28o NATURAL HISTORY OF AQUATIC INSECTS CH. 



gaping slit b\' which the fly escaped. The eyes, 



legs, jaws, and wing- 

 sheaths, as well as the 

 rings of the body, are 

 so perfect that the 

 outer form of the 

 larva can be described 

 from such a cast skin. 

 Not only these exter- 

 nal organs, but also 

 the lining membrane 

 of the great air-tubes, 

 and that of the fore 

 part of the alimentary 

 canal, including the 

 armature of the giz- 

 zard, can be made out 

 in the cast skin, a 

 proof that these struc- 

 tures are formed by 



Fig. 87. — Larva of Perla bipunctata 

 (liicaudat.i). 



Fig. 88.— Under side of he.id 

 of larva of Perla bipunctata 

 (bicaudata). 



folding of the outer integument into the respiratory 

 openings and the mouth. 



