Chapter XXV: Introduction: Habits–classification–structure–chilognatha–chilopoda (19)

The _Proctotrypidae_ are no doubt extremely numerous in species, but as yet they have been very little studied; a good work on the British species is much required. A valuable contribution has recently been made to the study of the family by Ashmead, in the book we have already referred to. This volume includes much information on the natural history of these Insects, and the outline figures give some idea of the great variety of external form.

Many entomologists include the Mymarides in Proctotrypidae, but Ashmead considers that they should be treated as a separate family. _Alaptus excisus_ Westw. (Fig. 354) has been frequently said to be the smallest known Insect, the {538}measurement given for it by Westwood[447] being a length of ⅙ of a millimetre—about 1/150 of an inch. Mr. Enock has recently examined Westwood's type in the Museum at Oxford, and from his information we may conclude that this Insect is probably the same as _Alaptus fusculus_ Hal., and that the measurement mentioned by Westwood is erroneous, the Insect being really about half a millimetre long. The Mymarides are, however, very minute, some of them not exceeding one-third of a millimetre in length. Whether any of them are smaller than the beetles of the family Trichopterygidae, some of which are only one-fourth of a millimetre long, may be doubted.

The Mymarides are recognisable by their very minute size, and by their peculiar wings. These are slender, destitute of nervures, fringed with long, delicate hairs, and stalked at the base. Probably Mymarides may all prove to be dwellers in eggs of other Insects. The group is remarkable from the fact that it contains some of the very few Hymenoptera with aquatic habits. Two species were discovered in their winged condition in the water of a pond near London by Sir John Lubbock[448]; one of them—_Polynema natans_ Lubbock—probably, according to Mr. Enock, the same as _Caraphractus cinctus_ Hal., uses its wings freely for swimming under water, while the other—_Prestwichia aquatica_—performs this operation by the aid of its legs. This latter Insect seems to be very anomalous, and its position quite doubtful. The embryogeny of _Polynema_ is very peculiar, and takes place in the egg of a dragon-fly—_Calepteryx virgo_—under water. According to Ganin,[449] in the earliest stages the developments of the embryos of the _Calepteryx_ and of the _Polynema_ progress simultaneously, but that of the dragon-fly does not proceed beyond the formation of the ventral plate. The _Polynema_ appears to leave its own egg at an extremely early stage of the embryonic development. It would appear, in fact, that there is no definite distinction between embryonic and larval stages. The information given by Ganin leads to the conclusion that a complete study of this remarkable mode of development is necessary before forming any general ideas as to the nature of Insect embryogeny and metamorphosis.

{539}FAM. III. CHALCIDIDAE.

_Pronotum with some freedom of movement, its angles not extending to the
insertion of the front wings. Antennae elbowed, consisting of from seven
to thirteen joints. Wings without a system of cells; with a single
definite nervure proceeding from the base near the front margin, or
costa; afterwards passing to the costa, and giving off a very short vein
more or less thickened at its termination. The species are, with few
exceptions, of parasitic habits._

The Insects of this family—the Pteromalini of Ratzeburg—are frequently of brilliant colours and of remarkable form; the species are very numerous, some 4000 or more having already been described. Of this number nearly 3000 are European, and as there is good reason for supposing that Chalcididae are quite as numerous in the Tropics and in the New World as they are in Europe, the family will probably prove to be one of the largest in the class. About twenty sub-families have already been proposed for the classification of the group; they are based chiefly on the number of joints in the tarsi, and the details of the antennae and of the ovipositor. This latter exhibits great variety in external appearance, due chiefly to the modification in form of the basal, or of the following ventral abdominal plates, one or more of which may be prolonged and altered in form or direction, giving rise in this way to considerable diversity in the shape of the abdomen. Correlative with this is a great variety in the mode of parasitism of the larva. Many live in galls, feeding on the larvae of the makers of the galls or on those of the inquilines; others attack caterpillars, others pupae only; some flourish at the expense of bees or other Hymenoptera, or of Coccidae {540}and Aphididae, and some deposit their eggs in the egg-cases of Blattidae. The details of the life-history are well known in only a few cases.

