REMARK on the noëtics of organic strategies or phases of evolution :
In the development of certain terebrants that lay their eggs into the early stages of the host, and having their complete development not until the host-larva is fully-grown or even when it has finally pupated, one has observed phenomena of polyembryony. Here, from one single egg laid by the terebrant several individuals develop, which in some cases reach complete development, but in other prematurely die. In the whole Class of Insects the phenomenon of polyembryonic development is observed only in Hymenoptera. But in this Order they are fairly widely distributed, namely in the terebrants of the family Braconidae, among the Serphoid terebrants of the family Platygasteridae, and among the Chalcidoid terebrants of the family Encyrtidae.
Polyembryonic development is also observed in wasp-forms [i.e. among Aculeata], namely in the family Dryinidae, about which we will speak farther below.
As was found out by Parker, 1931, the egg of the braconid Macrocentrus gifuensis Ashm., -- a parasite of the corn moth Pyrausta nubilalis Hb., [the egg] usually laid into the fat body of the prey -- first changes into a spherical pre-germ, which gives the primary germ, dividing into two. Secondary germs form tertiary ones transforming into morulae [looking like a blackberries] from which already develop embryos that give first-instar larvae. As a result, in one single caterpillar one whole brood of the terebrant Marcrocentrus gifuensis is formed, consisting on average of 16 female individuals, or of 24 individuals when they are all male. It has been found that from 200 broods, about which data had become available, 71 broods consisted exclusively of males, 54 of females, and 75 broods had a mixed content. The mixed offspring was, as was assumed, the result of a two-fold oviposition [each time] into one and the same caterpillar.
Interesting data were later obtained by Daniel, 1932, concerning another species -- Macrocentrus ancylivorus Roh. -- a parasite of many species of caterpillars. In this case, the first parasitic germ, having reached the larval stage, shows an inhibiting influence onto the other embryos, morulae, germs, and pre-germs, present in the body of the same host. As a result their development completely stops. In this braconid, consequently, polyembryonism does not give off a result, because from the layed egg only one terebrant larva develops here. It is supposed that M. ancylivorus in the past used to parasitize on much larger hosts, than it does today, and as a result might have produced a larger number of descendants in each one of them [hosts]. In recent conditions this terebrant, as far as known, is always single.
In the Platygasteridae polyembryonic development is, apparently, limited to just a series of species of the genus Platygaster, developing in larvae of gall-midges (Cecidomyidae). Because of the small size of the hosts the number of developing individuals is here not large. In its most simple form polyembryonism is represented by Platygaster hiemalis Forb., a parasite of larvae of the hessian fly. Here, from one laid egg develop no more than two individuals. In the usual oviposition of eggs in groups of 5-8 into one egg of the gall-midge, about 6 terebrant individuals develop. One third of them perish as a result of not enough feeding. In addition, several eggs develop in the usual way, mono-embryonic, which generally is typical of a series of other Platygasteridae. The maximum number of offspring being produced from one single laid platygasterid egg, namely 18, is reported for P. felti Fts., which develops at the expense of the gall-midge Rhopalomyia sabinae Felt. But the presence of both methods of reproduction within one and the same genus is not observed in any other hymenopterous insect, and in this fact one sees the clue for resolving the whole question of the origin of polyembryonism.
In the most complex and evolved form polyembryonism is represented by chalcidoidea-Encyrtidae. Here it is encountered in the group of related genera -- Ageniaspis, Litomastix, Paralitomastix, and Copidosoma. There is good reason to suppose that the majority of, if not all, species of these genera, reproduces in this way, and, as far as known, without exception at the expense of caterpillars of butterflies. All of them are extremely small, although many of them attack large caterpillars, and then an enormous number of their offspring may attain complete development in one single prey. Thus it is found that even from one single egg of Litomastix may develop up to 2000 descendants. A development of a larger number of desendants (3000) is, presumably, connected with a repeated infection of the prey.
