyia (and also  Protaxymyia, Mesaxymyia) was (up to 1964) not so clear. However, new data provided by KRIVOSJEINA, 1969, might throw new light on the problem. These data are about the morphology of the respective larvae.
However, before we extract these data (together with imaginal features, such as the wings, antennae, legs, etc.) we will say something about the general features of larval morphology. For a complete understanding they will not be totally sufficient, but will ease the reading of the ensuing text (of  Krivosjeina,  pp 229) on these families and associated genera. In this preliminary we will consider the folllowing :

General appearance of the larvae of Bibionomorpha.

Generally, a larva of a fly of the Bibionomorpha has a head, which is more or less sclerotized, three thoracic segments, and about nine abdominal segments. The segments do, or do not, carry projections, spinelets or hairs. These larvae, like the larvae of all diptera, do not possess genuine legs. In most cases the appearance is more or less worm-like. See Figure 2 above .

Tracheal system of the larvae of diptera.

Often the sides of the segments carry spiracles, which are the exits of the tracheal system, guaranteeing contact with the air. Depending on which segments carry spiracles (one on the right side and one on the left side) we can distinguish six types of tracheal (pneustic) systems in dipterous larvae :

Figure 6 :  Types of the tracheal system in dipterous larvae.
1 -- Holopneustic.
2 -- Peripneustic.
3 -- Amphipneustic.
4 -- Metapneustic.
5 -- Propneustic.
6 -- Apneustic.
s = spiracle.
(After KRIVOSJEINA, 1969)

Let us describe -- following KRIVOSJEINA, 1969, pp. 141 -- these types.
The holopneustic type (Figure 6, 1 )  is characterized by the distribution of spiracles on the fore- and hind-segment of the thorax and on the first 8 abdominal segments. Such a type of tracheal system is characteristic for only a few groups of diptera  ( Hesperinidae, Pachyneuridae). In some cases in more specialized groups an  'inversion'  takes place, that is, a shift of the last pair of spiracles from the 8th to the 9th abdominal segment (Bibionidae, Pleciidae). But even in the most simple case we can observe differences of the sizes of the spiracles :  The first and the last pair have spiracles that are significantly larger than the others. There are nine well-developed abdominal body-segments.
The peripneustic type (Figure 6, 2 )  is characterized by the presence of spiracles on the first thoracic segment and on the first 8 abdominal segments. This distribution we see in the larvae of Ditomyiidae, Cecidomyiidae, Scatopsidae, and Hyperoscelididae.
In the larvae in some families a reduction has taken place of the last pair of spiracles. Only the first seven pairs of abdominal spiracles remain  ( Mycetophilidae, Sciaridae).
In a whole series of related families a further reduction of spiracles is observed, connected with a hidden way of life of the larvae. The larvae of Diadocidiidae which live in silk-like tubes, possess in all only one pair of thoracic spiracles :  Such a tracheal system belongs to the propneustic type (Figure 6, 5 ).  It is characteristic for the majority of dipterous pupae.
The apneustic type of tracheal system (Figure 6, 6 )  is characterized by the complete absence of spiracles on the body. It is typical of free-living larvae of Chironomidae and Ceratopogonidae (dwelling in damp places and in the water), Macroceridae and Ceroplatidae (living in narrow slimy tubes, or on the surface of fungi, or under bark). All nine abdominal segments are well-developed.
The amphipneustic type of tracheal system (Figure 6, 3 )  is characterized by the presence of only two pairs of spiracles, one on the first, the other on the last body-segment. Such a type of tracheal system is typical of the larvae of Anisopodidae  [ = Rhyphidae, = Phryneidae], Trichoceridae  [ = Petauristidae], the majority of Psychodidae, and of a large part of the Brachycera Ortho- and Cyclorrapha. For all, the development of only eight clear abdominal body-segments is typical.
The metapneustic type of tracheal system (Figure 6, 4 )  is characterized by the presence of only one pair of spiracles which lies at the end of the body. It is typical of the majority of larvae that live in the water  ( Tipulidae, Limoniidae, Culicidae,  Sycorax HAL. from the Psychodidae, and others).
    So the greatest number of spiracles in dipterous larvae is ten pairs. In larvae with a holo- or peripneustic tracheal system the spiracles are placed laterally on the body-segments or shifted to the dorsal side of the segments. For the majority of larvae with amphi- or metapneustic tracheal system we see a shift of the last pair of spiracles to the end of the body.

Head and mouthparts of larvae of (nematocerous) Diptera

Prior to presenting the data (given by KRIVOSJEINA) concerning the families Pleciidae, Hesperinidae, Pachyneuridae, and Axymyiidae, we will briefly expound the general structure of the head and its appendages (mouthparts) of the (nematocerous) dipterous larvae.
But before we do this we say something about the  g e n e r a l  structure of the mouthparts of insects (whether adults or larvae) as such.  See next Figure, which depicts the mouthparts of a very generalized ( = primitive, original) but still recent (primarily wingless  [ = apterygote] )  insect. Here the main parts, such as the maxillae (= lower jaws) and the labium (= lower lip), are still complete, that is, no parts of them have been evolutionarily disappeared or changed beyond recognition.

Figure 7 :  Mouthparts of  Petrobius  maritimus  (Apterygota, Machilidae).
1 -- Mandible.  2 -- Maxilla.  3 -- Hypopharynx (h) and superlinguae (sl).  4 -- Labium (= lower lip).
The Labrum (= upper lip, not drawn) only functionally belongs to the mouthparts. Morphologically it is associated with the head-shield.
Maxilla :  c = cardo,  s = stipes,  l = lacinia,  g = galea,  pf = palpifer,
mx.p = maxillary palp.
Labium :  m = postmentum,  pm = prementum,  pgr = palpiger,  gl = glossa,  pg = paraglossa,  lp = labial palp.
(After RICHARDS & DAVIES, Imms' General Textbook of Entomology, 1977)

Essentially, the mouth-apparatus comprises three pairs of appendicular jaws :
The anterior jaws, the mandibles, followed in turn by the maxillae, and a second pair of maxilla-like structures that fuse medially during embryonic development to form the labium or lower lip.
Closely associated with them are two unpaired, non-appendicular structures, the labrum or upper lip and the median, tongue-like hypopharynx.
The mouthparts vary in form to a greater extent than almost any other organs do, the variation being correlated with the method of feeding and other uses to which they may be subjected. An examination of the structure of the mouthparts will therefore give a clue to the feeding mechanism and frequently to te nature of the food of an insect.
Let us describe these main elements of the mouth-apparatus in insects (in the description of which, consulting  Figure 7  might prove useful) : The mouthparts of adults and also of larvae are strongly modified or even reduced in different insects, and it will often not be easy to recognize the mentioned parts in dipterous larvae. The mouthparts of course belong to the head, which in larvae of diptera generally consists of the head-capsule, the mouthparts and the tentorium  ( The tentorium is the internal skeleton of the head).
Before we generally describe the head of the dipterous (especially the nematocerous) larva, following KRIVOSJEINA, 1969, we first present the general scheme and topography of that head and the location of the mouthparts in it :

The larvae of most representatives of the suborder Nematocera possess a developed (and sclerotized) head-capsule. This head-capsule is like a short tube enclosing the internal parts of the head, first of all its internal skeleton -- the tentorium, but, of course, also the suboesophageal ganglion, the oesophagus (gullet), and the other internal organs. Often the head-capsule is dorsally extended forwardly, more or less forming a roof under which lie the mouthparts-proper -- mandibles, maxillae, and labium. The  labrum,  which functionally belongs to the mouthparts, is the distal part of the mentioned roof. The dorsal part of the head-capsule consists of a median frontal plate + clypeus (= anterior head-shield), often fused, resulting in one single plate, the fronto-clypeal plate), and distally the labrum is attached to the clypeus. Laterally attached (often by a seam) to the fronto-clypeal plate are the  side-plates (pleural plates) of the head-capsule (one plate on the right, the other on the right). They bend down around the sides of the head, and then approach each other again at the ventral side of the head. There they fuse together along a shorter or longer line or remain separated (only connected by a non-sclerotized patch). The eyes and antennae are placed more or less fronto-laterally on the head-capsule. Also the maxillae possess antenna-like sense organs :  maxillary palps (one on each maxilla).

