The origin and development of the wings of Coleoptera. 539 



growth is possible between the hypodermis and cuticula without causing 

 any inconvenience to the larva. In the Coccinellidae and Chryso- 

 melidae, where the larval period is short and ecdyses occur in rapid 

 succession, some arrangement must be made whereby the wings which 

 are already well formed in the half grown larva can pass from instar 

 to instar without becoming external structures. This has been ac- 

 complished by the retreat of the wing inward from the surface and 

 its becoming surrounded by a sac or pocket of the hypodermis which 

 protects the organ from injury and allows ecdysis to go on and the wing 

 not become prematurely an external structure. This has resulted in 

 the establishment of a type of wing development almost exactly like 

 that of the Lepidoptera, and probably by similar causes. 



In these beetles the chitin which is secreted by the wing in the 

 wing sac is cast at ecdysis like that of the rest of the surface, and 

 not infrequently after this has been drawn out the opening of the wing 

 sac fails to close completely, so that the cavity of the wing sac opens 

 freely to the outside world. It is quite evident that insects which 

 possess wings in the early larval stages and which have frequent ec- 

 dyses must have some method of preventing these organs from be- 

 coming external and appearing prematurely, perhaps to the great in- 

 jury of the insect. In this may lie the secret of the origin of the 

 highly specialized imaginai discs of Lepidoptera, Diptera and some 

 Coleoptera. I think we may conclude that in the Holoraetabola, 

 where, for any reason, the larval period becomes shortened, the 

 imaginai disc retreats from the surface, more and more, and in the 

 pockets thus formed the imaginai organ being well protected may go 

 on developing uninterrupted by any of the activities of the larva or 

 vicissitudes of the environment. While duration of larval life is un- 

 doubtedly important, the degree of specialization of the larva as a 

 whole is probably also a potent factor in the production of specialized 

 types of imaginai discs. These two, length of larval life and type of 

 larva are closely associated and are almost certainly two manifestations 

 of the eifect of the environment upon larval life and development. 



4. Behavior of the Hypodermis. 



In the early stages of the wing fundament the cells differ in no 

 way from those of the hypodermis immediately about it, but when 

 the disc becomes thickened the cells have a characteristic polygonal 

 shape, such as has been figured by Pancritius (1884) and occurs so 

 frequently in insect tissue. The volume of the cell has not increased. 



