ANATOMY AND PHYSIOLOGY 



6 9 



nucleus. The cells are limited outwardly by the cuticula and inwardly 

 by a delicate, 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. 91). Hairs and bristles are frequently 

 tactile in function, over the general integu- 

 ment or else locally; or olfactory, as on 'the 

 antennae of moths; or occasionally auditory, 

 as on the antennas of the male mosquito; 

 these and other sensory modifications are 

 described beyond. The hairy clothing of 

 some hibernating caterpillars (as Isia isa- 

 bella) probably protects them from sudden 

 changes of temperature. Hairs and spines 

 frequently protect an insect from its enemies, 

 especially when these structures are glandu- 

 lar and emit a malodorous, nauseous or 

 irritant fluid. Glandular hairs on the pul- 

 villi of many flies, beetles, etc., enable these 

 insects to walk on slippery surfaces. The 

 twisted or branched hairs of bees serve to 



. . FIG. 94. Various forms of 



gather and hold pollen grains; in short, scales. A,E.thysanuran,Mac- 



these simple structures exhibit a surprising 

 variety of adaptive modifications, many of 

 which will be described in connection with other subjects. 



A hair arises from a modified hypodermis cell, formative cell or 

 trichogen (Fig. 92), the contents of which extend through a pore canal 

 into the interior of the hair (Fig. 93); sometimes, to be sure, as in 

 glandular or sensory hairs, the hair cell is multinucleate, representing, 

 therefore, as many cells as there are nuclei. The wall of a hair is 

 continuous with the general cuticula and atNnoulting each hair is 

 stripped off with the rest of the cuticula, leaving in its place a new hair, 

 which has been forming inside the; old one. 



Scales. Besides occurring throughout the order Lepidoptera and 

 in numerous Trichoptera, scales are found in many Thysanura and 