Let us now succinctly describe the basic (and often modified or reduced) structure of the head of dipterous larvae as this (general) structure is expounded and analyzed by KRIVOSJEINA, 1969, pp. 55, and pp. 58 :

Up until today [1969] there does still not exist one definite understanding about the number of metamers (body-segments) and the morphological interpretation of the appendages of the insect head. Generally acknowledged is the fact that the head of the arthropods (insects, spiders, lobsters, etc.) is formed by the acron (head-lobe [-blade, -sheet], homologous with the prostomium of the annelids [ring-worms] and some segments coming [morphologically] after it. However, the question about the number of segments which, together with the acron take part in the formation of the head of the arthropods has been differently answered. The majority of investigators is inclined to believe that in the formation of the head of insects five segments are involved :  the antennal, intercalary, mandibular, maxillar, and labial segment, where the intercalary segment is only present in embryos.
The head-capsule proper (epicranium) is just the total of outer sclerotized elements of the head, excluding the eyes and the mouth-appendages.
Within the order Diptera we can observe in the larvae all transitions from a completely developed head-capsule to a totally reduced one, where even whatever trace of the capsule and mouthparts can be absent.
Depending on the stage of reduction one can usually distinguish three types of head-capsule :  (1) a well-developed massive head-capsule (Eucephala), (2) a partly reduced capsule (Hemicephala), (3) a totally reduced head-capsule (Acephala).
This division in its most general form expresses the chief direction of the evolution of the [larval] head in the order Diptera. But, while the first and last category are sufficiently definite, groups with partial reduction of the head-capsule include very heterogeneous forms.
The change and refinement of the ability to feed in dipterous larvae have in the evolutionary process led to the origin -- within the Order -- of the most diverse types of head-capsule and mouthparts [oral apparatus]. The overall tendency detected within the Order Diptera is a general simplification, and in a whole series of cases the reduction, of individual elements of the head-capsule and of the mouth-apparatus or of the head as a whole. There takes place a replacement of many elements of the mouth-apparatus (well-developed in the majority of insects and lower diptera) by just one pair of mouthhooks in the cyclorraphic diptera. Such changes lead to a fundamental transformation of the musculature of the head and of the very head-capsule.
One can, as does KRIVOSJEINA, pp. 58, distinguish within each of the three above mentioned groups  ( Eucephala, Hemicephala, Acephala )  a number of structural types of the head-capsule and mouth-apparatus in the larvae of diptera. For our preliminary understanding of these larval strucures in the families Pleciidae, Hesperinidae, Pachyneuridae, and Axymyiidae it is sufficient to describe -- as a whole -- the first group, the Eucephala, (consisting of several types), as it is defined by KRIVOSJEINA :
Group I -- Larvae with a well-developed and [thus] not reduced head-capsule (Eucephala).
In the capsule the central part is well-distinguished (frontal plate) and also the lateral plates of the head, which ventrally fuse with each other, that is, touch each other in one or several points or, on the other hand, are more or less separated. The internal skeleton of the head (tentorial rods), if present, lies within the confines of the head-capsule. Mouth-apparatus well developed, of the biting type, all its parts expressed, and the mandibles and maxillae completely [morphologically] independent of each other.
To this group belong the larvae of the majority of nematocerous diptera [midges, mosquitoes, crane-flies, etc.].
Depending on the nature of the structure of the head-capsule and the mouth-apparatus within the range of this group we can distinguish seven basic types of head-capsule :  the Trichocerid-Anisopodid, the Chironomid, the Ptychopterid, the Culicid, the Ditomyiid, the Mycetophilid, and the Bibionid type.

Without describing all these types one by one (which is done by KRIVOSJEINA), we will describe one type only, namely the Bibionid type, which is especially relevant for interpreting the genera  Plecia, Penthetria, Pachyneura, Hesperinus, Axymyia (and also  Protaxymyia, Mesaxymyia). In it we will describe and depict the structure of the head-capsule and the mouthparts of the larvae of a species of the genus  Plecia  and a species of the genus  Bibio  ( Bibionidae).

Bibionid type (KRIVOSJEINA (1969), pp. 70).
Head-capsule with massive side-plates which are shaply narrowed on the ventral side. The elements of the mouth-apparatus are heavily sclerotized. The head-capsule expressed by this type is characteristic of the larvae of Bibionidae, Pleciidae, Pachyneuridae, and Hesperinidae. Let us, as an example, analyze the morphology of the head of the little known larva of  Plecia  nigra LUND.  See next Figures.

Figure 8 :  Head of the larva of  Plecia  nigra LUND.  ( Pleciidae).
1 -- head, dorsal view.  2 -- head, ventral view.
lbr = labrum (upper lip),  cl = clypeus,  md = mandible,  fr = frons,  lb = labium (=lower lip: the small light-colored more or less circular [as drawn] part at the center of the ventral side of the head-capsule),  mx = maxilla. (After KRIVOSJEINA, 1969)

Figure 8a :  Mouth-apparatus of the larva of  Plecia  nigra LUND.  ( Pleciidae).
1 -- Maxilla, ventral view.  2 -- Maxilla, dorsal view.  3 -- Mandible.  4 -- Labium and Hypopharynx, ventral view.
crd = cardo,  hyp = hypopharynx,  lac = lacinia,  lb = labium, = maxillary palp,  st = stipes.
(After KRIVOSJEINA, 1969)

In the central part of the head a small triangular frontal plate is clearly delimited. The side-plates of the head-capsule are large, massive, fused at the ventral side. The seam at the place of their unification is black. The clypeus is represented by a massive rectangular plate making an angle (in a lateral view) with the frontal plate and is clearly distinguished from it. The antennae are strongly reduced. They present themselves as elongate-oval lightly-colored humps lying at the base of the clypeus. The eyes are positioned behind them at the sides of the head. They are reprepresented there by small transparent vesicles.
The mouth-apparatus is of the biting type. The upper lip (labrum) is represented by a broad rounded plate with two sclerotized projections laterally at its fore-margin. At the anterior end of the lip are placed three large beaker-shaped sensillae, some stick-like sensillae, and hairs. The whole ventral surface of the lip is also covered with hairs and spinelets, and in addition there are two large sensillae in its middle part at both of its sides. The mandibles are well developed, massive, with four clearly delimited teeth at its tip. The maxillae are represented by cardo, stipes, and lacinia with galea. The cardo presents itself by an elongated rod. The stipes presents itself by a small sclerite carrying a one-segmented palp, itself carrying many sensilla at its end. The lacinia is broad, with a large lateral plate carrying teeth and hairs. The lower lip (labium) is heavily sclerotized along its periphery. It has a rectangular shape. Its fore-margin with two large rounded projections. The central light-colored part of the labium carries sensilla and a groups of hairs, which indicates that it belongs to the prementum. On its dorsal side the labium borders at the transparent plate of the hypopharynx, carrying on its surface many thin hairs. The anterior margin of this [hypopharyngeal] plate is even, without individual lobes. As a whole, the complex of lower lip and hypopharynx is unified with the head-capsule by an elongated projection. All parts of the mouth-apparatus, as well as the head-capsule, are strongly sclerotized and black.
Thus, characteristic of the head of the larvae of  Plecia  nigra LUND. are :  a short frontal plate, clearly distinguished from the clypeus, ventrally fused and strongly narrowed side-plates, heavily sclerotized elements of the mouth-apparatus including the lower lip and the maxillae.

Most similar to the head-capsule of  Plecia  nigra LUND. is the head-capsule of the larvae of the family Bibionidae. See next Figure, where in its subscript one can click to some magnified parts of this Figure :

Figure 9 :  Details of the structure of the head of the larva of  Bibio  pomonae FABR.  ( Bibionidae).
1 -- head, dorsal view (see HERE, upper figure ).
2 -- head, ventral view (see HERE, lower figure ).
3 -- upper lip (labrum), right :  dorsal view, left :  ventral view.
4 -- mandible.
5 -- lower lip (labium) and hypopharynx.
6 -- maxilla (see HERE ).
cr = comb.  crd = cardo.  fr = forehead (frons).  lac = lacinia.  lb = lower lip.  lbr = upper lip.  md = mandible.  mx = maxilla. = palp of maxilla.  st = stipes.
(After KRIVOSJEINA, 1969)

The head-capsule of the larvae of  Pachyneura ZETT. and  Hesperinus WALK. shows a series of essential differences, but still preserving the same structural type.
In the larvae of Bibionidae the side-plates of the head-capsule are fused ventrally, [morphologically] pressing forward the lower lip (labium) which is firmly attached to the head-capsule through a [more or less] elongated outgrowth [projecting forwardly from the posterior margin of the head-capsule] (see HERE, lower figure ). The head-capsule has a fronto-clypeal plate which is one single whole [that is, which is not divided into a frons and a clypeus (see HERE, upper figure ) ].  The upper lip (labrum) is broad, transverse.

