The decrease of the size of (certain) terebrants as a result of malnutrition of their larvae in small egg-clutches of their hosts led in further evolutionary development the terebrant-egg-eaters to their (individual) development at the expense of the content of only one single egg of the host. This fact was, apparently, inseparably connected with the ability, appearing in these terebrants, to lay their eggs inside the eggs of the host.
The transition from predatory external feeding to endoparasitism opened up a series of new possibilities, first of all the possibility of the development of several and even many very small egg-eating larvae inside a single host-egg. In this way among the terebrants the smallest forms evolved, and indeed, not only (the smallest) among terebrants, but among the whole Class of insects.
Such are the terebrant-egg-eaters of the family Mymaridae ( Fairy Flies), of which some have a length of only  0.1 mm, that is, in spite of the fact that they possess the complete organizational complexity of Hymenoptera, they are three times smaller than the unicellular infusorium  Paramecium.
While in previous phases the terebrant-larvae were only in a position to attack eggs having a sufficiently weak shell [because they were already enclosed in galls and the like], now, for terebrants developing  inside  the eggs of the host [the egg of the terebrant now being laid inside the egg of the host], the quality and condition of the shell of the host-egg became less important for these terebrants, because the adults obtained the ability to successfully bore with their ovipositor through the shell of the egg that had to be infected. This gave the terebrants the possibility to widen their range of potential hosts, including those that carry eggs possessing a hard shell -- Lepidoptera [butterflies and moths], herb-bugs [Hemiptera], many grasshoppers, and others. And so in this way new relationships may be created. The terebrant-egg-eater of the phase presently under discussion, while (still) attacking larger egg-clutches (for example the egg-capsules of grasshoppers), infected in them only some individual eggs to which also the feeding of its larva was limited.
This parasitic egg-eating, or endo-oophagous, phase, is represented by very small forms of terebrants, chiefly belonging to two large groups :  to the Chalcidoidea, to which belong the above mentioned Mymaridae,  and to the Proctotrupoidea (=  Serphoidea). In some cases, as we shall see, also among the gall-wasps, Cynipidae, one encounters development at the expense of the egg-stage of a host.

Although not perhaps precisely belonging to the present phase, we here depict a typical chalcid,  Blastothrix  sericea,  in order to provide the reader with some image of this large group (Chalcidoidea), that, as a superfamily or family can be found in many evolutionary phases. They are very small, generally dark shiny stocky wasps, often having wings with a very reduced venation with only one vein.

Above we reproduced Malyshev's short exposition of aquatic Hymenoptera. And because the ecological transition from a terrestrial life -- typical of Hymenoptera -- to a (more or less) aquatic life is, in a certain respect, a sharp transition (because, among other things, the respiratory conditions become quite different), we will dwell upon it longer, and reproduce an exposition of aquatic Hymenoptera given by Wesenberg-Lund.
Wesenberg-Lund is an authority on aquatic insects, and therefore his exposition will represent a valuable contribution to what has already been said above. It was written in German, and starts on page 576 of his book on aquatic insects. We will translate (and comment where necessary) into English :

