Microscopical Essays. 185 



or fkin, which in thefe anfwer the purpofe of bones. The ffein 

 ferves as a continent to the whole infect, covering the body, con- 

 necting the parts, and maintaining them in their proper places. 

 This extern") covering is very flrong in thofe infects, which, by 

 the nature of their life, are expofed to ftrong friction, or violent 

 compreffion ; but is more tender and delicate in thofe which are 

 not fo expofed. The [kin of infects, like that of larger animals, 

 is porous ; the pores in fome fpecies are very large. Many 

 infects often change or caft off their (kin ; this exuvia forms an 

 excellent object for the microfcope. 



Another diftinguifiiing criterion of infects is the colour of their 

 blood, which is never red; this, at fir ft fight, feems liable to 

 fome objections, on account of the drop of red liquor which is 

 often procured from fmall infects when fqueezed or prefled to 

 pieces. It does not appear, however, that this is the blood of 

 the little animal ; when it exifted as a worm there was no fuch 

 appearance, and when they exiit as a fly it is only found in the 

 eye, and not in the body, which would be the cafe if it circulated 

 in the veins of the infects. It is probable there is a circulation of 

 fome fluid analagous to the blood in molt infects: with the 

 afliffance of the microfcope this circulation may be perceived in 

 many ; but the circulating liquor is not red* 



To thefe difcriminating characterises we may alfo add the fol- 

 lowing particulars : 1. That the body of infects is divided by 

 incifurse, or tranfverfal divifions, from whence they take their 

 name. 



Z 



2, That 