The life habit of the genera of the mentioned group of chalcidoids is very similar, and, according to Nikolskaja (1952) as follows : The female of the parasite lays its egg into the egg of a butterfly. Here, in the formed embryo of the caterpillar it goes through the first stages of development and hibernates. In spring, when the caterpillar emerges from the egg and begins to feed, the parasite, while not delaying its growth, continues just slowly developing. Not until the end of the summer when the caterpillar prepares for pupation, the larvae of the parasite reach the last developmental stage, and after having quickly consumed the contents of the prey conclude development. At this time the body of the caterpillar is strongly stretched and sometimes by two times surpasses its normal size. Its surface becomes uneven, and from the body project oval cells, each containing a parasite, included in its already formed skin. We do not need to dwell further on the details of the embryological process taking place here, but only deal with its general course. The egg of the terebrant encyrtid (Encyrtidae), laid inside the embryo of the host, during the remaining period of incubation of the latter, and after that also inside the body of the developing caterpillar, divides into a different number of cells, all in all forming an extended, and sometimes even a branched, body, covered by a shell formed by itself. This is the so-called "germ chain". For some time it freely floats in the body-cavity of the caterpillar, but after that disintegrates into its constituent parts, each one of them attaching itself to one or another organ of the host, and then transforms into an embryo, and finally into a larva of the usual hymenopterous type, if we do not yet think of the asexual larvae, which never will reach maturity. Sometimes in one single host there may be several such "chains" present, each having developed from one egg, and then the content of even a large caterpillar may be destroyed totally by thousands of very small encyrtids, having developed from these "chains".
Of special interest, from the ethological point of view, are the encyrtids of the genus Coelopencyrtus Fimb. Of them six species are known only from the Hawaiian islands, two from North America, and, finally, one described not long ago from the Palaearctis [Europe and Northern Asia] namely C. malyshevi Trptz. The latter was more than once encountered by the author [Malyshev] in nests of the primitive bees Hylaeus sp. This encyrtid gnaws a small circular opening through the membraneous wall of the newly-constructed cell of the Hylaeus, through which [opening] it enters the cavity of the cell. In one observed case (Chopersky nature reserve, 31 VII 1959), after first having penetrated into the anterior cell of the nest of the Hylaeus, and after that also into the next cell, the encyrtid quickly left the nest. After 10 days, upon opening the cells [to see what's in them], in one and the other, encyrtid-larvae shined through the [skin of the] Hylaeus-larvae, and these encyrtid-larvae had already finished feeding, and had given off black spots of excrements.
Thus, in spite of the geographical isolation of the few known species of Coelopencyrtus they are similar in their development at the expense of special hosts -- larvae of the primitive bees Hylaeus, that is, hosts also characteristic of the above mentioned archaic terebrants Gasteruptiidae. In this fact one must see a definite indication of the phylogenetic connection between the two terebrant families.
The indication in the literature (Trjapitsin, 1960) that four Hawaiian species of Coelopencyrtus parasitize on the larvae of solitary wasps of the genus Odynerus Latr., demands confirmation. As was shown, a similar assertion with respect to the terebrants Gasteruptiidae turned out to be wrong : All biologically known Gasteruptiidae develop at the expense of solitary bees, not of wasps (Malyshev, 1947, 1965).
Field observations revealed that the effectiveness of the polyembryonic way of reproduction, apparently unusual, is strongly decreased by a series of factors. The chief one of them is the oviposition into a very early developmental stage of the host, and this in such a time when appropriate feeding of the parasitic larvae can be realized only in host-larvae that have concluded their development, and sometimes also in the pupal stage of the host. In these conditions, together with the ordinary death of the host in its early developmental stage, in the very germ [of the host] also its parasites perish. Based on this, it is supposed that the demand for compensation for the great loss of offspring at an early stage of its development turned out to be one of the factors that had caused polyembryonic development [That is, when many hosts perish for one or another reason, a few of them, having survived, will, when infected, still yield a thousand or more wasps]. In other cases this solution -- avoidance of prematurely death of the parasite -- was accomplished, as we will see, also in other ways (see the "immediate-parasitic phase").
Indeed, it is impossible to suppose that earlier, at the time of origin of the polyembryonic way of reproduction, the egg of the terebrant was laid not into an early stage, as it is the case today, but in the last larval developmental stage of the host. Such a state of affairs not only is not observed in nature, but is, evidently, impossible by reason of the special conditions of the initial incubation of such a polyembryo, and of the feeding of the larvae emerging from it, chiefly at the expense of only the developmental end stage of the host, when the latter already has prepared for pupation. In this way, the polyembryonic method of reproduction provides yet another indication of the fact that the laying of the egg in the early developmental stage of the host is primary (in the sense of original).
With all this we conclude our exposition of the Polyembryonic-Parasitic Phase.
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