342 HOUSEHOLD INSECTS 



color (Fig. 117). At the posterior end of the body is a 

 disk-like sucker fringed with minute hooks by which the 

 larva fastens itself securely to the rocks. The larvse 

 attached by the posterior ends stand upright in the water 

 unless the current is too swift, in which case their bodies 

 incline downstream. They are so numerous in many 

 places that they form a black moss-like carpet over the 

 rocks for large areas. The head of the larva bears two 

 fan-shaped organs (Fig. 117), each one having about 60 

 rays. These organs are evidently for the purpose of 

 creating currents of water directed toward the mouth and 

 bearing particles of food. When disturbed, the fans are 



drawn backward and 

 folded up like an ordi- 

 nary fan. Just back 

 of the head on the ven- 



Fig. 117. — Larva of the black-flv . i -j £ x i 1 j 



OS. victipes), enlarged. tral Slde ° f the bod y » 



a fleshy proleg termi- 

 nating in a sucker fringed with hooks similar to the 

 posterior sucker already described. By means of the 

 anterior and posterior suckers the larva is able to walk 

 with a looping gait similar to that of a measuring worm. 

 Moreover, the larva possesses the power of spinning 

 silk from its mouth. This is undoubtedly an adaptation 

 to its environment or the situations in which it lives. 

 When a larva leaves its old position for a new one, it 

 spins out a silken thread from its mouth, which is se- 

 curely attached at the free end. To this thread the larva 

 tightly clings as it loops along, lest it be washed away 

 by the swift current before it has reached its destination 

 and becomes fastened to the rock again by the posterior 

 sucker. The larva breathes by means of three much 



