280 NATURAL HISTORY OF AQUATIC INSECTS CH. 



gaping slit by 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 

 (bicaudata). 



FIG. 88. Under side of head 

 of larva of Perla bipunctata 

 (bicaudata). 



folding of the outer integument into the respiratory 

 openings and the mouth. 