One might assume that insects that have adapted themselves to living in fresh water, escape the attacks of ichneumon-flies [parasitic hymenoptera-terebrantia], to which terrestrial insects are so heavily subjected. This is, however, not the case at all. The better we come to know the aquatic insects, the more we get the impression that almost every species of them is ecologically connected with a particular ichneumon-fly, that parasitizes on it and [thus] lives of it. In saline water there are only a few wasps. However, the larvae of  Ephydra (Diptera-Cyclorrapha, Ephydridae) are often heavily attacked.
Until now (1943) little was known about aquatic ichneumon-flies. The first ones were described in 1863 by Lubbock. Then many years went by, until recently in further investigations one had found numerous aquatic ichneumon-flies. But the way of life of many of these is unfortunately still little known. Our knowledge of this group chiefly comes from Hendriksen (1918 and 1922), whose works formed the source of most of our expositions here.
Attacks of ichneumon-flies in water or on land apparently differ by the fact that aquatic insects almost always are infected in their egg-stage, and only exceptionally in their larval stage. Most ichneumon-flies of the water are only a few millimeters long. They bore their ovipositor into the eggs of aquatic insects, from which then, instead of an individual of the involved species of the latter, emerge one or many very small ichneumon-flies. Many insects of which the larvae and pupae live in the water, as for instance certain Diptera (Stratiomys, Eristalis a.o.), lay their eggs above the water, often on Phragmites. Therefore, many parasitic wasps (ichneumon-flies) live in the reed zone. Wasps that infect eggs not submerged in water cannot be considered aquatic insects, because they never volontarily go into the water, while they sometimes parasitize animals that are in certain of their stages aquatic insects. Many aquatic insects deposit their eggs directly onto the water's surface. Also these eggs can become infected without the wasps needing to go under water. Other aquatic insects, for instance many dytiscids (predator aquatic beetles), bore their eggs into living plant tissue under water. When in the spring the plants (Iris, a.o.) grow out above the water, the eggs now end up above the water-surface (while they were laid below it) and here go through the largest part of their development. Such eggs are almost always heavily infected by ichneumon-flies. In many places in north Seeland (Denmark) I have collected hundreds of such eggs. Often of about 50 eggs not a single one was not infected, while eggs that develop under water are less often infected. So it seems that the parasites of such eggs chiefly of them infect those eggs that find themselves (now) above the water-surface.
So it is not surprising that the ichneumon-flies of the water only rarely possess features pointing to a relationship of the animals with water. Their body, it is true, often is non-wettable, but this is not a result of special hairs or the like, but rather of the structure of the chitin. However, in many of them the underside of the tarsi (feet) is covered with felt. The wings are somtimes absent or are reduced. The wasps generally are bad swimmers. They rather crawl than swim. Their tracheal system is normal. Certain forms (Smicra) infect, it is true, eggs (of Stratiomys), but the egg of the ichneumon-fly only begins to develop when the host-larva has emerged. But most ichneumon-flies [of this 'watery group'] develop in the host-eggs [that is, consume them when they are still eggs]. Braconidae and Ichneumonidae do not parasitize eggs but lay their eggs into the larvae of their hosts.
Many parasitic wasps, and chiefly those that parasitize eggs of Odonata (damselflies and dragonflies), look for such eggs under water. They are in several respects clearly adapted to living in the water and are skilled swimmers. The ichneumon-flies of the water are distributed into five [terebrant] families :  Chalcididae, Proctotrupidae, Ichneumonidae, Braconidae, and Agriotypidae.

Chalcididae
Chalcididae and Proctotrupidae always are very small insects, almost all smaller than 1 cm and often only a few millimeters long. They are distinguished from each other especially by the fact that the ovipositor of the females in chalcids is placed far anteriorly on the underside, in Proctotrupids at the tip of the abdomen. Further, the antennae ar in chalcids always, in Proctotrupids not always, bent [the parts forming an obtuse angle].
The forewings have no spots (macula). The whole venation consists of just a single vein at the anterior wing-margin. Winglessnes is widely distributed. Further, the chalcids are characteristic by their metallic colors. The majority of them parasitize terrestrial insects [and many of them are phytophagous], and only a small minority of them can be considered aquatic wasps, and also these chiefly infect eggs or cocoons, placed above the water level, of aquatic insects. Some of these have been reared from cocoons of Gyrinidae [beetles living as adults on the surface of water], a single species  Pleurotropis  facialis  Gir.  from eggs of Dytiscidae (predatory water-beetles) bored into Alisma-stems (Hendriksen, 1918). However, only those eggs had been infected that had ended up above the water as a result of evaporation of the pond. From each egg numerous wasps emerged. Many of such forms can be encounterd in matters washed ashore at lakes. Many emerge from fly-pupae. These originally were lying in plant tissue, in which they as larvae gnawed channels, but became free when the plant tissue decayed.
Of the large division of Trichogrammae only the species  Trichogramma  evanescens  Westw.  has certain relationships with fresh water (Kryger, 1918). It is often encountered on the brown eggs of  Sialis (Alder Flies). Their females are winged, the males usually wingless. The small males crawl over the  Sialis eggs and wait for the females to mate with them. The animals do not limit themselves to  Sialis eggs though, but lay their eggs also into those of Stratiomyidae (certain Diptera, with aquatic larvae), Megaloptera other than  Sialis, butterflies, as also into the eggs of the beetle  Rhynchites  betulae (Cuculionidae). So these wasps are in fact terrestrial animals that parasitize eggs at the water (rather than in the water).
Only a single species, the famous  Prestwichia  aquatica  Lubb. (see Figure 1 above ),  really goes  under  water and infects there the eggs of numerous aquatic insects, as for instance water-beetles, dragonflies, and aquatic bugs (Hemiptera). Numerous treatises on the life history of this animal have appeared. The male is wingless. The females appear in no less than three forms, of which one has well-developed, the second shortened, and the third rudimentary wings. The very small size of these animals is best expressed by the fact that from one single  Dytiscus-egg or an egg of an aquatic bug up to 34, but at least 11-16 wasps emerge (Ruschka and Thienemanm, 1913). The wasps have a length of less than 1 mm. When they lay their eggs in those of Odonata, then from each one of them only one wasp emerges. Males and females swim with the help of their legs (Rousseau, 1907), which are, however, not provided with special swimming hairs. The winged females do not use their wings for swimming. Most often they crawl on plants or walk on the water-surface. But it is also observed that they go under water, crawling under the water-surface and then slowly swimming downward. The animals can be held for five days in aquaria without leaving the water. Their tracheal system is normal, with open spiracles at the metathorax. Probably also respiration through the skin plays an important role. Their body is certainly unwettable. Under water the animals are always surrounded by air, and instantly dry when they have come at the surface. The females are much more abundant than the males. Rimsky-Korsakow (1917) asserts that often, when many wasps develop in one host-egg, mating takes place in that egg, before the wasps emerge. The correctness of this observation is disputed by other authors (Heymons, 1908, Hendriksen, 1918). Development lasts only a short time. Several generations in a year develop.

