The composition of the head in the Hymenoptera.—Ratzeburg stated in 1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, and Formica) does not correspond to that of the larva, but is derived from the head and the first thoracic segment of the larva. Westwood and also Goureau made less complete but similar observations, though Westwood afterwards changed his opinion, and the same view was maintained by Reinhard. Our own observations (as seen in Fig. 38) led us to suppose that this was a mistaken view; that the larval head, being too small to contain that of the semipupa, was simply pushed forward, as in caterpillars. Bugnion, however, reaffirms it in such a detailed way that we reproduce his account. He maintains that the views of Ratzeburg are exact and easy to verify in the chalcid genus Encyrtus, except, however, that which concerns the ventral part and the posterior border of the prothoracic segment.
As the time of transformation approaches, the head of the larva, he says, is depressed and soon concealed under the edge of the prothoracic segment; the latter elongates, becomes thicker and more convex, and within can be seen the two oculo-cephalic imaginal buds. The head of the perfect insect is derived not only from the head of the larva, but also from the portion of the prothoracic segment which is occupied by the buds, i.e. almost its entire dorsolateral face. But the hinder and ventral part of this segment (which contains the imaginal buds of the first pair of legs) takes no part in the formation of the head; these parts, according to Bugnion, towards the end of the larval period detaching themselves so as to become fused with the thorax and constitute the pronotum and the prosternum.
Fig. 39.—Anterior half of larva of Encyrtus, ventral face, showing the upper (wing) and lower (leg) thoracic imaginal buds: b, mouth; ch, chitinous arch; gl, silk gland; g, brain; n, nervous cord; a1, bud of fore, a2, bud of hind, wing; p1–p3, buds of legs; st1–st3, stigmata.
Fig. 40.—Anterior part of Encyrtus larva, 1.2 mm. in length; dorsal face; the cellular masses beginning to form the buds of the wings, eyes, and antennæ: o, eye bud; e, stomach.
Fig. 41.—Older Encyrtus larva, lateral view, showing the buds of the antennæ (f), legs, and wings; oe, œsophagus; q1, q2, q3, buds of the genital armature; x, rudiment of the sexual gland (ovary or testis); u, urinary tube; i, intestine (rectum); a, anus.
Fig. 42.—A still older larva, ready to transform. The imaginal buds of the antennæ, eyes, wings, and legs have become elongated; lettering as in Fig. 41.—This and Figs. 39–41 after Bugnion.