556 W. L. TOWER, 



the hypoderraal cells. After growth has ceased the protoplasmic pro- 

 cesses are withdrawn and the canals are empty or filled with air. 



2) Chitinous lamellae. The chitinous lamellae of the elytron 

 vary greatly in thickness in different species, in different individuals 

 of the same species, and at different periods in the life of the same in- 

 dividual. The dorsal and ventral layers are also of different thick- 

 nesses. In the pupa the hypodermis secretes a thin homogeneous 

 layer of chitin (PI. 17, Fig. 33 da^\ PI. 19, Fig. 58), the primary 

 cuticula, which remains soft and pliable until the wing has become 

 fully expanded, when it hardens. This primary cuticula is of nearly 

 uniform thickness over the entire wings and body. After the expansion 

 of the wings and the development of the chitiuous columns the hypodermis 

 begins to deposit the secondary cuticula which forms the backing and 

 strength of the wing. This characteristic of this layer I have given 

 in a preceding paragraph. 



The deposition of this secondary cuticula continues for some time 

 after the beetle begins to fly about and in some species (Carahidae) 

 may be carried to the extent of almost obliterating the cavity of the 

 elytron. The deposition of this layer goes on just as long as the 

 hypodermis receives proper nourishment, but when the sexual products 

 begin to develop, the food supply is largely diverted from such non- 

 essential structures as the wing hypodermis, and its cells " soon 

 become starved and degenerate rapidly. I have not, however, found 

 any hypodermal cells becoming included in the cuticula, as is recorded 

 by CoMSTOCK & Needham (1899). 



3) Fate of the hypodermis. The hypodermis remains active 

 just as long as it is properly nourished, but when the food supply ceases 

 the hypodermal cells become dense, granular, and stain deeply without 

 much nuclear differentiation and form a dense but thin matrix on the 

 lower surface of the chitinous lamellae. I did not find, however, any 

 case where the hypodermis entirely disintegrated. 



4) The tracheal system. The tracheation and the tracheae 

 of the adult elytra differ in no way from the condition of the 

 prepupa or pupa and show no modifications of interest. They never 

 disintegrate or become so modified as not to be able to conduct air 

 into the wing. In many species of beetles the taenidia of the larger 

 tracheae develop numerous fine chitinous hairs, (PI. 17, Fig. 33) the 

 function of which I do not know. 



5) Veins. In the elytra of Coleoptera the veins are best seen 

 in the pupal stage or in the early imago, where they exist as relatively 