The career of _Leucospis gigas_ has been investigated by Fabre, and exhibits a very remarkable form of hypermetamorphosis.[450] This Insect is of comparatively large size and of vivid colours, wasp-like, black contrasting with yellow, as in the case of the wasps; and like these it has the wings folded or doubled. The female bears a long ovipositor, which by a peculiar modification is packed in a groove on the back of the Insect. This species lives in Southern Europe at the expense of _Chalicodoma muraria_, a mason-bee that forms cells of a hard cement for its nest, the cells being placed together in masses of considerable size; each cell contains, or rather should contain, a larva of the bee, and is closed by masonry, in the construction of which the bee displays much ability. It is the mission of the _Leucospis_ to penetrate the masonry by means of its ovipositor, and to deposit an egg in the cell of the bee. The period chosen for this predatory attack is the end of July or the beginning of August, at which time the bee-larva is in the torpid and powerless condition that precedes its assumption of the pupal state. The _Leucospis_, walking about leisurely and circumspectly on the masonry of the nest, tests it repeatedly by touching with the tips of the antennae, for it is most important that a proper spot should be selected. The bee's cell is placed in a mass of solid masonry, a considerable part—but a part only—of whose area is occupied by the group of cells; every cell is closed by hard mortar, making an uneven surface, and the face of the masonry is rendered more even by a layer of hardened clay outside the rougher material; it is the task of the _Leucospis_ to detect a suitable spot, in the apparently uniform external covering, and there to effect the penetration so as to introduce an egg into a cell. By what sensations the fly may be guided is unknown. After a spot has been selected and the ovipositor brought into play, the masonry is ultimately pierced {541}by patient work; sometimes a quarter of an hour is sufficient for the purpose, but in other cases three hours of uninterrupted effort are required before the end is attained. Fabre expended much time in watching this operation, and after the Insect had completed it, he marked with a pencil the exact spot of the masonry that was penetrated, and the date on which it was done, and he states that he afterwards found that without any exception a proper spot had been selected, and a cell consequently penetrated. Admirable as the instinct of the parasite appears from this point of view, it is nevertheless accompanied by a remarkable deficiency in two other respects. The first is that though the spot selected by the _Leucospis_ invariably gives entrance to a cell, yet in the majority of the cases the selected cell is not a suitable one; a large number of the cells of the _Chalicodoma_ are not occupied by living larvae on the point of pupation—though in that case only can the egg of the _Leucospis_ hatch and successfully develop—but by dead and shrivelled larvae, or by mouldy or dried-up food. And yet, in each case of penetration, Fabre believes that an egg is deposited, even though it may be impossible that it can undergo a successful development. Strange as this may appear, it is nevertheless rendered less improbable by the second deficiency in the instinct of the parasite. The Insect has no power of recognising a cell that has been previously pierced either by itself or by another of its species. One bee larva can only supply nourishment for a single larva of the parasite, and yet it is a common occurrence for a cell to be revisited, pierced again and another egg introduced; indeed Fabre, by means of the cells he had marked, was able to assure himself that it is no uncommon thing for this to be done four times; four eggs, in fact, are sometimes deposited in a cell that cannot by any possibility supply food for more than one larva. The egg of the _Leucospis_ is a curious object (Fig. 357, A), very elongate oval, with one end drawn out and bent so as to form a hook; it is not placed at random in the cell of the bee, but is suspended on the delicate cocoon with which the _Chalicodoma_ larva is surrounded at the period of pupation.

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Fabre allowed sufficient time to elapse for the hatching of the larvae from the eggs, and then opened some cells where Leucospis eggs had been deposited, in order to obtain the larvae; when doing this he was surprised that he never found more than one _Leucospis_ larva in a cell. Even in cells where he had observed more than one act of oviposition, and which he had marked at the time, only one larva existed. This induced him to think that it was possible that no egg was deposited by the _Leucospis_ at the second penetration. He accordingly examined cells soon after the eggs were laid, and thus discovered some that contained more than one egg,—indeed in one cell he observed no less than five eggs suspended from the cocoon of the _Chalicodoma_; he was also able to demonstrate that eggs were actually deposited in some cells that contained no means of support for the larva. How then could these two facts be reconciled—four or five eggs deposited in a cell, only one larva present afterwards? It is of course impossible to observe the operations of a larva shrouded in the obscurity of a cell formed of masonry, so he transferred some bee larvae with their destructive companions to glass tubes, in which he was able to note what took place. He found that the egg deposited by the _Leucospis_ hatches and produces a very peculiar larva, having little resemblance to the _Leucospis_ larva that he had found eating the _Chalicodoma_ larva. The primary larva (Fig. 357, B) of the _Leucospis_ is an arched worm, moderately deeply segmented, a millimetre or a little more in length, with a remarkably large and abruptly-defined head. The body bears erect setae, the most remarkable of which are a pair on the ventral aspect of each of the segments, each of these ventral setae being borne on a small conical prominence. These prominences and setae serve as ambulatory organs, and are supplemented in their function by a protuberance at the posterior extremity. The little creature has considerable powers of locomotion; it moves, after the fashion of many other larvae, by contracting and arching the body so as to bring the posterior part nearer to the anterior; then fixing the hinder part, the anterior is extended and fixed, the posterior being again brought nearer to the front. The _Leucospis_ larva when hatched does not at once attack the bee larva which is to be its future food, but every few hours makes excursions over its surface, and even explores the walls of the cell; returning, however, always to the cocoon for repose. The object of these {543}excursions is, Fabre believes, to ascertain if another _Leucospis_ egg has been laid in the cell, and in that case to destroy it. For the food, as we have said, being only enough for one larva, and the mother _Leucospis_ frequently laying more than one egg in a cell, it is necessary that all the eggs except one should be destroyed. Fabre did not actually observe the act of destruction, but he found repeatedly in his glass tubes that the supernumerary eggs were destroyed, being, in fact, wounded by the mandibles of the first-hatched larva. After several days of this wandering life the tiny destroyer undergoes a first moult, changing its skin and appearing as a very different creature (Fig. 357, C); it is now completely destitute of any means of locomotion, very deeply segmented, curved at one extremity, with a very small head, bearing extremely minute, scarcely perceptible, mandibles. The sole object of its existence in this state is to extract the contents of the _Chalicodoma_ larva, and appropriate this material to the purposes of its own organisation. This it accomplishes not by wounding, tearing, or destroying the larva, for that apparently would not answer the purpose; the contents must be conveyed while still in their vital state to itself; and this it effects by applying its mouth to the extremely delicate skin of the victim, the contents of whose body then gradually pass to the destroyer, without any visible destruction of the continuity of the integument. Thus the _Leucospis_ larva gradually grows, while the bee larva shrinks and shrivels, without, however, actually suffering death. The process of emptying the bee larva apparently does not occupy the _Leucospis_ more than two or three weeks, being completed by about the middle of the month of August; afterwards the larva remains in the cell by the side of the shrivelled skin of its victim for ten or eleven months, at the end of which time it assumes the pupal condition, and very shortly thereafter appears as a perfect Insect.

