ANATOMY AND PHYSIOLOGY 



59 



canals, and being the seat of the cuticular colors; (2) an inner layer, 

 " thickly pierced with pore canals, and always in layers of different re- 

 fractive indices and different stainability." (Tower.) These two layers, 

 respectively primary and secondary cuticula, are radically different in 

 chemical and physical properties. The chitinous cuticula is secreted, as 

 a fluid, from the hypodermis cells. Each layer arises as a fluid secretion 

 from the hypodermis cells, the primary cuticula being the first to form and 

 harden. 



The fluid that separates the old from the new cuticula at ecdysis is 

 poured over the hypodermis by certain large special cells, which, accord- 

 ing to Tower, "are not true glands, but the setigerous cells which, in early 

 life, are chiefly concerned with the formation of the hairs upon the body; 

 but upon the loss of these, the cell takes on the function of secreting the 

 exuvial fluid, which is most copious at 

 pupation. These cells degenerate in the 

 pupa, and take no part in the formation 

 of the imaginal ornamentation." 



Histology. The chitinous cuticula 

 owes its existence to the activity of the 

 underlying layer of hypodermis cells (Fig. 

 88). These cells, distinct in embryonic 

 and often in early larval life, subse- 

 quently become confluent by the disap- 

 pearance of the intervening cell walls, 

 though each cell is still indicated by its 

 nucleus. The cells are limited outwardly 

 by the cuticula and inwardly by a deli- 

 cate, hyaline basement membrane; they contain pigment granules, fat- 

 drops, etc. 



Externally the cuticula may be smooth, wrinkled, striate, granulate, 

 tuberculate, or sculptured in numberless other ways; it may be shaped 

 into all manner of structures, some of which are clearly adaptive, while 

 others are unintelligible. 



Hairs, Setae and Spines. These occur universally, serving a great 

 variety of purposes; they are not always simple in form, but are often 

 toothed, branched or otherwise modified (Fig. 89). Hairs and bristles 

 are frequently tactile in function, over the general integument or else 

 locally; or olfactory, as on the antennae of moths; or occasionally audi- 

 tory, as on the antennae of the male mosquito; these and other sensory 

 modifications are described beyond. The hairy clothing of some hiber- 



- k 



FIG. 88. Section through integu- 

 ment of a beetle, Chrysobothris. b, 

 basement membrane; c 1 , primary 

 cuticula; c 2 , secondary cuticula; h, 

 hypodermis cell; n, nucleus. After 

 TOWER. 



