IMAGINAL BUDS OF WINGS, LEGS, ETC. 653 



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 larvae) ; in others for 

 the growth and the transformation of organs already existing (legs, 

 antennae, 1st and 2d maxillse 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 debris, as shown by Weismann, Viallanes, 

 and Van Rees in the Muscidse. On this point Gonin disagrees with Dewitz, 

 who stated that the walls of the wing-sacs are not destroyed, but are only gradu- 

 ally withdrawn at the time of pupation, in order to allow the orifice to distend 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 Kiinckel) serves at first to attach the appendage, while form- 

 ing, 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, 1 while that 

 of the abdominal segments persists, at least in a modified way, and only under- 

 goes (at the end of the pupal period) transformations as regards the appearance 

 of the scales and pigment. 



The wings. The imaginal buds of the wings do not participate 

 in the larval moults. Gonin has observed, contrary to Dewitz, that 

 their surface only forms a cuticle towards the end of the last larval 

 stage. 



The network of fine tracheae of the wing-bud is drawn out at 

 the time of pupation with the internal cuticle of the large tracheae. 

 The permanent tracheae of the wing have already appeared at the 

 time of the 3d moult under the form of large rectilinear trunks, the 

 position of which corresponds afterwards to that of the veins, but 

 they are not filled with air until the time of pupation. There are 

 from eight to ten of these tracheae in each wing (Fig. 159), and they 

 give rise in the pupa to a new system of fine tracheae (tracheoles) 

 which replaces that of the larva. (For further details the reader is 

 referred to pp. 126-137.) 



Development of the feet and the cephalic appendages. 2 - -In the apodous 

 larvee of Diptera and Hymenoptera the rudiments of the legs are, 

 like those of the wings, developed within hypodermal sacs; at 

 times they remain there up to the end of larval life, but sometimes 



1 In the regions where the imaginal buds are not present (dorsal aspect of the pro- 

 thorax, and abdomen), the epithelium (hypodermis) may proliferate independently of 

 these buds. 



2 We shall translate portions and, when the text allows, make an abstract of parts 

 of Gonin's clear and excellent account, often using his own words. 