(see Figure 2 and 3 ).

Apparently best adapted to an aquatic life of all wasps is  Polynema  natans  Lubb. (= Anaphes  cinctus  Halid.  =  Caraphractes  cinctus). See Figure 2 and 3 above .  Also this species is hardly one millimeter long. Both sexes are winged. For swimming they use their wings provided with long hairs. Earlier it was indicated that these animals do not possess a tracheal system, and that their wings, supposedly filled with blood, serve as gills. This turned out not to be so. The tracheal system of these animals is well developed. These tiny creatures belong to the prettiest microscopic objects. When a larger number of them swimms in a fish-tank they are most lovely to watch. It is certain that mating also takes place under water. The animals, it is true, come, from time to time, at the surface, but are able to live certainly at least several days under water (see Figure 3, above ).  This species parasitizes eggs of damselflies (Calopteryx) and aquatic Hemiptera (Notonecta, backswimmers). According to the investigators, having enquired into the parasites of eggs of Odonata, from each one of the infected eggs only one ichneumon-fly emerges. Also when several ichneumonfly-eggs have been laid in one odonate-egg, only one of them will develop. In the much larger Hemiptera-eggs one finds in one such egg 4-5 wasps, often 2 males and 3 females. When the whole odonate-egg has been consumed, which lasts only a few days, the wasp-larva pupates. The pupal stage lasts 10-12 days. In north Seeland (Denmark)  P. natans  is very common in odonate-eggs. One can recognize infected eggs already by their blue-black color. Leaf-stalks of Nuphar, densely populated by eggs of  Erythromma  najas (Odonata-Zygoptera, damselflies), often take a bluish shine as a result of the wasp-attacks. Where, and in what stage, the animals hibernate is not known. When in autumn the shores of the Furesee [a lake] are, after a storm, covered with foam, I often have found in it thousands of 1 mm long ichneumon-flies. They, however, have not yet been determined (as to what species they belong).
The genus  Polynema  contains numerous species, that chiefly parasitize Hemiptera-eggs. One sometimes had assumed that the long hairs along the wing-margins increase the value of their wings as swimming devices. Contrary to this, one legitimately points to the fact that such hairs are present in all species of the genus, and that they will serve at least just as well when used in the air in flying.

Two species of the closely related genus  Anagrus  parasitize in odonate-eggs laid under water. For a representative of this genus see Figure 4, above .

The Proctotrupidae are characterized by the fact that inside the host-egg they go through several very peculiar larval stages, belonging to the most adventurous forms of the insect world. Because they develop inside the insect-egg, these larval stages naturally, are very small indeed, being only some fractions of a millimeter long.

Ichneumonidae
Among the family Ichneumonidae, including so many large forms, and that preferably parasitize in larvae and not in eggs, there still are a few that parasitize aquatic insects. But they most often develop in those stages of them that live above the water :  Hemiteles  in cocoons of Gyrinidae (beetles, as imagines living on the surface of the water) (Thienemann, 1916, Butcher, 1933), see next Figure.

Figure 5 :    Hemiteles  biannulatus  Gray,  female.  Family  Ichneumonidae.
(After R. and H. Heymons, 1909, from WESENBERG-LUND, 1943)

Atractodes  develops in pupae of flies (Fliegenpuppen) (Diptera) (Ruschka and Thienemann, 1913), or in  Stratiomys-larvae (Diptera-Stratiomyidae) that always float on the surface. Also in larvae of Trichoptera (caddis flies) now and then are found parasites, chiefly, it is true, in those that ended up outside the water. Thus  Hemiteles  biannulatus  Gray  was reared from  Limnophilus-larvae (Siltala and Nielsen, 1906), Trichocryptus  aquaticus  Thomson  from  Hydrocampa  nymphaeata (Hendriksen, 1918). At the same place where Hendriksen had found this species, I have, during several years, observed numerous  Hydrocampa-cases, containing pupae of about 1 cm long of parasitic wasps. Probably they belong to the mentioned species.

