Fig. 357.—Leucospis gigas. A, Egg; B, primary, C, secondary larva. (After Fabre.)
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 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 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."
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 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.