In the larvae of  Pachyneura ZETT. (see HERE )  and  Hesperinus WALK. (see HERE )  the side-plates of the head-capsule are not fused ventrally, The lower lip (labium) is morphologically distinguished from the elements of the head-capsule. The frontal plate and the clypeus are almost fused. They are separated by a weakly expressed seam and clearly distinguished from the upper lip (labrum). The elements of the mouth-apparatus of  Hesperinus WALK. are strongly sclerotized just as in the families Bibionidae and Pleciidae. In  Pachyneura ZETT. the fore-margin of the lacinia is membraneous. Only a small part of the cardo is sclerotized.
Characteristic of the head-capsule of the larvae of the present type are the strongly sclerotized elements of the mouth-apparatus, the peculiar rod-like cardo, and the rectangular lower lip.
All this indicates the significant specialization of the head as a whole, which has reached its strongest expression in the larvae of Bibionidae.
Of the [mentioned] outgrowth, unifying the head-capsule of the larvae of Bibionidae and Pleciidae with the lower lip, do not exist homologous structures in other groups.

Of the larval head belonging to either the ditomiid, mycetophilid, or bibionid type, is characteristic the narrowing of the side-plates of the head-capsule on the ventral side, often touching each other in a single point only, which is expressed by the position of the head with respect to the body. In the larvae of Hesperinidae, Pachyneuridae, and Bibionidae, the head clearly is hypognathous, which is also characteristic of mycetophilid diptera, although in some cases a secondary expansion of the ventral walls of the head-capsule is observed, or supplementory massive sclerites develop which join up with the structure of the head-capsule.

Having expounded the general appearance, the tracheal system, and the general morphology of the head and mouthparts, of nematocerous larvae, we are now, finally, ready to give the exposition of KRIVOSJEINA (1969) pp. 229 [to which exposition we add things when necessary], dealing with the mentioned nematocerous families :  Pleciidae, Hesperinidae, Pachyneuridae, and Axymyiidae, that is, interpreting the genera  Plecia, Penthetria, Pachyneura, Hesperinus, Axymyia (and also  Protaxymyia, Mesaxymyia), all belonging to the infraorder Bibionomorpha in the neighborhood of the superfamilies Fungivoridea and Bibionidea  ( The Pachyneuridae might well belong to the Tipulomorpha, especially by reason of the imaginal features [that is, the features of the adult fly] ).
[When following the text -- which rightly considers not only features of the adult (= imaginal) fly, but also the features of the larva -- we must realize that KRIVOSJEINA determines the affinities, taxonomic rank, and systematic position, of the mentioned genera purely in terms of comparative morphology without attempts to  f u n c t i o n a l l y  interpret the discussed structures, and also without involving the  w a y  o f  l i v e  of especially the larvae. Moreover, the comparative morphology here is purely typological. It rarely considers the discovered morphological differences as representing survived stages of morphological  t r a n s f o r m a t i o n s  having taken place in certain evolutiononary  d i r e c t i o n s.
What we ourselves are after is to distinguish the  e s s e n c e s  (in the metaphysical sense, that is, the intrinsic kernels of the different substances {also in the metaphysical sense} of the different taxa (considered as substances), especially of families (Earlier we have considered to be genuine substances {in the metaphysical sense} not only individual organisms but also species of them and also the higher taxa.). This largely coincides with the 'typological' approach in taxonomical systematics, as we see it in KRIVOSJEINA (and also in ROHDENDORF, but here with much more emphasis on function and ecology), and as it contrasts with the 'phylogenetic systematics' as initiated by HENNIG. In the latter it is the  g e n e t i c  kinship and  g e n e t i c  affiliations that matter, in which it is, consequently, assumed that the  e s s e n c e  (also in the metaphysical sense) of an organism or of a taxon is entirely contained in, or represented by, its genome (that is, its set of genes). However, it has been found out (see for example, STEWART & COHEN, 1994, The Collapse of Chaos ) that there is no simple one-to-one relationship between (phenotypic) feature and gene (or a particular group of genes). And this means that a purely phylogenetic system, that is, a classification of organisms entirely and exclusively in terms of, and based on,  g e n e t i c  kinship relations, does not single out and compare the  e s s e n c e s  of the different taxa, that is, such a phylogenetic system does not express  what  they -- the different taxa -- essentially are.  A typological system of classification does so, as long as it is not purely formal, but interprets the features of the taxa  f u n c t i o n a l l y  and  e c o l o g i c a l l y.

Now the exposition of KRIVOSJEINA (with additions and small changes where necessary) :

The mentioned families are relict groups of diptera, known in the world-fauna by 1-3 genera :
Plecia WIED.,  Pleciidae.
Penthetria MEIG.,  Pleciidae.
Pachyneura ZETT.,  Pachineuridae.
Hesperinus WALK.,  Hesperinidae.
Axymyia Mc ATEE.,  Axymyiidae.
Protaxymyia MAM. et KRIV.,  Axymyiidae.
Mesaxymyia MAM.,  Axymyiidae.
Up until today [1969] the systematic position of the listed genera had remained very obscure.
The opinions of the greatest dipterologists of the world fundamentally differ with respect to the taxonomic rank of these groups as well as to their position in the taxonomic system. In the monograph of Duda (Duda, 1930) the representatives of the genera  Plecia WIED., Penthetria MEIG., Pachyneura ZETT., Hesperinus WALK., together with  Axymyia Mc ATEE., are considered within the confines of the family Bibionidae. Hennig, 1954, while investigating the phylogeny [in the sense of setting up a purely phylogenetic system exclusively based on genetic kinship relations] of the Diptera, considers  Hesperinus WALK. together with  Penthetria MEIG. and  Bibio GEOFFR.,  but points to their [taxonomic] independency. But  Pachyneura ZETT. and  Axymyia Mc ATEE. are included by him in the mycetophiloid complex [roughly coinciding with the superfamily Fungivoridea].
Accordingly, some investigators place the representatives of the mentioned genera closely to, or into, the family Bibionidae, while others place them closely to the mycetophiloid complex. The question concerning the taxonomic rank of these groups also remained in dispute. The difference of opinion becomes clear by the fact that in the majority of cases investigators attributed great phylogenetic significance to [mere] secondary features. But the correctness of this or that viewpoint cannot be established because of the absence of expositions concerning the ontogenesis [here it is meant :  the morphology of the larvae] of the representatives of these groups.
Our [Krivosjeina and Mamajev] [over many years] continued collecting of larvae have partly met with success. In the Coastal district [Primorski krai] in the years 1964 and 1967 a large amount of larvae of the representatives of these families have been found and collected.
[The larva as well as the imago will be investigated in the present text. The next Figure, taken from KRIVOSJEINA, depicts the wing-venation of the insects under investigation together with that of some other more or less closely related forms.]

