Microscopical Essays, <^S3 



ones ; fo that there are ninety -four principal nerves, whofe 

 ramifications are innumerable. 



The tracheal arteries of the caterpillar are two large 

 aerial velTels that creep under the (kin, clofe to the fpiracula, one 

 at the right fide of the infect, the other at the left, each of them 

 communicating with the air, by means of nine fpiracula ; they 

 are nearly as long as the body, beginning at the firft fpiraculum, 

 and going a little farther than the laft, terminating in fome 

 branches, which extend to the extremities of the body. 



Round about each fpiraculum the tracheal artery puflies forth a 

 great number of branches, which are again divided into final ler 

 ones, which further fubdivide, and fpread through the whole 

 body of the caterpillar. 



The tracheal artery, and it's numerous ramifkations, are all 

 open elaftic veffels, which may be p relied clofe together, or 

 drawn out considerably, but return immediately to their ufual fize 

 when the tenfion ceafes. They are naturally of a filver colour, 

 and have a very beautiful appearance in the microfcope. 



This vefl'el and it's principal branches are compofed of three 

 coats, which may be feparated one from the other. The exterior 

 covering is a thick membrane, furnifhed with a great number of 

 fibres, which defcribe a vaft variety of cir cles round it, commu- 

 nicating with each other by numerous {hoots. 



The fecond is very thin and tranfparent ; no particular veffel 

 Is diftinguifhed in it, The third is compofed of fcaly threads, 



W w which 