_Monodontomerus cupreus_ is another member of the Chalcididae that lives parasitically at the expense of bees of the genus _Chalicodoma_. Its habits have been sketched by Fabre,[451] and exhibit considerable difference from those of _Leucospis_. It is much less in size, and can accommodate itself to a greater variety of food; it will, in fact, eat not only the larva of _Chalicodoma_, but also that of another bee, of the genus _Stelis_, that is frequently found {544}shut up in the cell of the _Chalicodoma_, at whose expense the _Stelis_ also lives parasitically. The _Monodontomerus_ bores a hole through the masonry of the bee and deposits its eggs in the cell after the fashion of the _Leucospis_; one bee larva is, however, sufficient food for several individuals of the young of this smaller parasite. There is no hypermetamorphosis, the early larval condition resembling the later. This Insect attacks not only _Chalicodoma_ and _Stelis_, as already mentioned, but also other bees; and a single larva of some of the larger kinds will afford sufficient food for fifty young of the _Monodontomerus_. They feed on the bee larva, as the _Leucospis_ does, without wounding it. This fly has the power of recognising what is suitable provender for its young by the use of the antennae, even when the conditions are so changed that it is clear the sense of sight has nothing to do with the recognition. Fabre relates that he had extracted a number of the bee larvae from their cells of masonry, and that as they were lying on his table enclosed in their cocoons, the _Monodontomerus_ recognised the latter as containing the desired provender for its young by examining them with its antennae; after which, without hesitation, the _Monodontomerus_ pierced the cocoon with its ovipositor and deposited the eggs in a suitable position. This observation, together with those made on _Leucospis_, seem to indicate that it is neither by sight nor smell that these Insects discover the desired object, but by some sense we do not understand, though its seat is clearly in the antennae of the Insect.

Newport discovered a _Monodontomerus_, which he described as _M. nitidus_,[452] in the cells of the bee _Anthophora retusa_, and demonstrated that the alimentary canal, as is usual in Petiolate Hymenoptera, is closed behind until the Insect is about to enter the pupal state, when it becomes perforated and faecal matters are for the first time passed from it. "These matters were composed of the refuse of digestion and of epithelial cells accumulated during the period of feeding, and retained in the digestive sac until the period of its perforation. In this way the food and abode of the Insects are maintained pure and uncontaminated, and the digestive apparatus is completed, and the refuse of nutrition ejected only when the whole of the food has been consumed."

{545}In the cells of the same bee Newport discovered another curious parasitic Chalcid, _Anthophorabia retusa_.[453] The male has short wings, and the compound eye is replaced by an ocellus on each side of the head, the female having fully developed wings and eyes. A variation may occur in the metamorphosis of this Insect, inasmuch as when the growth is completed during the month of August, the Insect changes to a pupa, the imago appears ten or twelve days thereafter, and the perfect Insect then hibernates for seven or eight months; but should the completion of growth be deferred till after the end of August, hibernation takes place in the larval condition. A large and brilliant Chalcid _Eucharis myrmeciae_, has been described by Cameron as preying on the formidable Australian ants of the genus _Myrmecia_.

The development of _Smicra clavipes_ has been partially described by Henneguy.[454] This Insect lives in the interior of the aquatic larva of _Stratiomys strigosa_, a Dipterous Insect. As many as fifty eggs of the parasite are found in one larva, but a large number of embryos die during development, so that he has never found more than two or three well-grown larvae in one _Stratiomys_ larva. It has been ascertained that the eggs of many of these parasitic Insects are deficient in yolk, and the ovum of _Smicra_ is said to obtain the nutritive materials necessary for the development of the embryo from the blood of its host by endosmosis. For a long time after the assumption of the larval condition, the larva appears to nourish itself only at the expense of the blood of its host. The segmentation of the ovum is total, and a single embryonic membrane appears at an early period, before the formation of the embryo, by a process very different from that giving origin to the amnion of the majority of Insects.

A very interesting sketch of the development of _Encyrtus fuscicollis_ has been given by Bugnion.[455] This small parasite passes its earlier stages in the interior of the larva of _Hyponomeuta cognatella_ or other Lepidoptera. The female _Encyrtus_ deposits her eggs in the interior of a caterpillar, in the form of a series of 50 to 100 or more eggs enclosed in a sac; the origin {546}of the sac is obscure, but the embryonic development and the early part of the larval life are passed in the sac, which contains a supply of nutritive matter. The larvae of the _Encyrtus_ are at first entirely confined to this sac, but when they have consumed all the nutritive matter in it, they leave it and pass the remainder of their larval and pupal existence in the body-cavity of the caterpillar. They live at first on the lymph (blood) of the Insect, and apparently do it no harm; nevertheless the strength of the caterpillar is so much enfeebled that it fails to undergo the transformation to a pupa; the parasites then devour its interior, and use the empty skin as a nidus for their own pupation; they form cocoons which divide the area into compartments. Usually the individuals disclosed from one _Hyponomeuta_ are all of one sex, which may be either male or female. Unfortunately the most interesting points of this development, viz. the history of the common sac for the larvae, the nature of the eggs, the earlier embryonic stages, and the nutriment in the sac, are still without elucidation. The account given by Bugnion raises a great desire for information on these points.