Figure 10 :  Wings of the representatives of the families Ditomyiidae, Bibionidae, Pleciidae, Hesperinidae, Pachyneuridae, Axymyiidae, Mycetobiidae, Tipulidae, and Leptoconopidae. In order to ease the reading of the subscript the reader can click on "Figure".
1 -- Ditomyia  fasciata MEIG.  ( Ditomyiidae ). ( Figure )
2 -- Bibio  marci L.  ( Bibionidae ). ( Figure )
3 -- Plecia nigra LUND.  ( Pleciidae ). ( Figure )
4 -- Hesperinus  rohdendorfi KRIV. et  MAM.  ( Hesperinidae ). ( Figure )
5 -- Pachyneura  sp.  ( Pachyneuridae ). ( Figure )
6 -- Mesaxymyia  kerteszi DUDA.  ( Axymyiidae ). ( Figure )
7 -- Mycetobia  pallipes MEIG.  ( Mycetobiidae ). ( Figure )
8 -- Idiotipula  confluens ALEX.  ( Tipulidae ). ( Figure )
9 -- Leptoconops  borealis GUTZ.  ( Leptoconopidae ). ( Figure ).
an = anal vein,  cu = cubital vein,  m1-4 = medial veins,  m  and  rm  = radio-medial cross-vein [we might also write :  r-m ),  r1-5 = radial veins  [ r1 = Radius,  r2 + r3 + r4 + ... = Radial Sector ].
(From KRIVOSJEINA, 1969,  1-7, 9 after KRIVOSJEINA, 1969,  8 after HENNIG, 1954)

On the basis of this material, let us analyze the mutual connection of these very ancient groups with the families of the bibionid and mycetophilid complexes of diptera, first of all with the Ditomyiidae as representing the least specialized among the mycetophilids, and also with family Bibionidae, in which the representatives of the mentioned genera were placed by many investigators.
Let us consider the basic and most chararacteristic features of the structure of the larvae and imago in Ditomyiidae and Bibionidae.

Ditomyiidae,  larva.
The larvae of the representatives of the family Ditomyiidae possess a well-developed head-capsule, with side-plates which ventrally meet each other along a short line, and with a clypeus clearly distinct from the frons (frontal plate). See HERE for  Symmerus  annulatus.
The upper lip (labrum) is broad, transverse. The mandibles are massive and rectangular. The maxillae and lower lip (labium) blade-like, weakly sclerotized.
Body with clearly expressed 12 segments without secondary segmentation, naked, only with rare small spinelets on the dorsal as well as on the ventral side.
Tracheal system peripneustic (see Figure 6 (2), above ),  the spiracles are placed on the prothorax  [ = anterior part -- first segment -- of thorax] and on 8 abdominal segments, each with a large central moult seam and a grated peritrema [We will not expound these latter features as to what they precisely are.].
Middle gut with two blind sacs at its anterior section. The Malpighian tubules are simple, isolated.  [ To give the reader an impression of the digestive system of larvae of Nematocera, we have a drawing of this system as it is in  Mycetophila,  which genus does not, it is true, belong to the Ditomyiidae, but to the closely related family Mycetophilidae  ( = Fungivoridae). And both families belong to the superfamily Fungivoridea. The drawing, then, is  HERE ].

Ditomyiidae,  imago.
The imago of the representatives of the Ditomyiidae has usual, a little elongated legs.
The wing-venation is weakly costalized, and a clear radial sector is characteristic.  The long  r1  (Radius) ends (in the Costa) at the level of the distal half of the wing. See next Figure (In our text, in the signs for the veins, we do not use subscripts, we place the numbers just next to the letters).

Figure 10_1 :  Wing of  Ditomyia  fasciata MEIG..  (1 of Figure 10).

The Radial Sector branches (into two branches) distally from the cross-vein  r-m ,  and  R5  ends up at the wing-tip. The cross-veins  r-m  and  m  lie in the central part of the wing and make an angle with each other, where  r-m  lies (a bit) more distally than  m.  The medio-cubital cross-vein (mcu), like the medial cross-vein (m), makes an angle with  r-m  [In the interpretation of KRIVOSJEINA the short vein that connects M4 with Cu1 is the medio-cubital cross-vein (mcu), while in the interpretation of other authors it might just be the proximal part of the anterior branch of the cubital fork].  Both veins (m and mcu) look little like cross-veins and make only a small angle with the costal vein of the wing. The central trunk of the medial system of veins is absent. The discoidal cell is absent  [ For the discoidal cell, see Figure 10 (8), where this cell is present and marked in the drawing.]  The anal vein (An1) goes all the way to the posterior wing margin.

Bibionidae,  Larva.
The larvae of the representatives of the family Bibionidae have a massive head-capsule with its side-plates ventrally fused, and with a single whole, that is, undivided, fronto-clypeal plate. See Figure 9, above .  Upper lip (labrum) broad, transverse. Mandibles massive, one-segmented, triangular. The maxillae and lower lip (labium) more or less rectangular, strongly sclerotized. The body consists of 12 distinct segments with the prothoracic segment secondarily divided, and carries many soft conical projections  [See Figure 2 (above), top ].
Tracheal system holopneustic (see Figure 6 (1), above ),  the spiracles distributed over the pro- and metathoracic segments and over 8 abdominal segments. Here the last [pair of] spiracles are not placed on the eighth but on the ninth segment [which feature is called 'inversion']. These last spiracles are complex, with two or three moult seams.
The middle gut with three unpaired blind sacs on its anterior part and often with unpaired outgrowths on its posterior part. Malpighian tubules united at the base.

Bibionidae,  imago.
In the adult form of the representatives of the family Bibionidae the first antennal segments are clearly distinguished and short. Fore-tibiae are thickened and shortened, with combs of spines or massive spurs.
The wing-venation is strongly costalized. Only two long radial veins exist [Radius and an unbranched Radial Sector]. The first ends in the fore-margin past its middle, the second ends up at the costal vein, it does not continue down to the wing-tip. See next Figure.

Figure 10_2 :  Wing of  Bibio  marci L.  (2 of Figure 10). [ For comparison with the wing of  Ditomyia,  see Figure 10 (1)(2) ]

The cross-veins  r-m  and [especially]  m  are shifted towards the wing-tip, they are almost parallel to each other and make an angle with the costal vein of almost 90 degrees. The trunk of the medial system of veins is well developed, it forks at the level of the medial cross-vein (m). Discoidal cell is absent. Anal veins clearly do not reach the wing-margin.

Let us now compare to the Ditomyiidae (Fungivoridea) and Bibionidae (Bibionidea) representatives of the genera  Plecia WIED.,  Hesperinus WALKPachyneura ZETT.,  Axymyia Mc ATEE.,  Protaxymyia MAM. et  KRIV.,  Mesaxymyia MAM.,  that is to say, to the representatives of the families Pleciidae, Hesperinidae, Pachyneuridae, and Axymyiidae.

Family  P L E C I I D A E

The majority of dipterologists included and include  Plecia WIED. and  Penthetria MEIG. in the family Bibionidae, in which usually these genera were distinguished as representing a subfamily. However, detailed investigation of the larvae of  Plecia WIED. and articles in the literature concerning the larvae of  Penthetria MEIG., bear witness to the significant idiosyncrasy of the larvae of both genera, making it legitimate for us to consider the subfamily Pleciinae as being in fact a separate family, Pleciidae. This conclusion, which was based on a study of the larvae, is also confirmed by data concerning the imago.
In the larvae of  Plecia WIED., see Figure 8  and  Figure 8a ,  we can observe differences with the Bibionidae in the structure of the fronto-clypeal region, of the spiracles, and of the topography of projecions [outgrowths] on the body. The clypeus is clearly distinct from the frons [frontal plate] and makes an angle with it, while in Bibionidae we have to do with a single undivided [fused] structure. The lacinia is very peculiar, [in] having an extra side-plate (see Figure 8a-2 ).  The last [pair of] spiracles in this genus (Plecia), as in many diptera, are simple, represented by a plate carrying a moult seam in the center and grated holes at the periphery.
The middle gut with two lateral anterior blind sacs and with one posterior sac. Malphigian tubules close to each other.
Accordingly, in the structure of the head, of the mouth-apparatus, the spiracles, and of the digestive system, of the larva, we can observe clear differences from the larvae of  Bibio GEOFFR. and  Dilophus MEIG., which we consider to be representatives of the family Bibionidae. But the larvae of  Plecia WIED. possess a whole series of features which connect them with the Bibionidae. The similarity expresses itself in the global shape of the body, the development of soft conical projections on the segments (see Figure 2 ),  the degree of sclerotization of parts of the mouth-apparatus, and the position of the Malpighian tubules. But also these features have their specifity. Thus in the larvae of  Plecia WIED. along the posterior margin of the last segment, laterally and ventrally are placed eight soft conical outgrowths which are absent in the larvae of Bibionidae (see Figure 2 ).
The structure of the wing, as well as of the legs, of the imago, sharply distinguishes the representatives of  Plecia WIED. from those of the Bibionidae.
In the imago the antennae have short but clearly visible first segments. The legs are simple, without series of combs. Fore-tibiae normal, elongate, not thickened.
The wing-venation, see next Figure, is weakly costalized.

