Fig. 17, Egg of Platygaster (after Ganin). 18, Egg of Platygaster showing the central cell. 19, Egg of Platygaster after the division of the central wall. 20, Egg of Platygaster more advanced. 21, Egg of Platygaster more advanced. 22, Egg of Platygaster showing the rudiment of the embryo.
Ganin met with the larvæ of Platygaster in those of a small gnat, Cecidomyia. Sometimes as many as fifteen parasites occurred in one gnat, but as a rule only one of these attained maturity. The three species of Platygaster differ considerably in form, as shown in Figs. [23-25]. They creep about within the larva of Cecidomyia by means of the strong hooked feet, kf, somewhat aided by movements of the tail. They possess a mouth, stomach, and muscles, but the nervous, vascular, and respiratory systems do not make their appearance until later. After some time the larva (Fig. [23]) changes its skin, assuming the form represented in Fig. [26]. In this moult the last abdominal segment of the first larva is entirely thrown off: not merely the outer skin, as in the case of the other segments, but also the hypodermis and the muscles. This larva, as will be seen by the figure, resembles a barrel or egg in form, and is .870 mm. in length, the external appendages having disappeared, and the segments being indicated only by the arrangement of the muscles. slkf is the œsophagus leading into a wide stomach which occupies nearly the whole body, gsae is the rudiment of the supra-œsophageal ganglia, bsm the ventral nervous cords. The ventral nervous mass has the form of a broad band, with straight sides; it consists of embryonal cells, and remains in this undeveloped condition during the whole larval state.
Fig. 23, Larva of Platygaster (after Ganin)—mo, mouth; a, antenna; kf, hooked feet; z, toothed process; lfg, lateral process; f, branches of the tail. 24, Larva of another species of Platygaster. The letters indicate the same parts as in the preceding figure. 25, Larva of a third species of Platygaster. The letters indicate the same parts as in the preceding figures. 26, Larva of Platygaster in the second stage—mo, mouth; slkf, œsophagus; gsae, supra-œsophageal ganglion; lm, muscles; bsm, nervous system; ga, gh, rudiments of the reproductive glands. 27, Larva of Platygaster in the third stage—mo, mouth; md, mandibles; gsae, supra-œsophageal ganglion; slk, œsophagus; ag, ducts of the salivary glands; bnm, ventral nervous system; sp, salivary glands; msl, stomach; im, imaginal discs; tr, tracheæ; fk, fatty tissue; ed, intestine; ga, rudiments of reproductive organs; ew, wider portion of intestine; ao, posterior opening.
At the next moult the larva enters its third state, which, as far as the external form (Fig. [27]) is concerned, differs from the second only in being somewhat more elongated. The internal organs, however, are much more complex and complete. The tracheæ have made their appearance, and the mouth is provided with a pair of mandibles. From this point the metamorphoses of Platygaster do not appear to differ materially from those of other parasitic Hymenoptera.
An allied genus, Polynema, has also very curious larvæ. The perfect insect is aquatic in its habits, swimming by means of its wings; flying, if we may say so, under water.[15] It lays its eggs inside those of Dragon-flies; and the embryo, as shown in Fig. [28], has the form of a bottle-shaped mass of undifferentiated embryonal cells, covered by a thin cuticle, but without any trace of further organization. Protected by the egg-shell of the Dragon-fly, and bathed in the nourishing fluid of the Dragon-fly’s egg, the young Polynema imbibes nourishment through its whole surface, and increases rapidly in size. The digestive canal gradually makes its appearance; the cellular mass forms a new skin beneath the original cuticle, distinctly divided into segments, and provided with certain appendages. After a while the old cuticle is thrown off, and the larva gradually assumes the form shown in Fig. [29]. The subsequent metamorphoses of Polynema offer no special peculiarities.