We have in a previous page described the remarkable mode of oviposition of _Mantis_. Captain Xambeu[456] has made a very curious observation to the effect that a minute Chalcid, _Podagrion (Palmon) pachymerus_, shelters itself under the wings of the _Mantis_ so as to be in a position to oviposit in the eggs of the latter when it shall be forming its peculiar ootheca.

The genus _Isosoma_ consists of Insects that differ in habits from their congeners, being phytophagous instead of parasitic. _I. tritici_ and _I. hordei_ live in the stalks of corn, and in North America, where they are known to the agriculturist as joint-worms, are frequently very injurious to crops. They are sometimes obtained in large numbers without any males appearing, and a wingless as well as a winged form of the female occurs. Owing to the fact that the allies of these Insects are parasitic, it has been frequently maintained that this may also prove to be the case with _Isosoma_, but the observations of Riley[457] and others leave no doubt that the Insects of this genus are really plant-feeders.

{547}Riley has called attention[458] to some facts in connection with _I. tritici_ and _I. grande_, that make it clear that these two supposed species are really alternate generations, and that both generations are probably in larger part, if not entirely, parthenogenetic. Some species of the genus _Megastigmus_ are known to be of phytophagous habits.[459]

The most interesting of all the forms of Chalcididae are perhaps those called fig-Insects. A considerable number of species are now known, and amongst them we meet with the unusual phenomenon of species with wingless males, the females possessing the organs of flight normally developed. The wingless males exhibit the strangest forms, and bear no resemblance whatever to their more legitimately formed partners (Fig. 358, A, B). Many of the fig-Insects belong to a special group called Agaonides. Others belong to the group Torymides, which contains likewise many Chalcididae of an ordinary kind; possibly some of these may be parasitic on the Agaonides. Some of these Torymid fig-Insects have winged males, as is normal in the family, but in other cases winged and wingless forms of the male of one species may be present.

The most notorious of these fig-Insects is the one known as _Blastophaga grossorum_ (Fig. 358), this being the chief agent in the custom known as caprification of the cultivated fig-tree. This process has been practised from time immemorial, and is at the present day still carried on in Italy and the Grecian archipelago. The Greek writers who describe it say that the wild fig-tree, though it does not ripen its own fruit, is absolutely essential for the perfection of the fruit of the cultivated fig. In accordance with this view, branches from the wild fig {548}are still gathered at certain seasons and suspended amongst the branches of the cultivated fig-trees. The young fig is a very remarkable vegetable production, consisting of a hollow, fleshy receptacle, in which are placed the extremely numerous and minute flowers, the only admission to which is by a small orifice at the blunt end of the young fig; this orifice is lined with projecting scales, that more or less completely fill it up or close it; nevertheless inside this fruit the _Blastophaga grossorum_ develops in large numbers. The males are, as we have seen, wingless creatures, and do not leave the fruit in which they were bred, but the females make their way out of the wild fig, and some of them, it is believed, enter the young fruit of the cultivated trees and lay their eggs, or attempt to do so, therein; and it has been supposed by various writers that these proceedings are essential to the satisfactory development of the edible fruit. It is a curious fact that the _Blastophaga_ develops very freely in the wild fig—so much so, indeed, as to be a means of preventing it from coming to maturity; but yet the Insect cannot complete its development in the cultivated fruit. This is due to the fact that the fly must lay its egg in a particular part of the fig-ovule, so that when the egg hatches the larva may have a proper supply of food. In the cultivated fig the structure of the flower differs somewhat from that of the caprificus, as the wild fig is called, and so the egg, if deposited at all, does not reach a proper nidus for its development. Hence the _Blastophaga_ can never live exclusively on the cultivated fig, and if it be really necessary for the development of the latter, must be brought thereto by means of the caprifig. Whether the _Blastophaga_ be really of use, as has been for so long supposed, is, however, a matter for doubt. The reasons for this are (1) that those who think caprification beneficial do not agree as to the mode in which they suppose it to be so; (2) that there is but little reason for believing that when introduced amongst the cultivated figs the _Blastophaga_ occupies itself to any great extent therewith; and (3) that in some parts of the world caprification is not performed, but the cultivated fig nevertheless ripens its fruit there. Hence many writers on the subject— Solms-Laubach,[460] Mayer,[461] and Saunders[462]—entertain considerable doubt as to whether caprification is at present anything {549}more than an old custom destitute of practical utility. On the other hand, Riley states[463] "that the perfect Smyrna fig, the most esteemed of the edible species, can be produced only by the intervention of the _Blastophaga psenes_ [_grossorum_]."

Although the questions connected with the effect the _Blastophaga_ is supposed to produce on the fruit are of a botanical rather than a entomological nature, we may briefly say that two views have been held: (1) that, as in the fruit of the cultivated fig, only female flowers are produced, the _Blastophaga_ is necessary for their fertilisation and the subsequent development of the fruit; (2) that the Insects stimulate the fig by biting parts thereof or by burrowing in it, and so give rise to the processes that have as their result the edible fruit. There seems to be little doubt that the Insect agency is necessary to the fertilisation of some species of figs. Cunningham, who has recently carried out an elaborate investigation as to the fertilisation of _Ficus roxburghii_,[464] concludes that in this fig, and probably also in other kinds, the perfect development is dependent on the access of the fig-Insects to the interior of the receptacular cavity. Should access fail to occur, both male and female flowers abort, without the formation of pollen grains by the former or seeds by the latter. The access of the _Blastophaga_ is thus as necessary for the perfect evolution of the normal male and female flowers as it is for that of the modified ♀ or gall-flowers, with their contained ova and Insect-embryos. Whether the successful fertilisation of the flowers is really essential to the production of the edible fig is not a question for our discussion.