Figure 10_3 :  Wing of  Plecia  nigra LUND.  (3 of Figure 10). [ Next to the cross-vein (m) between M1+2 and M4 there is drawn the sign "m" ]. [ For comparison with the wings of  Ditomyia,  and  Bibio,  see Figure 10 (2)(3) ]

There are three, not two, radial veins [namely R1, R2+3, R5 ].  The first runs into the wing-margin past its mid-point. The third ends up before the wing-tip. The Radial Sector forks into two branches distally from the cross-vein  r-m  (radio-medial cross-vein). The cross-veins (r-m and m) lie in the middle of the wing, and  r-m  is parallel to  m,  making a right angle with the Costa. The common trunk of the medial system of veins is well developed. It forks distally from the medial cross-vein (m). The discoidal cell is absent. The anal veins do definitely not reach the wing-margin.
Accordingly, of the larvae of  Plecia WIED. the following features are characteristic :  Clypeus well marked of from the frons. The presence of an extra plate in the maxilla [namely on the lacinia], primitive spiracles. And for the imago :  Simple prothorax and fore-legs, three, and not two, radial veins, and a more or less uniform sclerotization of the veins of the wing. Similar features are possessed by the representatives of the genus  Penthetria MEIG.
The features of the larvae as well as of the imago first of all show that  Plecia WIED. and  Penthetria MEIG. are more primitive than Bibionidae, and also show their long independent development. The differentiating features of both stages [larva and imago] which concern basic traits of their organization, point to the appropriateness of erecting a separate family [instead of just a subfamily] for these two genera, the family Pleciidae, closely related to the Bibionidae.

Family  H E S P E R I N I D A E

This family is represented by only the one genus  Hesperinus WALK., which has 6 species.
For a long time the dipterologists did not agree concerning answering the question about the taxonomic rank of the genus as well as the question concerning its position in the taxonomic system of the Order. Hendel (1928, 1936) sees it as constituting the independent family Hesperinidae. Duda (1930) includes the genus  Hesperinus WALK. in the subfamily Pachyneurinae of the family Bibionidae. Edwards (1930) also considers the genus  Hesperinus WALK. as belonging to the family Bibionidae. Hardy (1945, 1958) repeats this same inaccurateness. Finally, Hennig (1948) and Rohdendorf (1964) placed  Hesperinus WALK. in an independent family next to the Bibionidae.
How can we clarify this when considering features of the larvae as well as of the imago?
The larvae [of  Hesperinus] superficially partly recall the larvae of the bibionids, and partly those of the mycetophilids.
The head-capsule is strongly sclerotized, almost black (see HERE, for a drawing ).  The side-plates of the head-capsule not touching each other ventrally [meant is apparently that they are not ventrally fused with each other]. The clypeus is clearly distinct from the frontal plate. The structure of the mouth-apparatus (see HERE, for a drawing )  has much in common with that of the larvae of Bibionidae :  Upper and lower lip of the same structural type, so also the mandibles and the maxillae. But the elements of the lower lip and of the hypopharynx are somewhat more independent. The body consists of 12 clearly distinct segments with a secondarily segmented prothorax. The segments lack projections [outgrowths] of any sort. Only series of spinelets are visible on the body. See next Figure.

Figure 11 :  Larva of  Hesperinus  rohdendorfi KRIV. et  MAM.  ( Hesperinidae), lateral view.
(After KRIVOSJEINA, 1969)

Tracheal system holopneustic (see Figure 6 (1), above ).  The spiracles lie on the first and last thoracic segments and on the first 8 abdominal segments (meaning that there is no inversion of the spiracles -- that is, no shift of the last pair from the eighth to the ninth abdominal segment). Such a spiracle is represented by an oval plate with a large central moult seam and a grated peritrema. The last body segment lacks spiracles. Middle gut with only two anterior blind sacs. Malpighian tubules close to each other.
Of the imago of  Hesperinus  the following features are characteristic :  Antennae 12-segmented. They are long, significantly surpassing the length of head and thorax. The first segment is the longest, the next one is almost twice as short. Further up the length of the segments gradually (but not significantly) diminishes. The eyes of the male are circular, separated by a broad frons. They are simple, that is, they consist of uniform facets. The legs are thin and ordinary.
For the wing see next Figure.

Figure 12 :  Wing of  Hesperinus  rohdendorfi KRIV. et  MAM.  ( Hesperinidae). (4 of Figure 10).

The wing-venation is moderately costalized. All in all there are three radial veins :  Of them R1 runs into the wing-margin past its center. R5 is long, it reaches the wing-tip. Radial Sector forks distally from  r-m.  This cross-vein  r-m  runs parallel to the cross-vein  m,  and both almost make a right angle with the costal vein. The common trunk of the medial veins is well developed. It forks distally from the medial cross-vein (m). Discoidal cell absent. Anal vein reaches the wing-margin.
Accordingly, for the larvae of  Hesperinus WALK. are, in contrast to the Bibionidae, the following features characteristic :  A clear separation of the side-plates of the head on the ventral side, division of the fronto-clypeal plate, simple spiracles in which no inversion of the last pair is observed, body without outgrowths, with long spinelets.
These features show a connection of the larvae of the genus  Hesperinus WALK. with those of the mycetophilid complex. Similarity with the larvae of Bibionidae is only observed in the structural type and sclerotization of the mouth-apparatus.  [ It might be instructive to compare the larval head of  Hesperinus (Hesperinidae), on the one hand, with that of  Symmerus (Ditomyiidae, mycetophilid complex) and  Bibio (Bibionidae) on the other :  Hesperinus HERE ,  Symmerus HERE ,  Bibio HERE . ( To return, press the BACK button of the browser) ]
In the imago there is in the wing-structure much similarity with the representatives of the genus  Plecia WIED. :  The same structure of the radial sector and the character of branching of the medial veins. In the structure of the very peculiar antennae the species of the genus  Hesperinus WALK. distinguish themselves from all closely related groups of Nematocera.
In the present case the following conclusion forces itself upon us :  First, the genus  Hesperinus WALK. is so idiosyncratic that it naturally must figure as an independent family. Secondly, the family is relatively close to the Pleciidae especially with respect to features of the imago, but nevertheless finding itself somewhere between the Pleciidae and the mycetophilid diptera, to which it parly connects by larval features.


The family Pachyneuridae is represented by the one genus  Pachyneura ZETT., which has five species, from which in Europe  P. fasciata ZETT. is known. Let us give some short information concerning the history of its interpretation. Hendel (1928, 1936) acknowledges the independent family Pachyneuridae with two genera  Pachyneura ZETT. and  Axymyia Mc ATEE.  Duda (1930) considers it, with that same content, within the family Bibionidae as a subfamily. Edwards (1928) again acknowledges the Pachyneuridae as an independent family, including also the genus  Axymyia Mc ATEE, and has them closely related to the Anisopodidae  [ = Rhyphidae]. Hennig (1954) also considers  Pachyneura  and  Axymyia,  but places them into the mycetophilid complex. Rohdendorf (1964) acknowledges the independent family (without Axymyia ),  but places it on the basis of the structure of the wings into the Tipulomorpha. Finally, in the compilation of North American Diptera of the American authors (Stone et al., 1965), the Pachyneuridae are taken in the earlier content (with Axymyia )  and are considered to be closely related to the Anisopodidae. Accordingly, a part of the authors considers the genus  Pachyneura  to be closely related to the Bibionidae, while the majority places it more closely to the Anisopodidae.
The larvae of  Pachyneura  are so peculiar that they superficially do not seem to be dipterous larvae at all. See next Figure.

