550 W. L. TOWER, 



wandering stage the wings become external and begin to grow 

 rapidly. This rapid growth in the wings is due, as has already 

 been pointed out, to the rearrangement of the contents of the 

 hypodermal cells and the entrance of haeraolymph into the wing (see 

 §§'s 4 and 5). 



The expansion of the wing, as viewed in "in toto" preparations, 

 is at first downward (PI. 14, Figs. 6 and 10), which growth continues 

 until the whole side of the segment is nearly hidden by the wing 

 (PI. 19, Fig. 55). Eventually the wing's downward extension is brought 

 to an end by the leg (PL 19, Figs. 54 and 56), which prevents further 

 growth in that direction. This produces, on further growth, much 

 folding and crumpling of the wing. Soon, however, the motion of 

 the insect within the now loosened, chitinous covering of the larva 

 and the position of the legs cause the wings to suddenly slide 

 posteriorly and assume their normal pupal position. In this latter 

 position the folds early become obliterated and the wings possess 

 nearly the adult form. 



If the leg be cut off during the prepupal stage the wing 

 grows downward and may even cross over to the opposite side and 

 never assume its normal pupal position. This result cannot be ob- 

 tained, however, unless both femur and trochanter are removed, so it 

 is quite as evident from this as it is from Figs. 53, 54 and 56 (PI. 19) 

 that the position of the trochanter and femur are the guides which 

 direct the wings at pupation to their ventro-posterior position. The 

 chitinous integument which surrounds the developing prepupa is 

 of great use to the larva. If the cuticula be removed during 

 the prepupal period the wings soon become sac-like and fail to de- 

 velop properly, showing that the presence of the larval cuticula is 

 necessary for the proper development of the wings. The wings at 

 this time are filled with haemolymph under considerable pressure, but 

 are held in position between the parallel surfaces of the body and 

 the cuticula, these causes them to expand in one plane. When, 

 however, the outer retaining layer is removed extension in three 

 directions is possible and occurs at once. The middle membrane at 

 pupation is perfectly passive, that is, the attachment of the hypodermal 

 cells is not strong enough to hold the two lamellae of the wings to- 

 gether and this is accomplished by the wing's being kept under pres- 

 sure between the cuticula and the body surface. During the pupal 

 stage, however, these basal ends of the hypodermal cells become 