Fig-Insects are apparently more numerous in South America than they are in any other part of the world; and Fritz-Müller has discovered[465] a number of species there of a very extraordinary character, several of them possessing two forms of the male, one winged like the female, the other wingless and so different in character that they were considered to belong to a different genus. The wingless male of a species found in Madagascar, _Kradibia_ _cowani_, has the peculiarity of possessing only four legs, the middle pair being represented merely by minute two-jointed rudiments. Some of these Insects live in galls on the figs. The fig-Insects {550}were formerly considered to belong to the Proctotrypidae or to the Cynipidae (gall-makers), but there can be no doubt, notwithstanding they differ so much in their habits from the parasitic Chalcididae, that they probably belong to the same family. If treated as different from Chalcididae, they should be separated as a distinct family rather than united with the Cynipidae.[466]

It is impossible for us to do more than allude to the extraordinary shapes exhibited by some Chalcididae. The genus _Thoracantha_ is specially remarkable in this respect. _T. latreillei_ is said to resemble a beetle of the family Mordellidae, and has the wings concealed by false wing-cases—really projections from the thorax—so that from above the Insect bears no resemblance to the other Insects of the Order it really belongs to.

Howard has called attention to some peculiarities in the pupation of Chalcididae.[467] Like the Cynipidae, they do not make a silken cocoon, but some of them that change to pupae inside the victims on which they were nourished have the power of forming oval cells in which to undergo their transformation, and they thus cause a peculiar inflation of the skin of their deceased victim, which after death still continues to serve as a protection to the destroyers. The statement made by Haliday, and repeated subsequently in various works, to the effect that Coryna spins a cocoon under the _Aphis_ in which it has lived, is an error, the cocoon being really formed by _Praon_, a Braconid that is a parasite of the _Aphis_, and on which the Chalcid _Pachycrepis (Coryna)_ lives as a hyperparasite. The pupae of some species differ from those of other Hymenoptera, in that the integument is hard, and the limbs are soldered to the body as in Lepidoptera. These forms pupate external to the victim.

Fritz Müller has recorded that the pupa of an unnamed species of Chalcid that attacks a Brazilian ant (_Azteca instabilis_ Forel) is suspended on the wall of the cell the ant lives in by its posterior extremity, just like the chrysalis of a butterfly.

{551}Notwithstanding the small size of Chalcididae, their remains have been detected in the tertiary strata of both Europe and North America.

FAM. IV. ICHNEUMONIDAE (ICHNEUMON-FLIES).

_Wings with a well-developed series of nervures and cells; the space on
the front wing separating the second posterior cell from the cubital
cells is divided into two cells by a transverse veinlet. The abdomen is
attached to the lower or posterior part of the median segment. Larvae
parasitic in habits._

The Ichneumonidae form a family of enormous extent, containing nearly 6000 described species. The study of the family is but little advanced, owing to their parasitic habits and to this bewildering multiplicity in their specific forms. Most of the species, in the larval state, live inside the larvae of Lepidoptera, and they thus keep the myriads of caterpillars within bounds, the number of these destroyed by Ichneumons being prodigious. Some of the family are, however, external parasites, and some are known to attack Spiders and Insects of other Orders than Lepidoptera. Their antennae are not elbowed and are many-jointed, the joints being closely compacted, especially towards the extremity. This character readily distinguishes Ichneumonidae from the families we have previously considered. The ocelli are well developed even in the apterous forms, and are placed in a triangular position on the vertex. The pronotum is small in front; and extends backwards at the sides to the points of insertion of the front wings; it is fixed to the mesonotum. The wings (Fig. 367, A) have a more complex neuration than those of most of the other parasitic Hymenoptera, but are occasionally absent in one or both sexes of a species. The metathorax is very small, and the middle and hind legs are placed close together. The propodeum is very large, and is frequently covered with a highly-developed sculpture.

{552}

The hind body springs from the lower part of the propodeum; it is usually of slender form, and its segmentation is very conspicuous. The females bear an ovipositor, which differs greatly in length according to the species, and is known in the case of one species to attain a length six times that of the whole of the rest of the body.[468] The egg is deposited by some species on the skin, by others within the body of the victim; it varies much in form and colour, some eggs being stalked and of peculiar shape. The larvae issuing from the eggs are legless maggots with a delicate integument of pallid white or creamy colour. If the eggs are laid on the surface of the body, the resulting larvae (except in the cases of the external parasites) soon bore into the interior of their victim, and disappear therein. The changes that take place in the lifetime of the larvae have been studied in only a few cases; but if we can judge from Ratzeburg's history[469] of the changes that take place in _Anomalon_, they are of great interest. From observation of the differences existing amongst a great number of larvae of _A. circumflexum_ he distinguished four stages. It is of course impossible to follow directly the growth of one individual, because it is concealed in the interior of the caterpillar in which it lives, and to open this involves the death of both caterpillar and Ichneumon-larva. The life history must therefore be constructed from a great number of separate observations; and it is not ascertained that the four instars described by Ratzeburg represent the number of moults of the larva that actually take place. He, however, entertained no doubt that all the forms he observed {553}were stages in the development of one species. In the earliest stage, when only one millimetre in length and about as thick as a horse-hair, the larva is free in the interior of the caterpillar's body, and has a small head armed only with a pair of mandibles. There are, in addition to the head, thirteen segments, and the last of these is an elongate tail forming nearly one-half the length of the creature. No trace of tracheae can be discovered. In the second stage the larva is still free, an elongate tracheal tube exists, the tail has diminished to half the length, the head has become much larger, and rudimentary antennae of one joint are visible; possibly stigmata are present at this stage, though they cannot afterwards be detected. In the third stage (Fig. 361, C) the larva is encysted, the head is large, the parts of the mouth are all developed, the tracheal system is extensive, and the caudal termination of the body is quite short; notwithstanding the extensive development of the tracheal system, no stigmata can be found. In the fourth stage the larva is still encysted, the tail has disappeared, the head and mouth parts are reduced in size and development, and the creature has now the appearance of a normal larva. The changes to pupa and perfect Insect take place within the body of the victim, in some cases, if not usually, after it has undergone its metamorphosis into a chrysalis. Very little information is extant as to the duration of the various stages, but it appears to be the rule that only one generation appears annually, though in some cases there are pretty certainly two.