Figure 13 :  Larvae of Pachyneura sp. ( Pachyneuridae ).
(After KRIVOSJEINA, 1969)

They look more like caterpillars with their large quadratic head and the long spinelets on the body. But this is just a mere superficial impression. In the structure of the body, its coverings, and the head-capsule, the larvae are certainly close to those of the Bibionidae as well as to those of the Hesperinidae. For the morphology of the larval head, see HERE .  The head-capsule is short and broad, the side-plates of the head do not ventrally touch each other. The frontal plate almost fused with the clypeus. It is distinct from it by a weakly expressed seam. The mouth-apparatus is structurally very close to that of Bibionidae  [ see HERE ][use BACK button to return]  and of Hesperinidae HERE ][use BACK button to return]  (blade-like upper lip, massive mandibles, sclerotized lower lip). The cardo of the maxilla is just a very small sclerite, but rod-like in shape. The anterior margin of the lacinia is not sclerotized, it is membranous. On the ventral side of the head, under the side-plates, there are massive sclerotized rods, which can be considered to represent the tentorium.
The body of the larvae [of the Pachyneuridae] consists of three thoracic and nine abdominal segments. The prothorax is secondarily divided into two rings. The segments of the body are clearly distinct dorsally as well as ventrally, without soft outgrowths, but with long dark spinelets, which well stand out against the light-colored background of the segments. Their topography is different from that in  Hesperinus WALK.
Tracheal system holopneustic (see Figure 6 (1) ).  The spiracles lie on the fore and hind segment of the thorax and on the first eight abdominal segments, that is, there is no inversion. The spiracle is represented by an oval plate with one large moult seam in its center. The last pair of spiracles is lightly shifted to the dorsal side of the segment. Middle gut only with two anterior blind sacs. The Malpighian tubules consist of two groups :  three tubules close to each other, and one isolated.
The imago of  Pachyneura ZETT. with its long narrow wings, long and thin legs, its elegant slender body, lets us think of it to be more like Anisopodidae and representatives of the families of the Tipulomorpha than like Bibionids.
Of the imago of  Pachyneura ZETT. the following features are characteristic :  Antennae 17-segmented, all segments (after the first) cylindrical and having about the same size. The eyes of the males are separated by the frons, they have a uniform structure [same type of facets]. Legs thin, ordinary. The wing-venation is moderately costalized. For this see next Figure.

Figure 14 :  Wing of  Pachyneura  sp.  ( Pachyneuridae ). (5 of Figure 10).
[In interpreting the wing-venation of  Pachyneura  we must realize that the common median trunk here is in fact only a weakly expressed vein (according to HENNIG, 1954)]

All in all there are four radial veins. R1 is long, it reaches the distal fourth of the wing. The Radial Sector forks at the level of the cross-vein  r-m.  R2+3 branches off from R4 almost at the wing-tip. R4 and R5 close to each other at the base, not parallel. R5 long, it reaches the wing-tip. The cross-veins  m  and  mcu  make an angle with  r-m,  and a small angle with the fore-margin of the wing. The cross-veins  r-m  and  m  make an angle with each other. The common trunk of the medial veins is developed, it forks distally from the medial cross-vein (m). Discoidal cell absent. Anal vein reaches the wing-margin.
Those features of the larvae, as the clearly expressed separateness of the side-plates [from each other ventrally] of the head, the presence of peculiar massive tentorial rods, the weak sclerotization of the maxillae, the remarkable shortened head-capsule, and, finally, the structure of the digestive and excretory systems, bear witness to a significant idiosyncrasy of the group [the genus  Pachyneura ].
The nature of the structure of the upper lip, the mandibles, and partially of the maxillae, further, the structure and distribution of the spiracles over the body, the global structure of the body, and, finally, its coverings, show the representatives of the genus  Pachyneura  to be closely related to the Hesperinidae.
The features of the imago are very idiosyncratic and point to the taxonomic independency of the genus from the Hesperinidae (and thus more so from the Bibionidae) as well as from the Ditomyiidae.
The genus  Pachyneura ZETT. has, in the wing-venation of its representatives, a well-developed medial trunk  [We should especially emphasize that the basal piece of the medial vein is -- as a genuine vein, and not just as a fold -- developed over its whole length, that is, from the wing-base to the medial cross-vein  m  and beyond.]. The Radial Sector forks at the level of  r-m  and consists of three branches (all in all 4 radial veins). In the, as to the wing-venation closest forms, namely the representatives of the genus  Ditomyia WINN., the medial trunk is absent, and there are all in all three radial veins. The Radial Sector forks distally from the cross-vein  r-m.  But nevertheless the general structural plan of the wings of  Pachyneura ZETT. and of those of the mycetophilid complex [to which  Ditomyia  belongs], as also Hennig (1954) had shown, are actually very similar. See next Figure.


Figure 15 :  Wing-venation.
Left -- Ditomyia  fasciata MEIG.  ( Ditomyiidae ).
Right -- Pachyneura  sp.  ( Pachyneuridae ).
(1 and 5 of Figure 10).

Similarity is not observed with respect to the number of veins, but to the general scheme of their mutual position, first of all to the relative position of the cross-veins. In  Pachyneura ZETT. as well as in  Ditomyia WINN. the cross-vein  r-m  makes an angle with the medial (m) and the medio-cubital (mcu) cross-vein, while the latter makes a very small angle with the wing margin, R5 runs into the Costa at the wing-tip.
It is easy to imagine the wing of  Ditomyia  as having originated from a wing close to the type of  Pachyneura.  But apart from some similarity,  Pachyneura  stands significantly [taxonomically] farther away from Ditomyiidae than the latter stand from the remaining groups of mycetophilid diptera.
Accordingly, the structure of the larva as well as of the imago of  Pachyneura ZETT. is in such a high degree idiosyncratic that the genus cannot be included in any nowadays existing family. Apparently we can confidently assert the existence of an independent family Pachyneuridae. In the larval features the genus  Pachyneura ZETT. stands most closely to the genus  Hesperinus WALK.,  but in imaginal features it is connected more to the mycetophilid complex than  Hesperinus  is.

Family  A X Y M Y I I D A E

Even more disagreement existed about the position of the genus  Axymyia McATEE  in the system of Diptera. Soon after its description Shannon (1921) placed the new genus in the family Anisopodidae  [ = Rhyphidae] as the subfamily Axymyiinae. After Duda (1930), Enderlein (1936) included  Pachyneura ZETT.  and  Axymyia McATEE  in the family Bibionidae. Hendel (1928, 1936) brings the two genera closely together constituting an independent family. The same position is held by Edwards (1925), Hennig (1948, 1954) and, finally, by the collective of American authors (Stone et al, 1965). A different position is held by Rohdendorf (1946). He lets  Axymyia McATEE  represent an independent family which is related to the extinct Paraxymyiidae and Eopleciidae. Newly available material concerning the present group of diptera pointed to the fact that there are at least three genera, namely  Axymyia McATEEProtaxymyia MAM. et KRIV.,  and  Mesaxymyia MAM.,  which are very closely related to each other in imaginal as well as in larval characters. Their structure is in such a degree idiosyncratic that there will not be any opposition against their forming one single family Axymyiidae. In the study of the larvae, which allegedly had to affirm the position held by the majority of investigators who considered the genera  Axymyia McATEE  and  Pachyneura ZETT.  to be closely related, totally unexpected biological features were clarified. [The close relationship between the Axymyiidae and Pachyneuridae is at least suggested by the wing-venation :

Figure 16 :
5 -- Pachyneura sp.  ( Pachyneuridae ).
6 -- Mesaxymyia  kerteszi DUDA  ( Axymyiidae ).
(After KRIVOSJEINA, 1969 (5 and 6 of Figure 10))

Obtained from yet another source, we again give here the wing-venation of representatives of the Pachyneuridae and the Axymyiidae] :

Figure 16a :
80 -- Pachyneura  fasciata ZETT.  ( Pachyneuridae ). (After DUDA, 1930).
81 -- Axymyia  furcata McATEE.  ( Axymyiidae ). (After CURRAN, 1934).
(From HENNIG, 1954)

The larvae, judging from their structure, do not have anything in common with any of the other groups under discussion. See next Figure.

