16 CRUSTACEA AND INSECTS. 



fibre. They are of two distinct sizes, whicli Moseley 

 proposes to call macroestlietes and microestlietes. 



In many animals, as in ourselves, the outer skin is 

 soft and susceptible to external impressions. In Insects 

 and Crustacea on the contrary, the inner skin, or hypo- 

 derm, is covered with a more or less thick layer of 

 horny substance known as chitine ; and, from the 

 nature of their chitinous integument, it naturally 

 follows that the sensations of insects, excepting that 

 of sight, are effected by means of variously modified 

 hairs. We know, however, so little, in the first place, 

 as to the real means by which animals, including 

 man, hear, smell, or taste, and, in the second, as to the 

 intimate structure of their minute organs, that we are 

 often in doubt, and there are still great differences of 

 opinion whether a given sense-hair serves for hearing, 

 smell, or touch. 

 The hairs of Arthropods belong to very different 



n [I n 



Fig. 18.— Diagram of forms of hairs in insects, a. Ordinary surface hair ; 5, plumose 

 natatory hair; c, hair of touch; d, auditory hair; e, olfactory hair;/, taste hair; 

 n, nerve hair. 



categories, some of which we may perhaps distinguish 

 as follows : — 



Those under which the chitinous integument is entire. 



1. Ordinary surface hairs (Fig. 18, a). 



2. Plumose natatory hairs (Fig. 18, h). 



Those under which the chitinous integument is per- 