It is very difficult to observe the act of oviposition; the Ichneumon-flies usually decline to notice caterpillars with which they are placed in confinement. Ratzeburg thinks they will only attack caterpillars that are in a deficient state of health or vitality. Occasionally we may by a happy chance observe the act in Insects at large, and from the records of observers it may be deduced with tolerable certainty that the sense of sight takes no part in the operation. Ratzeburg relates that he saw a _Pimpla_ alight on a leaf of _Rhus_ and thrust its ovipositor through the leaf. On looking to the under-side of the leaf he found that a cocoon of _Bombyx neustria_ was concealed there in such a position that it could not have been seen by the Ichneumon.

{554}

Among the most remarkable of the Ichneumon-flies are the Insects of the genera _Rhyssa_ and _Thalessa_. These fine Insects have an ovipositor three or four inches in length, and are parasitic on species of the family Siricidae, which, as we have previously described, live in solid wood. In order therefore to deposit the egg in a suitable place, the wood must be pierced by the Ichneumon. The ovipositor is not only of extreme length, but is also furnished with serrations on its apical part, so that it forms a very effective boring apparatus. It is brought into use by being bent on itself over the back of the Insect (Fig. 362), so as to bring the tip vertically down on to the wood, through which it is then forced by a series of efforts; the sheaths do not enter the wood. The egg is laid anywhere in the burrow of the _Sirex_; the young larva seeks its prey, and lives on it as an external parasite (Fig. 342, D). Erne, however, states[470] that the young larva of _Rhyssa persuasoria_ enters its victim, and remains within the latter till its death occurs. This happens when the young _Rhyssa_ is two or three lines in length, and it then makes its exit from the interior of the body and gradually eats it up. Should the larva it has attacked be of large size, it of itself affords sufficient food for the completion of the growth of the _Rhyssa_. Should the _Rhyssa_, however, have attacked a small larva, this does not furnish it with sufficient food, and it consequently dies without seeking another larva. Erne says, indeed, that it will not eat another if offered to it, so that in order to rear the _Rhyssa_ in captivity, the victim it has first attacked must always be given to it. The same observer states that the _Rhyssa_ larva is sometimes transported by the _Sirex_ deep into the wood, so that when it has completed its metamorphoses the Ichneumon-fly may find itself buried in solid wood to a depth of about two inches. In that case it excavates the wood with its mandibles, and should it fail to gain the exterior after {555}three days of work, it dies. In the case of _Thalessa_ it is stated that it sometimes bores into wood where there are no larvae, but Riley thinks this erroneous; it is, on the other hand, certain that the Insect after penetrating the wood is frequently unable to withdraw the ovipositor, and consequently dies.

Packard has recorded,[471] without mentioning the species, the oviposition of an Ichneumon of which the egg is deposited externally. It was placed on the head of the caterpillar, and speedily hatched; the young larva at once bored through the prothoracic segment of the victim, the head of the latter then became swollen, and covered the opening into the prothorax, made by the parasite.

