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Organic Evolution in terms of the Implicate and Explicate Orders.


Diptera (midges, mosquitoes, flies) (IV)

The evolutionary diversification in the Order Diptera revisited.

Lifting costalized (stratiomyoid) functional wing-type

Stratiomys  chamaeleon,  Stratiomyidae. 14-15.5 mm.
(After SEVERA, in Thiemes's insektengids voor West- en Midden-Europa, 1977)

(Description of stratiomyoid wing-type according to ROHDENDORF, 1951)

Representatives of the type
Lifting costalized (stratiomyoid) wings are typical to species of the brachycerous families Stratiomyidae, Chiromyzidae, many Cyrtidae and some Bombyliidae (the genera Empidideicus, Glabellula).

Size of the wings
The absolute size of the wings of this type is medium, more rarely small, from 3 to 15 mm, seldom larger. The relative size of the wings is short, almost always less than the length of the body, more rarely longer than it. The size of the surface area of the wings is known for some species of three genera of the subfamily Stratiomyinae, viz., of the genus Eulalia (0.151-0.287 cm2), of the genus Hirtea (0.416-0.713 cm2), and of the genus Stratiomys (0.529-0.769 cm2). Weight of the wings insignificant, reaching in the only species studied in this respect, a species of the genus Stratiomys, 1.34 percent of the weight of the whole insect. The load is studied in several species of the family Stratiomyidae and varies between 0.072-0.135 gr/cm2 in the small Eulalia and 0.116-0.204 gr/cm2 in the large Hirtea and Stratiomys.

Shape of the wings
Wings with a straight anterior margin, a small apex, individualized termen and a large anal lobe. The basiala is sharply distinguished from the wing-blade by the border of the anal lobe, and often carries a large alula which sometimes is absent (genus Glabellula and some others). Wings moderately elongate, usually 2.5-3 times longer than wide, more rarely a little longer. Seldom are the wings short, 2-2.33 times longer than wide (some Beredinae, and the genus Glabellula).

Skeleton of the wing
In resting position the wings are folded one above the other over the abdomen, sometimes obliquely so. Very characteristic of this type is the strong degree of costalization, consisting in strengthening, shift, and coalescence of veins in the anterior half of the wing, and reduction or weakening of the middle-part veins and a widening of the wing-membrane of the posterior half of the wing, free of veins. The Costal vein strong, almost never reaching the wing's apex. The Radial veins running closely together and thickened, all ending up at the anterior wing-margin at the Costal vein. Only sometimes the last Radial vein directed backwards (in the genera Oncodes and Corononcodes (see next Figure, third image))

Figure 1 :  Wings of representatives of the lifting costalized (stratiomyoid) type.  From top to bottom :

  • Archisargus  pulcher Rohd. (Archisargidae), Upper Jurassic of Karatau (Southern Kazachstan). Length 16 mm.

  • Pachygaster  minutissima Zett. (Stratiomyidae). Recent. Length about 2 mm.

  • Corononcodes  siculus Bezzi (Cyrtidae). Recent. Length about 3 mm.

  • Glabellula  nobilis palestinesis Eng. (Bombyliidae). Recent. Length about 1 mm.
(After ROHDENDORF, 1951)

The number of Radial veins [including the Radius proper] varies from 4 to 2. The Subcostal vein is weak, sometimes ending in the membrane of the costal field. Medial veins thin, in the form of 3 or 4 branches, usually forming the intermedial cell. Sometimes the number of Medial veins is reduced to two and the intermedial cell absent (in the genera Oncodes, Corononcodes, Philopota). The Cubital and first Anal vein are delicate, thin, and long, often coalesced at their end, forming the closed cubital cell (Stratiomyidae, Figure 1, second image), sometimes only converging or parallel (in the genera Glabellula and Oncodes). The cross-veins r-m and m-cu, as a result of costalization processes, sometimes completely disappear, or sharply change their position. Only r-m lies between the last Radial branch and the anterior Medial branch in the majority of Stratiomyidae.
The basiala consists of two upper [i.e. elevated] elements -- the base of the Radial vein and that of the second Anal vein -- and one lower element -- the base of the Cubital and first Anal vein, to which unites the strengthened membrane of the cell between the radial and cubital 'handles'. Phragma not developed. The Cubital and Radial veins sometimes coming closely together, but do not form a special firm separating ridge, a phragm. Chaetaria well developed, and placed on the radial and the very big handle of the second Anal vein.

