240 Journal New York Entomological Society. lvoI.xh. 



but two moults. In warm weather six to eight days elapse between 

 each moult, the whole life cycle being passed in five to six weeks. 

 The pupal period is passed in a cell of chips, formed in the gallery, 

 and requires from a week to ten days. 



The general appearance of the larva (Fig. 33) is much the same 

 as the larva of T. plastographus , but it is larger and each spiracle is 

 surrounded by a chitinized cresent, below which, on the abdominal 

 segments, is a large oval chitinized disc or tubercle. There is also a 

 strongly chitinized caudal plate on which are a number of short stout 

 spines. 



II . Development of the Wing. 



I. Formation and Growth. 



(a) Early Stages. — The wing fundament, in all the Holometabola 

 and probably in all the Heterometabola as well, becomes first recog- 

 nizable as a slight thickening of the hypodermis on the pleurum of 

 the meso- and metathorax (Figs, i, 2, 18). In the larvae of T. 

 plastographus and D. valens the cells that are destined to form the 

 wing begin to be differentiated from the rest of the hypodermis some- 

 time during the middle or latter part of the penultimate stage, the 

 time of first appearance varying somewhat in different individuals. 

 This thickening increases in size towards the end of the penultimate 

 stage and extends the greater part of the length of the segment as an 

 oval disc, becoming thinner on the edges and gradually merging into 

 the hypodermis. The cells become quite crowded together and elon- 

 gate and the nuclei come to lie at several levek in the disc. 



Just before the last moult the outer surface of the wing disc be- 

 comes thrown up into prominent folds and ridges (Figs. 3, 23) due 

 to the compression of the growing hypodermis by the old chitinous 

 covering. This folding and ridging of the hypodermis takes place to 

 a greater or less extent all over the body, but it is most pronounced 

 in the rapidly growing cells of the wing disc. The cells of the disc 

 are now quite long and narrow and there are traces of what later 

 becomes very pronounced — a narrowing and elongation of the bases 

 of the cells so that they become almost thread-like (Fig. 3). The 

 degree of this separation, however varies in different individuals. 

 The cytoplasm of the cells also seems to become thinner or lighter 

 colored near the center of the inner surface of the disc. After 

 moulting, the folds and ridges disappear from the surface of the wing 