Figure 17 :  Larva of  Mesaxymyia  kerteszi DUDA  ( Axymyiidae ).  Lateral view.
(After KRIVOSJEINA, 1969)

See HERE for the structure of the head and mouth-apparatus of the larva of  Protaxymyia  melanoptera MAM. et  KRIV.  (Axymyiidae).
Already a superficial look of the larvae will convince us that they cannot be closely related to the representatives of the previous groups. The head-capsule is for almost a third of its length retracted into the thoracic segments. It consists of a comparatively small frontal plate which does not extend to the posterior margin of the capsule. The two massive side-plates fuse ventrally over a large extent. These side-plates of the head-capsule on their dorsal side with large non-sclerotized elongate parts which reach almost to the middle of the head [that is, they project forwardly, their anterior margin almost reaching the mid-line betweem the anterior and posterior margins of the head.]. The elements of the mouth-apparatus (except the mandibles) blade-like and weakly sclerotized.
The body consists of three thoracic and only 7 clearly distinct abdominal segments. The thoracic and first abdominal segments are the most broad ones. Body-segments without outgrowths and spinelets. The last body-segment carries a long respiratory tube, equal to or surpassing the body-length, and two or four anal papillae. At the tip of the tube (on the stigmatic field) there are two spiracles, surrounded by five small blade-like projections.
The tracheal system is amphipneustic  ( See Figure 6(3), above ).  The first pair of spiracles lies on the first thoracic segment [prothorax], while the second pair lies at the tip of the respiratory tube. The spiracles are circular, with a central moult seam, and many spiracular openings. Middle gut of the tubular type, simple. Malpighian tubules simple, separate from each other.
Of the imago the following basic features are characteristic :  Antennae 15-segmented. In the males the antennae can consist of a lower number of segments because the last segments sometimes fuse together. The eyes in the males touch each other along their whole length and are clearly divided into two parts which have facets of different sizes. The abdomen is massive [that is, not slender].
Wing-venation moderately costalized. See next Figure.

Figure 18 :  Wing of  Mesaxymyia  kerteszi DUDA  ( Axymyiidae ).
(6 of Figure 10).

All in all there are four radial veins. R1 long, it reaches the distal fourth of the wing. Radial Sector forks proximally from the radio-medial cross-vein  r-m.  R2+3 branches off from R4 almost at the apex of the wing. It ends up at the costa very close to [or at]  R1.  R5 independent, it reaches the wing-tip.  R4 and R5 run parallel to each other. The common trunk of the medial system is present, it forks distally from the medial cross-vein (m). Discoidal cell absent. The anal vein reaches the wing-margin [The russian text says it does not, but that is probably a printing error, judging from the above Figures.].
Such features of the larvae, as the partial reduction of the head-capsule, blade-like upper and lower lip and maxillae, the long respiratory tube, distinct anal papillae, amphipneustic tracheal system, simple mid-gut without blind sacs which are so characteristic of bibionids and mycetophilids, sharply distinguish the larvae of the representatives of the Axymyiidae from all the above considered groups. This conclusion is also supported by the structure of the adult insects.
The most characteristic features of the imago are :  Branching off of R4 proximally from  r-m,  R4 and R5 clearly distinct at their base, the presence of the branch R2+3, strong development of the eyes in the male with their distinct subdivision into two parts, and a massive abdomen. These features are not shared by any of the above considered groups. In the present case we have to do with a very independent group of diptera, which possesses a whole series of idiosyncratic traits.
But why then were many investigators so stubbornly placing  Axymyia McATEE  and  Pachyneura ZETT. closely together, or closely to [one of] the other groups? Even the analysis of those features which were at the disposal of other authors, such as the wing structure, only confirm the opinion that the group is independent.
In considering the mutual connections that obtain between Axymyiidae and the dipterous complex to which this family was, and is, connected by different authors basing themselves on the wing-venation, let us discuss a number of features.
The one and only similarity that exists between Axymyiidae and Pachyneuridae is the number of veins. But such a feature cannot represent a basis for the establishment of genetic connections.  [ In the Axymyiidae] the overall plan of the position of the veins, their mutual connection, and the nature of the branching, do not have anything in common with what is observed in  Pachyneura ZETT.,  Mycetobia MEIG. and in the mycetophilid complex. [ For the wing-venation of  Mycetobia (belonging to the independent family Mycetobiidae, a family more or less detached from the mycetophilid complex (Fungivoridea)),  see Figure 10 ( 7 ) ].

Remark :
[ This latter statement of KRIVOSJEINA's, p. 238, is not very convincing.
As far as I can see, the overall or general plan of the wing-venation in Pachyneuridae and Axymyiidae is identical, and this plan in turn is very similar to that in  Mycetobia  as well as to that in the mycetophilid complex (for instance in  Ditomyia WINN.). The wing-venation of  Mycetobia  only differs from both Axymyiidae and Pachyneuridae in having a Radial Sector with only two branches, and in clearly having lost the common trunk of the medial veins -- but  Pachyneura  is in the process of loosing it. Further -- as one more difference -- the wings of  Mycetobia  possess a broad anal lobe (in contrast to those of  Pachyneura  and  Axymyia ).  And, apart from the reduction of the Subcosta, the venational plan in  Ditomyia  (of the family Ditomyiidae, belonging to the mycetophilid complex (Fungivoridea) as one of its least specialized forms)  is identical to that in  Mycetobia ].
[ For the wing-venation of  Ditomyia WINN., see Figure 10-1 above ].
The general plan of the wing-venation as we see it in  PachyneuraAxymyiaMycetobia,  and in  Ditomyia,  can be described as follows :

As a further development of this plan we see the reduction (growing pale) of the common trunk of the medial system in the wings of  Mycetobia  and in those of the mycetophilid complex (prepared already in  Pachyneura , where this vein is weak).
End of Remark ).

The Axymyiidae (see Figure 18, above )  have four radial veins, and R2+3 ends up in the Costa at the endpoint of R1 or directly into that vein.  R4 and R5 run parallel to each other, they do not come closer to each other,  r-m  is placed significantly distally from the fork of R4+5. Finally,  r-m  and  m  lie significantly far away from each other, and  r-m  lies in the distal half of the wing.
All listed features are characteristic neither of Bibionidae, Hesperinidae, and Pachyneuridae, nor of the mycetophilid diptera. On the contrary, elements of similarity in these respects with a whole series of groups of tipuloid diptera are observed.
Of many groups of Tipulomorpha, characteristic or even typical is the approach of R2 to R1, which is observed for example in  Antocha  alpigena MIK (Limoniidae), and in many Tipulidae.
See next three Figures and comments of mine.

Figure 19 :  Venation of apical part of the wing of  Antocha  alpigena MIK  ( Limoniidae ) (the cross-vein  tp,  drawn as a dashed line, is absent in this species, but is present in other species of this genus. R2 is only very weakly expressed).
(After HENNIG, 1954).

Figure 20 :  Venation of apical part of the wing of  Dolichopeza  albipes STRöM  ( Tipulidae ).
(After HENNIG, 1954).

The position of  r-m  distally from the separation of R4 and R5 (that is, the point where the Radial Sector first forks) is also typical of many groups of the Tipulomorpha  ( Tipulidae, Limoniidae, Ptychopteridae), see Figure 19.  In Figure 20 (Dolichopeza) we can see this feature also, but here the Radial Sector itself branches off very distally from the Radius (R1).
For the Ptychopteridae see next Figure.

Figure 21 :  Venation of   Bittacomorphella  nipponensis ALEX.  ( Ptychopteridae ).
(From HENNIG, 1954, after ALEXANDER, 1927).

In  Bittacomorphella  r-m  lies only a little bit distally from the primary fork of the Radial Sector. And also here -- as well as in  Liriope  scutellaris MEIG.  (also belonging to the Ptychopteridae) ( HENNIG, 1954, Figure 38) -- R2 approaches R1 such that it ends up in it.

The same [that is, similar situation in Axymyiidae and certain Tipulomorpha] can be said about the position in the wing of  r-m,  namely that it lies in its distal half.
As a result [of enquiries] we [KRIVOSJEINA, p. 238] can find many examples demonstrating the similarity in the structure and distribution of veins in the wing of  Axymyia McATEE and certain tipulomorphs. Only the presence of an extra cross-vein [which in HENNIG, 1954, is called  tp,  and which closes the discoidal cell of the wing-venation] distinguishes [for example] the wing of  Idiotipula  confluens ALEX.  ( Tipulidae) from that of  Axymyia. See Figure 10 (8), above .  This we can find also in other Tipulomorpha, such as in  Pales  dorsalis FABR. ( Tipulidae),  Xiphura  atra L.  ( Tipulidae),  Phalacrocera  replicata L.  ( Cylindrotomidae),  Limnophila  ferruginea MEIG.  ( Limoniidae), and  Trichocera  regelationis L.  ( Trichoceridae).  See next two Figures.