The next Figure presents some more wings of the present type :

Figure 2 :  Representatives of the Lifting Costalized (stratiomyoid) functional type of wings.
157 :  Solva  marginata Meig. (Solvidae).  158 :  Chiromyza  vittata Wied. (Stratiomyidae).  159 :  Cibotogaster  fumipennis Kert. (Stratiomyidae).  160 :  Craspedometopon  frontale Kert. (Stratiomyidae).  161 :  Hermione  locuples Lw. (Stratiomyidae).
In the wing of  Solva  the costalization process has hardly begun. So the wing in fact still belongs to the traction-lifting many-veined (tabanoid) functional wing-type (where this genus is indeed mentioned). (After HENNIG, 1954)

Figure 3 :  Some more representatives of the Lifting Costalized (stratiomyoid) functional type of wings.  Cyrtidae [= Acroceridae = Oncodidae].
206 :  Pterodontia  misella O.S.    207 :  Acrocera  convexa Cole.    208 :  Philopota  aenea Meig.    209 :  Oncodes  zonatus Erichs.   (After HENNIG, 1954)

Coverings of the wing
The wings on a large part of their surface are usually devoid of microtrichia and bristles. Only on certain veins and on a few places of the membrane there are minute microtrichia, with a length varying between 12 and 35 mu (Stratiomys  chamaeleon L.). Scales also are absent.

Functional characteristics
Only the general aspects of the way of life of these peculiar diptera are known. The chief group of the type, the Stratiomyidae, is connected with water-basins and rotting material, in which their larvae live. The winged insects are characteristic visitors of flowering plants, where they eat nectar. Other representatives of the type, Cyrtidae [= Acroceridae], develop in the body of spiders or in their egg-cocoons. The winged flies are encountered on flowers, and apart from this these flies are partly aphagous, having a reduced proboscis.

Figure 4 :  Acrocera  globulus, male, 4 mm. Cyrtidae
(After VERRALL, in Imms' General Textbook of Entomology, 1977)

Finally, representatives of the family Bombyliidae, namely the species of the genus  Glabellula,  undoubtedly belonging to the present type, are very peculiar, distinguishing themselves by their extreme smallness (from 1.25 to 2.5 mm), and are found on flowers. The individual development of these bee-flies is not known [ROHDENDORF, 1951]. In all probability we may assume that their larvae are parasites of certain insects.
The nature of flight is not especially investigated in any representative of the type. Wing-beat frequency and maneuverability are unknown. But, taking into account the big load, the wing-structure, and data of visual observations of the flight of these insects, we may say something about the nature of the flight-function of the representatives of the stratiomyoid type. First of all certain is the high wing-beat frequency :  evident from the extreme degree of costalization. On the other hand, the high degree of integration of the body of the representatives of the type, together with the high load exerted on the wings, point to a perfection of aerodynamic qualities, a good maneuverability in flight. Observing the behavior of Stratiomyidae on flowers, we may directly note the clumsiness and languidness of these flies when crawling on the plant.
Entirely different things must be said about the nature of their flight. Stratiomyidae fly in a remarkable way :  sometimes a fast rectilinear flight takes place from afar, and the fly then directly sits down on a particular flower. Sometimes, on the other hand, a relatively slow 'hovering' flight is observed amidst the vegetation whereby the insects freely move along an entangled fly-path, making many turns. The flight itself in this case is slow and as it were 'awkward' (the latter characterization is not well-founded because in fact there is no such 'awkwardness', i.e. there is no strain and difficulty in flight at all, evident by the great maneuverability in flight).
Evaluating the significance of the flight-function in the life-activities of these insects, we can definitely say that it is very important in the life of the representatives of the stratiomyoid type. These flies move around almost exclusively by their wings. Their legs are relatively weak and do not sustain running. Feeding on flowers, visiting of water-basins for reproduction, and, finally, the peculiarity of development in Cyrtidae in the body of living spiders, -- all these things point to the great significance of well-developed flight for the insect. Nevertheless this general conclusion is insufficiently based. The nature of the stratiomyoid type, its extreme forms of costalization of the wings, demand a more precise and causal explanation. To solve this rather hard problem it is, apparently, necessary to follow the same road as in discussing the evolutionary directions of the tabanoid wings, i.e. to study first of all the basic physical facts of these flight-apparatuses -- general dimensions [size] of the body and its weight, speed of flight, and wing-beat frequency.
Development of costalization takes place only when the sizes of the insect [and its parts] are sufficiently small. As we saw in the tabanoid wings, large size has strongly prevented costalization to take place in tabanoid wings.
It is therefore natural to assume that the formation of the stratiomyoid type went along significantly different ways. The features of this type began to appear first in minute flies in which flight with a high-frequency wing-beat with costalized wings began to develop. Increase of size took place slowly in certain groups (Stratiomyidae), while in others, apparently, generally did not take place (Cyrtidae and Pachygasterinae  ( Figure 1, third and second image )  or even, on the contrary, the size decreased (in the genus  Glabellula Figure 1, last image ).
Perfection of flight in all these flies went through the working out of costalization, to the development of which contributed the small weight of the body, which distinguished this type from the tabanoid type. Consequently, the features of this type were determined by the interaction of the same factors which also had determined the tabanoid type. The essential differences between the types were the result of different intensity of the interacting factors, or, more precisely expressed, the result of a different history of change of conditions of living in evolution. In the history of the stratiomyoid wings the perfection of flight (expressed as costalization) took place very early on, while increase of size was slow or not at all. In the tabanoid type these processes took place in reverse order.

