On opening a caterpillar entering on the semipupa state (Fig. 605), the relative position of the germs (imaginal buds or folds) of the wings and of the legs are seen.
Fig. 606.—Section through thorax of a tineid larva on sycamore, passing through the 1st pair of wings (w): ht, heart; i, œsophagus; s, salivary gland; ut, urinary tube; nc, nervous cord; m, recti muscles; a part of the fat-body overlies the heart. A, right wing-germ enlarged.
Fig. 607.—Section of the same specimen as in Fig. 606, but cut through the 2d pair of wings (w): i, mid-intestine; h, heart; fb, fat-body; l, leg; n, nervous cord.
These imaginal buds in a more advanced stage are seen in our sections of a tineid larva (Figs. 606, 607).
The number of 12 imaginal buds found by Weismann in the thorax of Muscidæ does not occur in Lepidoptera, since, as in the Hymenoptera (Bugnion), the dorsal buds of the prothoracic segment are wanting. Gonin finds in Pieris that the ventral buds of the three thoracic segments are each represented by several distinct folds attached to the femoro-tibial bud and to the tarsal joints.
The imaginal buds serve in some cases for the formation of new organs (wings, legs of insects with apodous larvæ); in others for the growth and the transformation of organs already existing (legs, antennæ, 1st and 2d maxillæ of Lepidoptera).
As to the peripodal sac or hypodermic envelope which contains the imaginal bud, a portion persists and is regenerated, while the other part becomes useless and is detached under the form of débris, as shown by Weismann, Viallanes, and Van Rees in the Muscidæ. On this point Gonin disagrees with Dewitz, who stated that the walls of the wing-sacs are not destroyed, but are only gradually withdrawn at the time of pupation, in order to allow the orifice to distend and let the wing pass out to the exterior.
The portion of the sac which persists (basal portion, peripheral pad of Bugnion, or annular zone of Künckel) serves at first to attach the appendage, while forming, to the hypodermis of the larva, then afterwards to more or less completely regenerate the adjoining portion of the integument. In this way the hypodermis of the thorax is partially, that of the head is almost entirely, replaced by the imaginal epithelium which proliferates at the base of the appendages,[[113]] while that of the abdominal segments persists, at least in a modified way, and only undergoes (at the end of the pupal period) transformations as regards the appearance of the scales and pigment.