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 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.
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.
Fig. 355.—Eurytoma abrotani, male. Britain. Hyper-parasite through Microgaster of Liparis dispar, and according to Cameron, parasite of Rhodites rosae and other gall-flies in Britain, × 10. (After Ratzeburg.)
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 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.
Fig. 356.—Leucospis gigas, female. Gibraltar.
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 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.