History and transformation of the type.
Fossil remains of the lifting costalized (stratiomyoid) type are few in number and until now [1951] limited to only two jurassic finds of single species of the families Archisargidae Figure 1, first image )  and Protocyrtidae, when not taking into account the many Tertiary forms of recent families and genera. To the latter belong many remains from the Oligocene of western Europe and North America, and from the Miocene of Eurasia and North America, belonging to species of the genera of the subfamilies Beridinae, Geosarginae, Stratiomyinae. The known mesozoic faunae of flies (diptera) (Karatau) include true representatives of the type, namely the highly specialized Archisargidae Figure 1, first image )  and Protocyrtidae. Certain jurassic forms from Karatau, namely species of the genera  Palaeostratiomyia  and  Rhagionempis  (See  Figure 4 in Part LXXI )  possessing wings with rich venation of the tabanoid type, do show such features that point to their belonging to a special subtype, and together with this point to the evolutionary direction of the wing-venation directly leading to the stratiomyoid type. Another source of this type undoubtedly was formed by the representatives of the cyrtosioid subtype of tabanoid wings, in which costalization developed, which distinguished it from the traction-lifting many-veined (tabanoid) type and led to the formation of the stratiomyoid type. These forms are known as various, for a great part relict, genera in the recent fauna.
The different sources of the stratiomyoid type and its phylogenetically diverse content determine the existence of subtypes reflecting the systematic content of the type.
The largest of these, the stratiomyoid-proper subtype, includes species of the family Stratiomyidae and the relict Chiromyzidae [or Chiromyzinae, a subfamily of the Stratiomyidae, see Figure 2, above]. Despite the manifold of genera distributed among the many subfamilies of Stratiomyidae, there is no reason to further subdivide this subtype into subordinated categories. The variability of the wing-venation in Stratiomyidae only shows the different stages of costalization, chiefly connected with the absolute size of the insects and the larger or smaller intensity of flight. Precise analysis of the features of this subtype is a special task which I shall not perform here. We only may note that the group Beridinae could be characterized as having the least costalized wings.
The other, cyrtoid subtype, includes phylogenetically alien, very few forms, and is characterized by the extreme degree of costalization of the wing-venation (see Figure 1, third image  and Figure 3 )  which has lost the majority of cross-veins (and with it having lost the closed cell in the middle of the wing!) and is markedly strengthened at the anterior margin. The larvae of the representatives of this subtype are highly specialized internal parasites of spiders.
The last, glabelluloid subtype consists of only two genera of Bombyliidae, Empidideicus  and  Glabellula  (see Figure 1, fourth image ),  which are minute insects (not larger than 2.5 mm), possessing extremely costalized venation, in which the Costal, Radial, Cubital, and cross-veins are significantly thickened and partly shifted distad. [Rohdendorf does not give a drawing of the wing-venation of  Empidideicus.  The only drawing we have so far seen is taken from HENNIG, 1954, see  HERE.  As one can see, this wing not at all belongs to the stratiomyoid type, because there is no trace whatsoever of any costalizaton of the venation]. The shape of the wing is truncated, reminding of certain Nematocera, namely various Oligoneura -- Scatopsidae and many fungivoroids. Until now [1951] the way of life of these peculiar diptera is unknown :  the winged insects are found on flowers of various plants, and their larvae undoubtedly parasitize in whatever insects as is the case in all other species of the large family Bombyliidae.
A general evaluation of the evolutionary directions of the stratiomyoid type was already succinctly given earlier. We only may, concludingly, add that all features of this type evidently bear witness of the progressive development of the flight-function. Flight in the representatives of this type is a very important function in their life-activities, apparently connected with obtaining food and with reproduction. The chief causes, however, of the origin of this costalized type remain more or less unclear, i.e. the very significance and role of the development of costalized wings. As is known, such a wing-structure is widely distributed among diptera, and, probably, its origin was determined by certain conditions of perfection of the flight-function, which included concrete relationships between the weight of the insect and the demands of its way of life. These changes of the living-conditions also determined the formation of costalized wings.

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