534 W. L. TOWER, 



and veutral layers are everywhere in contact except for a thin 

 chitinous lamella which lies between them (PI. 15, Fig. 18 cta^ ; 

 PI. 20, Figs. 65 and 66). The peduncle by which this imaginai disc 

 is attatched to the body wall now elongates slightly (L. decemlineata) 

 so that the proximal side of the wing is closely approximated to the 

 dorsal tracheal trunk, and from this the wing now receives several 

 small tracheoles which have developed at various points on the tracheal 

 trunk near the wing. These are not the original tracheal supply of 

 the wing disc, for, as I have already shown, in the earliest stages the 

 fundament has a well marked tracheal branch from the tracheal trunk 

 (PI. 15, Figs. 14, 16 and 17; PI. 17, Figs. 26, 27, 28 and 29; PI. 20, 

 Fig. 64). 



At the stage shown in Fig. 18 (PI. 15) the two layers of the 

 wing are ditierentiated, the future dorsal layer {h^drm.j) being thicker 

 than the ventral one {h'drni.^), and this is true of both elytra and 

 hind wings. Further growth of the wing is in a downward and back- 

 ward direction, so that the wing disc becomes an oval body projecting 

 into the body cavity but attached to the body wall by a short, thick 

 peduncle. Sections taken at the anterior end of the wing show a 

 structure like that seen in Fig. 21 (PI. 16) or Fig. 66 (PI. 20), while 

 sections at the posterior end show the wing entirely cut off from the 

 hypodermis, as in the Lepidoptera (PL 17, Fig. 32; PI. 20, Fig. 67) 

 During this growth the cavity of the wing becomes more and more 

 marked, especially near the posterior portion of the opening into the 

 wing. The anterior part of the wing fundament still has the two 

 layers in close contact (PI. 16, Fig. 21). Tracheoles, embryonic cells, 

 and leucocytes have increased in number during the growth of the 

 wing and now form a tangled mass near the opening of the wing 

 cavity. 



During most of the larval stage the wings grow slowly and remain 

 in much the same condition as that shown in Fig. 21 (PI. 16). When 

 the larva ceases feeding and begins the prepupal stage the wings in- 

 crease rapidly in size, which is largely due to a rearrangement of the 

 contents of the hypodermal cells and the entrance of haemolymph into 

 the wing cavity. The wing soon becomes too large for the wing sac, 

 and as it grows it pushes up and out of the sac which has begun to 

 open and at the same time — the dorsal motion being limited — 

 pushes the hypodermis of the wing pocket down and out of the way 

 (PI. 16, Fig. 22). 



The wings of these beetles all leave the wing sac in the 



