242 Nerve-winged Insects; Scorpion-flies; Caddis-flies 
abdomen is a pair of strong hooks pointing outward. These hooks can 
be 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 larvae 
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 flew away several feet. On attempting to catch the specimen Com 