The history of an Ichneumon larva that feeds as an external parasite has been sketched by De Geer and Newport. The observations of the latter[472] refer to _Paniscus virgatus_; he found small, shining, black bodies attached to the skin of the larva of a moth, _Mamestra pisi_; these were the eggs of the Ichneumon. They are furnished with a short peduncle, which is implanted in the skin of the victim; the egg, according to De Geer, being retained more firmly by the peduncle subsequently swelling, so as to form two knobs. The hatching takes place by the egg-shell splitting longitudinally, while from the split protrudes the little head of the destroying larva. This becomes fixed to the caterpillar, from which the nutriment is to be drawn; the _Paniscus_ larva does not, however, leave the egg-shell, but, on the contrary, becomes adherent to it, so that the parasite is in this manner fixed by the two ends to its victim. In fifteen days the parasite was full-grown, and had become half an inch in length. At first no tracheae were to be seen, but these were detected after the second day. Moulting took place three times, and in a peculiar manner, very different from that described by Ratzeburg as occurring in the internal parasites (which, he states, change their very delicate skin by detaching it in almost imperceptible fragments). In the external parasite the {556}skin remains entire, and is shuffled down to the extremity of the body, but cannot be completely detached owing to the anchoring of the posterior part of the body to the caterpillar; the cast skins thus remain as envelopes to the posterior part of the body. Newport states that if the mouth of the parasite be detached, it usually cannot again seize hold of the victim, and consequently perishes. It is a curious fact that more eggs than one caterpillar can support are habitually placed on it, and some of the resulting larvae of necessity perish during the period of growth. Poulton, who has recently made some additional observations on the development of _Paniscus_,[473] says that if three larvae are close together, it is the middle one that perishes, and suggests that this is due to some simple physical condition. From Newport's account it may be gathered that the _Mamestra_ retains sufficient vitality to form its cocoon, and that the _Paniscus_ larvae likewise construct their own cocoons within that of the _Mamestra_. In the case of _Paniscus cephalotes_ feeding on _Dicranura vinula_, Poulton relates that the latter died after the twelfth day of attack. The parasites, having relaxed their hold on the victim just previous to this event, then thrust their heads into the dead body, and devoured the larva, leaving only a dried and empty integument. These larvae span a loose sort of web in which to undergo their metamorphosis. In a natural state, however, they form cocoons inside the cocoon of the _Dicranura_. The period passed in the pupal condition was about four weeks. This parasite only attacks the Lepidopterous larva during the last stage of its existence as a larva, but the eggs may be laid on the victim in an earlier stage; and in such case De Geer has stated, and Poulton has confirmed the observation, that though the larva sheds its skin it does not get rid of the eggs.

The little Ichneumons of the genus _Pezomachus_ are quite destitute of wings and somewhat resemble ants; they are common Insects in Britain. Only the female sex is known, and it is believed that the winged Ichneumons assigned to the genus _Hemiteles_—of which no females are known—are the males of _Pezomachus_. Repeated efforts have been made to place this beyond doubt, but they have usually failed, for when a brood of these parasites is reared, the individuals generally prove to be {557}either all _Hemiteles_ or all _Pezomachus_. It is to be hoped that this interesting case will be fully elucidated.

Although the Ichneumonidae are perhaps the most purely carnivorous of all the great families of Hymenoptera, there is nevertheless reason for supposing that some of them can be nourished with vegetable substances during a part at any rate of the larval existence, Giraud and Cameron[474] having recorded observations that lead to the conclusion that some species of the genus _Pimpla_ may inhabit galls and live on the substance, or juices thereof.

Over 1200 species of Ichneumonidae are known to inhabit Britain, and there can be no doubt that this number will be increased as a result of further observation. Unfortunately no general work has yet been published on this department of our fauna, and the literature is very scattered.[475] The species of North America have not received so much investigation as those of Europe, and the Ichneumon fauna of the tropics remains almost uninvestigated. Six sub-families are recognised: Agriotypides, Ichneumonides, Cryptides, Tryphonides, Pimplides, Ophionides. Of these the first is the most remarkable, as it consists of an Insect having aquatic habits. It has for long been known that the unique species _Agriotypus armatus_, a rare Insect in our islands, is in the habit of going under water and remaining there for a considerable period, and it has now been satisfactorily ascertained that it does this for the purpose of laying its eggs in the larvae of Trichoptera.[476] The resultant larva lives inside the cases of species of _Silo_, _Goëra_, etc. It undergoes a sort of hypermetamorphosis, as its shape before assuming the pupal condition {558}is very different to what it was previously. It changes to a pupa inside the case of the Trichopteron in a cocoon attached to the walls of the case. Previous to making this, however, the _Agriotypus_ forms a curious, elongate, string-like process attached to the anterior extremity of its cocoon. The use of this is unknown. Full information as to the life-history of this aquatic Hymenopterous larva, especially as to its respiratory functions, would be of great interest. The affinities of this remarkable Insect are still doubtful. It may probably prove to be between Proctotrypidae and Ichneumonidae.

Remains of Insects that may be referred with more or less certainty to Ichneumonidae have been found in some abundance in various tertiary strata both in Europe and North America, but nothing indicative of the existence of the family has yet been found in the older rocks.

FAM. V. BRACONIDAE—SUPPLEMENTARY ICHNEUMON-FLIES.

_Antennae with many (nearly always more than fifteen) joints, not
geniculate. Wings with a moderate number of cells, which on the anal part
of the front wing are more or less imperfect, the anal (i.e. the second
posterior) cell being separated from the cubital cells by a large space
in which there is no cross-nervure. Abdomen with but little mobility
between the segments; the suture between the second and third usually
{559}absent, or obsolete. Larvae living parasitically in—possibly
exceptionally outside—the bodies of larvae or pupae of Insects._

The Braconidae are the Ichneumones, or Ichneumonides, adsciti of the older Hymenopterists. They are extremely similar to the Ichneumonidae, but the hind body has a much less degree of mobility of its segments, and there are some constant distinctions in the wings. Although there is a great deal of difference in the various forms of each of the two families, yet there are two points of distinction easily appreciated; the series of cells running across the wing (Fig. 367) being only three in the Ichneumonides (Fig. 367, A), but four in the Braconids (Fig. 367, B); besides this the space _a_ of the Braconid wing is divided into two (_a_, _b_) in the Ichneumonid wing. A glance at these characters enables us at once to separate correctly the thousands of species of the two families.