Braconidae
Of the braconids numerous genera and species live near fresh water. Many were reared from Diptera-pupae (Fliegenpuppen) from matters washed ashore. However, the number of species that can really be called water-wasps, and that go under water, is in fact very small. These aquatic braconids are chiefly parasites of larvae of  Hydrellia (Diptera-Cyclorrapha) living in Potamogeton-leaves : The wasp Chaenusa  conjugens  Nees, see next Figure :

Figure 6 :    Chaenusa  conjugens  Neer.  Family  Braconidae.
(After R. and H. Heymons, 1909, from WESENBERG-LUND, 1943)

Further we have (among aquatic braconids) Liposcia, Gyrocampa (see next Figure), Dacnusa, and others.

Figure 7 :    Gyrocampa  stagnalis  Heym., male.  Family  Braconidae.
(After R. and H. Heymons, 1909, from WESENBERG-LUND, 1943)

And finally we have  Hygroplitis  rugulosus  Nees reared from  Hydrocampa-cases (Hendriksen, 1918, Ruschka and Thienemann, 1913, Schulz, 1907, Rousseau, 1907, Brocher, 1910). One has observed that  Dacnusa  and other species swim in the water using their wings. The wings of these and other species living nearby the water often are hairy, or their margin is provided with long hairs. Also the tarsal segments are widened, in order that their legs can be used as swimming devices.

Agriotypidae
One of the most remarkable of water-Hymenoptera,  Agriotypus  armatus  Walk. -- see next Figure -- of the family Agriotypidae, parasitizes in larvae of various caddis flies (Trichoptera), especially of  Goëra  and  Silo.

Figure 8 :    Agriotypus  armatus  Walk.,  female.  Family  Agriotypidae.
(After R. and H. Heymons, 1909, from WESENBERG-LUND, 1943)

On the first warm days of spring toward the end of April in north Seeland (Denmark) above rapidly flowing forest-brooks one sees almost-black wasps of about 1 cm long flying around. They also sit on rocks and from there they crawl -- surrounded by an air-envelope -- into the water, where their trail quickly vanishes in the turbulence of the water. At the same places numerous  Silo-cases (i.e. cases of certain caddis flies) are present. Surely, the wasps look for the larvae living in these cases in order to lay their eggs onto them. The emerging wasp-larva feeds on the caddis fly larva. The latter, however, dies not directly, but still manages to pupate. Only then the  Agriotypus-larva totally consumes its prey. It has no anus and no tracheae. The fully-grown larva carries two short hooks at its last abdominal segment. When the host-larva is paralized [The original text reads "verzehrt", which certainly must mean here "paralysiert", as this expression is indeed used (by Wesenberg-Lund) in the subscript of his accompanying Figure 490 (see next Figure here), despite the fact that the ability to paralyze the prey is more typical, not of terebrants (parasitic wasps), but of aculeates (sting-possessing hymenoptera)], the wasp-larve provides the  Silo-case with a wide flat band, of which the length varies from 1 to 5-6 cm. See next Figure.

Figure 9 :  Case of  Silo,  paralyzed by  Agriotypus  armatus.
(After Brocher, 1913, from WESENBERG-LUND, 1943)

The cocoon [i.e. the case] anteriorly has, between the side-wall and the anterior wall, a cleft. Through this and the sieve-disc [Siebplatte] the band [which Wesenberg-Lund now calls the tube (Röhre)] emerges. How the larva manages to push the tube out is at least to me [Wesenberg-Lund] difficult to understand. If one cuts off this band the parasite dies. The band is filled with air. The fully-grown parasite is surrounded by a layer of air. Hendriksen (1918) notes to this that the  Agriotypus-pupa, having an open tracheal system, must provide itself with air, because the caddisfly-pupa has tracheal gills and therefore not stores air in its case. The band would then be the means by which air is led in [for the benefit of the wasp-larva]. It mysterious to me [W-L] how this very flat tube would be able to lead the air contained in the water into the pupa-case. Moreover, one is then forced to hold that countless other aquatic parasites also would need such an air-pipe.  I have seen countless cases ['houses'] of  Silo and  Goëra  that were occupied by  Agriotypus,  where these chiefly were, as it seems to me, cases that were outside the water. The tube is certainly produced by the silk glands of the wasp-larva. It is natural to compare this band with the mast on the  Hydrophilus- (a vegetarian water-beetle)-cocoon. The presence of this bands indicates that a [caddis fly] case is occupied by  Agriotypus.  In autumn the larva spins its cocoon and before the beginning of winter changes into the adult insect. As such the animal rests 4-5 monthes in its cocoon inside the caddisfly-case, and only at the end of April frees itself from its prison.
(end of extract from Wesenberg-Lund, 1943, on aquatic Hymenoptera)

With all this we have come to an end of our exposition of the Parasitic egg-eating (endo-oophagous) phase of the evolution of the Hymenoptera.
In the next document we will describe the Secondary Phytophagous (phyto-oophagous) Phase.

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