Mar. I90C.] MeRCER : DEVELOPMENT OF WiNGS IN LePIDOPTERA. 13 



feet. From the very nature of this growth and the extent of it the 

 peripodal membrane on the internal ventral side is swung dorsad in 

 the direction of the inside arrow. Finally the ventral fold has 

 straightened out as the dorsal fold did and the hypodermis has 

 assumed the position shown in Figure 29, j'. The wing is now on 

 the outside of the body. The fused portion of the hypodermal wall 

 is broken down and disappears with the development of the adult 

 hypodermis. The ventral hypodermis approaches the dorsal hypo- 

 dermis as the larva increases in age until there is only a narrow open- 

 ing into the lumen of the wing (Fig. 29, ho) which corresponds more 

 nearly to the lumen of the adult wing. 



Gonin's objection to the idea of Landois, who thought the folds 

 entirely straightened out thereby placing the wing on the outside of 

 the body-wall, is not sustained. Gonin observes correctly that the 

 thorax is not larger after the transition than it was before. Accord- 

 ing to my observations given above there would be no necessity for 

 the enlargement referred to, for what slack there is is needed to form 

 the ventral wall (Fig. 29, _r) after the transformation has taken place. 

 It will be seen also from the above description that the part of the 

 fold that disappears is a small portion of the peripodal membrane and 

 a piece of the hypodermal wall. 



During the time of the last larval stage and especially during the 

 latter part of it (prepupal period) the wings have been growing veiy 

 rapidly. Now after the Avings are on the exterior of the body this 

 growth is even more rapid. The wings have reached the region of the 

 feet and yet the growth is increasing. In order to allow this growth 

 and still have the wings confined in the limits of the segments of their 

 origin they become folded and wrinkled, so that the surface appears 

 convoluted (Fig. 28). During this stage the wings become free 

 and conspicuous in the section. A thin layer of cuticle is laid down 

 upon the surface and follows the contour of the wings (Fig. 28, pc). 

 This cuticle is thicker on the portions of the wing that will come to 

 the air upon pupation, at the same time it must be very different in 

 texture from the outside cuticle of the body for upon pupation it nmst 

 needs straighten out as the wing expands. 



On studying the wing-buds as a whole the trachea can be seen, 

 not because they are filled with air but on account of their chitinous 

 intima (Fig. 22, T). Tracheoles (//) are shown in the wing with 

 their point of attachment in the axis of the wing. At the time the 