The habits of the Braconidae are similar to those of Ichneumonidae, it being believed that all are parasites. Usually they attack larvae, but they are bred in great numbers from pupae, and even from imagos of other Insects. _Elasmosoma_ is one of the few parasites known to attack ants. As many as 1200 specimens of _Microgaster_ have been reared from a single Lepidopterous larva. Although such parasitism raises a feeling of repulsion, yet there is reason for supposing that there may be {560}little or no cruelty or acute suffering connected with this mode of life. The victim attacked is not eaten, the parasites in the interior taking in the lymph of the caterpillar either by the mouth or by endosmosis, but not biting their host. The latter displays no sign of sickness, but eats voraciously, so that it serves merely as a sort of intermediary between the juices of the plant and the larvae inside itself. It is only when the metamorphosis is at hand that the host sickens, but this does not always happen: parasitised larvae frequently change to pupae, and they may occasionally even become perfect Insects. Cases are known in which imagos have appeared with some of the small parasites embedded in some of the outer parts of their bodies. These cases are, however, very rare; in the enormous majority of instances the host is destroyed either when it is in the larval stage or before the pupa has advanced to any great extent on its metamorphosis to an imago. Particulars as to various species will be found in the valuable work of Ratzeburg we have already referred to.[477] Reference may also be made to Goureau's account of _Microgaster globatus_,[478] this latter including some suggestions by Dr. Boisduval on some of the difficult physiological questions involved in the lives of these parasites.

The metamorphosis of _Microgaster fulvipes_ has been studied by Ratzeburg, and an epitome of his observations is given by Marshall.[479] The larva goes through a series of changes somewhat similar to those we have already sketched in _Anomalon circumflexum_. Usually these Insects after emerging from the body of their host spin a mass of cocoons more or less loosely connected together. A most curious case has, however, been recorded by Marshall[479] of a stalked cocoon (Fig. 368) being formed as an exceptional act by _Apanteles formosus_. Mr. Marshall has recently received other specimens of this cocoon as well as the Insects reared therefrom in France, and inclines to the opinion that the stalked cocoon may be the usual form, and is sometimes departed from by the Insect for unknown reasons.

{561}This family is of enormous extent; we have several hundred species of it in Britain,[480] and there are no doubt many thousands of undescribed exotic forms. To _Apanteles glomeratus_ we are indebted for keeping our cabbages and kindred vegetables from destruction by the caterpillars of the white butterflies. The larvae of the various species of _Pieris_, as well as those of other Lepidoptera, are attacked by this little Insect, the masses of whose cocoons may frequently be found in numbers in and near cabbage gardens. The tropical species of Braconidae are greatly neglected, but many large and remarkable forms—some of brilliant colours—have been brought from there, so that we are justified in believing that Insects of this family will prove to be very numerous. There are but few apterous Braconidae. Both sexes of _Chasmodon apterus_ are destitute of wings; the females of one species of _Spathius_, and also those of _Pambolus_ and _Chasmodon_ are apterous; in a small number of species of various genera the wings are so minute as to be incapable of serving as organs of flight. In the genus _Alloea_ the wings of the male are shorter than those of the female.

FAM. VI. STEPHANIDAE.

_Antennae composed of many (thirty to seventy) joints; hind body attached
to the lower and posterior part of the median dorsal {562}plate. Wings
with a distinct costal cellule; head globose, posterior femora frequently
toothed._

This is a doubtful family, consisting of a few anomalous Insects. Schletterer assigns to it only two genera, _Stephanus_ and _Stenophasmus_;[481] both have a wide distribution over the world, though we have no species in Britain. Nothing is known of their habits, and they are apparently all very scarce Insects. The definition is compiled from those of Cameron and Schletterer. There seems very little to distinguish these Insects from Braconidae.

FAM. VII. MEGALYRIDAE.

_Hymenoptera with short broad hind body, which is not separated by a
pedicel from the thorax. The female has a very long bristle-like
ovipositor. Antennae with fourteen joints._

This family is constituted by the Australian genus _Megalyra_,[482] one of the most interesting of the numerous extraordinary Insect-forms found in that region; the species appear to be very rare and not numerous. Apparently nothing is known as to their habits. It is quite possible that these Insects will prove to be anomalous Braconidae.

FAM. VIII. EVANIIDAE.

_Petiole of the abdomen attached to the upper part of the median dorsal
plate; antennae not elbowed, of thirteen or fourteen joints. Wings with a
moderate number of nervures. Larva of parasitic habits._

This family is composed of only three genera—_Evania_, _Gasteruption_, and _Aulacus_, each possessing a considerable number of species; they agree in the characters mentioned above, and may be readily recognised by the peculiar insertion of the hind body. This character occurs outside the limits of the Evaniidae only in one or two genera of Chalcididae and Braconidae; it is to this latter family that the Evaniidae must be considered most closely allied.

The species of the genus _Evania_ are believed to live at {563}the expense of cockroaches (Blattidae), and to deposit their eggs in the egg-capsules of those Insects. The species of _Gasteruption_ live, in the larval state, on the larvae of other Hymenoptera, more especially of such as form nests in wood. Very little is known as to the habits of the species of _Aulacus_, but it is believed that they are parasitic on members of the Hymenopterous families, Siricidae and Oryssidae. Only the most meagre details as to the life history of any of the Evaniidae have been recorded. The species of _Evania_ are met with most freely where cockroaches abound, and are said, hence, to be frequently observed on board ship. Two or three species of each of the two genera _Evania_ and _Gasteruption_ occur in Britain. The latter genus is more widely known under the name of _Foenus_.[483]

FAM. IX. PELECINIDAE.

_Sexes very different; the female without exserted ovipositor, but with
extremely long abdomen. Articulation between the femur and trochanter
oblique and elongate, but without division of the trochanter._