Dec, 1904.] Powell : Wings of Certain Beetles. 241 



disc (Fig. 6) and the hypoderm, by the expansion of the body 

 after becoming free from the old cuticle. There does not seem to be 

 any sudden and great proliferation of the cells of the disc just before 

 moulting, as was found in the Lepidoptera by Verson (1904) and the 

 growth of the wing appears to be even and constant up to the prepupal 

 period, when it becomes accelerated. 



(/?) Types of Wing Development. — Tower (1903), v/orking on 

 the development of the wings in Coleoptera, found three types of 

 wing development present in that order, which he designated as the 

 simple, the recessed and the enclosed. The simple type, in which the 

 wing merely evaginates and lies between the cuticle and the hypoder- 

 mis, he found to be the prevailing type among the Coleoptera. The 

 recessed type he found only in the Scarabseidae. In this the disc first 

 invaginates, then evaginates and lies in the shallow open pocket thus 

 formed. This is the type found by Gonin in the Dipteron, Corethra. 

 In the third, the enclosed type, after the invagination the opening be- 

 comes closed and the wing evaginates downward into a closed sack, 

 formed by the lower wall of the invagination. This type was found 

 by Tower to occur in the Coccinellidse and the Chrysomelidas. There 

 are two other types, which have been found only in the Diptera, the 

 stalked and the detached. They are similar to the enclosed type 

 except that the walls of the invagination in the stalked type become 

 very thin and the evaginated part, which forms the wing, is pushed 

 well into the body cavity, while in the detached type the Aving bud 

 is entirely separated from the hypodermis and lies free in the body 

 cavity. 



In all the Coleoptera, according to Tower (1903), after the wing 

 disc becomes well thickened, a pit-like invagination forms in the cen- 

 ter of the disc, which rapidly widens into a groove extending nearly 

 the length of the disc. In those insects in which the wings develop 

 within the body, this invagination becomes much extended and the 

 hypodermal layers thus extended form the peripodal sack into which 

 the wing is evaginated. Tower found this primary evagination form- 

 ing, even in those beetles which have a simple type of wing develop- 

 ment. I find, however, that in both T. plastographus and D. valens 

 this primary evagination is not formed. The only other record among 

 Coleoptera in which the wing evaginates without this preliminary in- 

 vagination is that of Needham (1900) in the flag-weevil {Mononychus 

 vulpeculus Fab.). 