Figure 22 :  Venation of   Pales  dorsalis FABR.  ( Tipulidae ).  Discoidal cell present (that is, closed by the cross-vein  tp ).
(After HENNIG, 1954).

Figure 23 :  Venation of   Trichocera  regelationis L.  ( Trichoceridae  [ = Petauristidae] ).  The Trichoceridae belong to the superfamily Tipulidea of the Tipulomorpha. Discoidal cell present (that is, closed by the cross-vein  tp ).
(After HENNIG, 1954).

In  Antocha  alpigena MIK.  ( Limoniidae) -- see Figure 19, above -- there is even no such cross-vein. The discoidal cell [therefore] is absent, although present in other species of this genus.
But, all in all, the structure of the wing and the topography of its veins is very similar [in the Axymyiidae and certain Tipulomorpha].
We must now also consider some questions concerning the ecology of the larvae.
The larvae of Bibionidae, Pachyneuridae, Hesperinidae, and Pleciidae, develop in various rotting vegetable remains, principally in wood rot. The larvae of Axymyiidae are connected with moist, soaked through with water, firm rotting wood of different wood sorts. They gnaw short (5-7 cm), oval in diameter, tunnels, partly free from wood rot. Withdrawing into the depth of the tunnel -- not rarely filled with murky fermenting thin pulp -- the larva extends its respiratory tube, of which the end is always held at an exit of the tunnel. This exit is always open :  superfluous wood rot material is ejected from the tunnel, forming characteristic oozes. There is some basis to hold that in the larval feeding fungi might play a certain role, fungi which multiply in the fermenting liquid. However, investigation of the gut revealed that it is filled with fine wood rot. Notwithstanding the presence of the respiratory tube and anal papillae, the larva is fairly mobile. It can strongly lengthen or shorten its respiratory tube, not withdrawing it into the body.

Analysis of the larval and imaginal features of the Axymyiidae demonstrates that they cannot be closely related to  Pachyneura ZETT. or to bibionids. The larval features of Axymyiidae show them to be most closely related to the group Anisopodidae-Mycetobiidae, but this is very conditional. One must admit that an analysis of imaginal features (especially of the wing) and larval features do not provide us with the possibility to judge about to whatever known group of diptera the Axymyiidae are [phylogenetically] related. All this at least indicates the existence of this group as a whole, not only as an independent family, but also as a superfamily.

* * *

After having dealt with the genera  PleciaHesperinusPachyneura,  and  Axymyia, and having shown them to represent independent families, it is perhaps instructive to discuss yet another problematic genus, namely  Mycetobia,  normally considered to belong to the mycetophilid complex. It represents the family Mycetobiidae.

Family  M Y C E T O B I I D A E
(KRIVOSJEINA, p. 240-241)

The family consists of the single genus  Mycetobia MEIG. with a small number (less than 10) of species. Based on imaginal features, the genus was for a long time included into the Mycetophilidae. The study of the early stages of [individual[ development had shown that this was totally wrong. The amphipneustic larvae (see Figure 6 (3), above ),  with their elongate mouthparts and 2-segmented mandibles, and, finally, with secondary segmentation of the body, do not have anything in common with the larvae of mycetophilid diptera. See next Figure.

Figure 24 :  Larva of  Mycetobia sp.  ( Mycetobiidae).
Compare the general appearance of this larva with that of a representative of the family Anisopodidae  [ = Rhyphidae], namely  Sylvicola  fenestralis SCOP.,  HERE .  Compare it also with a larva of the family Mycetophilidae  [ = Fungivoridae] (belonging to the mycetophilid complex) : HERE .
(After KRIVOSJEINA, 1969)

For details of the head of the larva of  Mycetobia,  see HERE .  To compare this with the larva of  Symmerus  ( Ditomyiidae, Fungivoridea  [ = mycetophilid complex] ),  see HERE  [press BACK button to return]. [We see here the great difference between the head of the larva of a representative of the genus  Mycetobia  and that of a representative of the mycetophilid complex, being an indication that  Mycetobia  taxonomically stands far away from the mycetophilid complex of diptera.]
Edwards (1928), while taking into account the knowledge of the biology of the group, included them into the family Anisopodidae  [ = Rhyphidae] [Indeed, as we saw, at least the outer appearance of the larvae of Mycetobiidae and Anisopodidae is essentially the same]. In recent times the predominant opinion of the dipterologists was that we have to do with an independent family, the Mycetobiidae, but many authors stubbornly see them to be close to the mycetophilid diptera (Hendel, 1938,  Hennig, 1954). These latter authors base their conclusion on the structure of the wing.
Let us now turn to these features.
Of the wing-venation in  Mycetobia MEIG. the following features are characteristic. See next Figure.

Figure 25 :  Wing-venation of  Mycetobia  pallipes MEIG.  ( Mycetobiidae).
( 7 of Figure 10)

The Radial Sector consists of two branches, so all in all there are three radial veins. Forking of the Radial Sector takes place at the level of the cross-vein  r-m.  R5 runs into the Costa at the wing-tip. The common trunk of the medial system of veins is absent. The cross-veins  r-m  and  m  lie almost along a straight line, that is, they do not make an angle, and they make an almost right angle with the anterior wing-margin.
The absence of the medial trunk, three, and so not four, radial veins, make  Mycetobia MEIG. and  Ditomyia WINN.(see HERE ) come close together. To  Pachyneura ZETT. (see HERE )  Mycetobia WINN. is only similar in the mutual position of  r-m  and the veins of the Radial Sector : [ in both genera] the latter forks [precisely] at the level of  r-m.  But the absence of the medial trunk, and of the vein R2+3 [present in  Pachyneura  as the short anterior branchlet of the Radial Sector], indicate that  Mycetobia  and  Pachyneura  stand significantly far away from each other [We must, however, take into account the fact that the trunk of M is -- according to HENNIG -- weak in Pachyneura ].
In order to compare the wing-venations of  Mycetobia,  Ditomyia  and  Pachyneura,  let us depict them together :

Figure 26 :
7 -- Wing-venation of  Mycetobia  pallipes MEIG. ( Mycetobiidae).
1 -- Wing-venation of  Ditomyia  fasciata MEIG. ( Ditomyiidae).
5 -- Wing-venation of  Pachyneura sp. ( Pachyneuridae).
( 7, 1, 5 of Figure 10)

The idiosyncratic posistion of the cross-veins indicates that the representatives of  Mycetobia  stand far away not only from the representatives of  Pachyneura,  but also from those of  Ditomyia.  [For all of them, the cross-vein r-m connects the Radial Sector with the Media (M1+2),  while the cross-vein m (often called mcu) connects the Media (M1+2) with the Cubital Fork, and this Cubital Fork itself consists of Cu1 and M4 (so the cross-vein m in fact connects M1+2 with M4). But the three genera differ among each other in the position and direction of these two cross-veins.]
And so the analysis of the wing-venation confirms the idiosyncrasy of the group  Mycetobia MEIG.,  that is, their standing taxonomically far away from the mycetophilid complex of diptera.
It is certain that the features of the larvae as well of the adults point to the meaningfulness of establishing for  Mycetobia  an independent family, significantly independent of the mycetophilids.

This concludes the discussion concerning certain problematic nematocerous genera, as we had found it in KRIVOSJEINA, 1969, who was determining the taxonomic position of them on the basis of larval morphology and wing-venation.

In the next document we will continue with the superfamily Bibionidea of the infraorder Bibionomorpha, namely determining to what functional wing-types the wings of their representatives belong.
As has been explained earlier, the wing-type is the morphological expression of the flight-regime of the possessors of such wings, that is, a particular functional wing-type expresses a particular way of flying of the insect. And this particular way of flying -- flight-regime -- is compatible with, that is, neatly fits into (in the sense that it may not interfere with) the precise way the insect experiences its environment by means of its antennae, but especially by means of its eyes (which are well developed in almost all diptera). In this way, the functional structure of the wings expresses the environment of the fly (as it is experienced by it).

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