The 2d maxillæ (labium) is formed of two separate parts. The imaginal buds of the lower lip appear on each side of the median line, with a fissure indicating the differentiation of the palpus. On each side are to be seen the 1st maxillary buds, bearing each a rudimentary palpus, and, farther in front, the buds of the mandibles.
The buds of the ovipositor.—The six stylets of the ovipositor arise from six small imaginal buds which become visible in the second half of the larval period, on each side of the median line, on the lower face of the three last segments (Fig. 620, q1, q2, q3). The bud is differentiated into a central discoidal bud, a furrow, and a marginal, rather thick swelling or pad. Afterwards, these buds elongate and form small papilliform projections directed backwards (Fig. 621); but only during the pupal period do they, as already observed in Bombus, approach each other and assume their definite shape as an ovipositor.
Fig. 620.—End of larva of Encyrtus of 2d stage, showing the three pairs of imaginal buds of the ovipositor q1, q2, q3.
Fig. 621.—The same in an older larva ready to transform: i, intestine; x, genital gland; a, anus.—After Bugnion.
Finally, Bugnion states that while metamorphosis in the Hymenoptera is less highly modified than in the Muscidæ, it is more marked than in the Coleoptera and Lepidoptera. In these orders the pupa moves the abdomen, but in Hymenoptera it is absolutely immovable throughout pupal life, as long as the integument is soft.
DEVELOPMENT OF THE IMAGO IN THE DIPTERA
The flies, particularly the Muscidæ and their allies (Brachycera), are the most highly modified of insects, their larvæ having undergone the greatest amount of reduction and loss of limbs, this atrophy involving even most of the head. The following account has been prepared in part from the works of Weismann, Ganin, Miall, and Pratt, but mostly from the excellent general summarized account given by Korschelt and Heider.
In the holometabolic orders of insects, with their resting pupal stage, during which no food is taken, the entire activity of life seems to be turned to deep-seated and complicated internal developmental processes. These inner changes involve an almost complete destruction of many organs of the larva, and their renewal from certain germs (the imaginal buds) already present in the larva, as will be seen in the highly modified Muscidæ. Only a few larval organs become directly transferred into the body of the pupa and imago. Such are the rudiments of the genital system. The heart also, and the central portion of the nervous system, suffer only slight and unimportant, almost trivial, internal changes. On the other hand, most of the other organs of the larva become completely destroyed: the hypodermis, most of the muscles, the entire digestive canal with the salivary glands; while their cells, under the influence of the blood corpuscles (leucocytes), which here act as phagocytes, fall into pieces, which are taken up by them and become digested. Simultaneously with this destructive, histolytic process, the new formation of the organs by the imaginal buds, already indicated in the embryo, is accomplished in such a way that the continuity of the organs in most cases remains unimpaired. This process of transformation can only be understood by considering that of the embryonal germs of the organs, (1) only a part is destined for the use of the larva in growth, and for the performance of certain functions which exhaust themselves during larval life, so that it is no more capable of farther transformation, and finally becomes destroyed; while (2) a second part of the embryonal germs or rudiments persists first in an undeveloped condition, as imaginal buds, in order to undertake during the pupa stage the regeneration of the organs.