250 NATURAL HISTORY OF AQUATIC INSECTS CH. 



the different species, but are usually wanting or im- 

 perfect in the thorax and the first and last abdominal 

 segments ; they are in several rows and either simple 

 or branched. Fine air-tubes lead from them to the 

 main air-trunks of the body.] 



" At the hinder end of the body is a pair of horny 

 hooks, which take firm hold of the under side of the 



I 



Fig. 8i. — Phryg.inea varia. A, Larva ; B, its case ; C, a tracheal gill. 



sheath. Indeed, if we attempt to remove the larva 

 by force, it can almost be pulled in pieces before 

 it will let go ; to remove it without violence from 

 its sheath, it is best to split open the sheath. 

 [Anglers know of a readier method than this; 

 A pin, gently inserted into the narrow end of the 



