554 W. L. TOWER, 



and leucocytes are fairly abundant. During the pupal stage the 

 glands, scales, hairs and other ornamental structures arise. The 

 development of these I shall take up in the next section. 



When the time for the final transformation of the insect arrives 

 the body and appendages become surrounded by a thin layer of 

 exuvial fluid which acts as a lubricator and allows the animal to slip 

 out of the pupal case without injury to the rather delicate body sur- 

 face. After emergence the wings expand and reach their adult size 

 and shape in a short time. The beetle now has the form and size 

 of the adult but lacks the coloration and firm texture of the body 

 wall, and a considerable period elapses after emergence in which the 

 beetle undergoes considerable development. In this period the color- 

 ation, glands, scales, hairs, etc. become completed and functional, 

 the body wall is thickened, the sexual products are matured and 

 the insect brought into condition to exist in the outside world and 

 preform its adult functions. In the following section I shall trace 

 the changes which occur in this period, many of which begin in the 

 pupal stage, but are completed after emergence. 



11. The Mature Wings. 



In the wings the gross changes which occur after the final trans- 

 formation are few, and consist in the final expansion, a decrease in 

 the thickness, and a stifiening of the wings. These changes are, 

 however, caused by the growth and modification of the hypodermis and 

 the hypodermal structures, so that an account of the changes of these 

 parts will also include the grosser changes. 



a) The Elytra. The Chitinous Columns. These struc- 

 tures have been frequently described in the adult elytra, but not 

 accurately, and their mode of growth has not, as far as I know, been 

 studied, although Comstock & Needham (1899) have figured one rather 

 late stage in their development in H. ISpunctata. 



Early in the pupal stage there are found in the elytra of all of 

 the beetles examined paired groups of hypodermal cells, — one of the 

 pair being in the dorsal and the other in the ventral layer of hypo- 

 dermis. The groups of cells of the upper and lower layers of hypo- 

 dermis are so closely united that they appear as a single structure 

 reaching from lamella to lamella of the wing. The cells in the dorsal 

 layer have, however, become differentiated from those of the rest of 

 the hypodermis (PI. 18, Fig. 49), being larger and having a large nucleus 

 with a well developed chromatin reticulum and a dense granular 



