242 Nerve-winged Insects; Scorpion-flies; Caddis-flies 



abdomen is a pair of strong hooks pointing outward. These hooks can 

 b? fastened into the sides of the case and thus hold the larva safely in its 

 house. Numerous thread-like tracheal gills are borne on the abdomen 

 and by a constant undulatory or squirming motion of the body a stream of 

 fresh water is kept circulating through the case, thus enabling the gills to 

 effect a satisfactory respiration. The caddis-worm crawls slowly about 

 searching for food, which consists of bits of vegetable matter. Those larva? 

 which have a fixed case have to leave it in search of food. Some of them 

 make occasional foraging expeditions to considerable distances from home. 

 Others have the interesting habit of spinning near by a tiny net (Fig. 335), 



FIG. 334. Hydropsyche scalaris. (After Needham; enlarged.) 



fastened and stretched in such a way that its broad shallow mouth is directed 

 up-stream, so that the current may bring into it the small aquatic creatures 

 which serve these caddis-fishermen as food. The caddis-flies live several 

 months, and according to Howard some pass the winter in the larval stage. 

 When the caddis-worms are ready to transform they withdraw wholly 

 into the case and close the opening with a loose wall of stones or chips and 

 silk. This wall keeps out enemies, but always admits the water which is 

 necessary for respiration. The pupae in the well-made cases have no other 

 special covering, but in the simple rough pebble houses attached to stones 

 in the stream they are enclosed in thin but tough cocoons of brown silk 

 spun by the larvae. The free cases are also usually attached just before 

 pupation to submerged sticks or stones. When ready to issue the pupa 

 usually comes out from the submerged case, crawls up on some support 

 above water and there moults, the winged imago soon flying away. Some 

 kinds, however, emerge in the water. Comstock observed the pupa of one 

 of the net-building kinds to swim to the surface of the water (in an aqua- 

 rium) by using its long middle legs as oars. The insect was unable to crawl 

 up the vertical side of the aquarium, so the observer lifted it from the water 

 on a stick. At this time its wings were in the form of pads, but the instant 

 the creature was free from the water the wings expanded to their full size 

 and it flew away several feet. On attempting to catch the specimen Com- 



