230 



Aquatic Organisms 



Fig. 137. The larva of a horsefly, Chrysops. 



Fig. 138. The eggs 

 of a horsefly on 

 an emergent bur- 

 reed leaf. 



P- 359) whose cylindric tough- 

 skinned larvs have their heads 

 retracted within the prothorax, 

 and bear on the end of the abdo- 

 men a respiratory disc perforate 

 by two big spiracles and sur- 

 rounded by fleshy radiating fila- 

 ments ; minute moth-flies — Psycho- 

 didae, (see fig. 214 on p. 359) 

 whose slender larvse live amid 

 the trash in both brooks and 

 swales. Swaleflies ( vSciomyzidce) 

 whose headless and appendage- 

 less larvae hang suspended by 

 their posterior end from the sur- 

 face in still water; and others 

 less common. 



It is a vast array of forms this 

 order comprises, this mighty group 

 of two-winged flies, that is still so 

 imperfectly known; and some of 

 the most highly diversified of its 

 larvs are among the commoner 

 aquatic ones. 



