<>MY AND I'll 



6g 



nucleic. The cells arc limited out wardly hy the . uii< nl.i and in.vardly 

 hyaline h.iscmnit ninnhr.uir; \ hey < out ain pi-.'mrnt i/r : ; 



t'at drops, etc. 



;ernally the cuticula may le Smooth, wriukl. 



tuluTi ulate. or sculptured in luimherle-s other way^; it may he >haped 

 into all manner of structures, some of which are dearly adapt i\ ,, while 

 others are unintelligible. 



Hairs, Setae and Spines. These occur universally, >erving a great 

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

 lied, hranched or otherwise modified 

 QI). Hairs and bristles are frequently 

 tactile in function, over the general integu- 

 ment or else locally; or olfactory, as on the 

 antenna' of moths; or occasionally auditory, 

 as on the antennae 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 

 ; s to walk on slippery surfaces. The 

 twisted or branched hairs of bees serve to 



. FIG. 94. Various forms 



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



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 at moulting 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 ami 



of 



