322 INTRODUCTION TO ZOOLOGY chap. 



ing at the back is often very small, and all are made of 

 material picked up by the larva and gradually built into 

 a case. The larvae live usually in the shallow, clear water 

 round the margin of a pond or stream, and by visiting 

 different pieces of water in which the nature of the pond- 

 floor varies greatly, a large variety of cases may be quickly 

 obtained. The case often so closely resembles the ground 

 it lies on, that it may be some time before the eye detects it, 

 for even when moving it often appears at first merely as if a 

 little piece of the debris at the bottom were shifting. It is 

 very interesting to watch the manufacture of a case, or the 

 adding of a new length to it when it has become too small 

 for its growing inhabitant. If it is a new case that is to be 

 made, two pieces of the material chosen are seized by the 

 front legs, which are turned forward on either side of the 

 head ; the pieces are bound together by silk threads given 

 out from the salivary glands below the mouth and a third 

 is bound to them in the same way. The process is repeated 

 until a complete ring is formed, which is then pushed over 

 the larva's head, and a fresh row of fragments added in front, 

 until the case is of sufficient length. Many of the cases are 

 very rough and irregular on the outside, but all are lined 

 with a smooth silken layer within. 



The cases of the genus Phryganea are perhaps the most 

 beautiful, for they are made of small, more or less rectangular 

 pieces of green leaf, which have been cut by the lavva, and 

 so arranged that they form a spiral band round the case 

 (Fig. 242, P). 



The little Triaenodes makes a similar case, but much smaller 

 and more tapering (Fig. 242, T). It is a form which can 

 swim freely in the water, for it is small and light, and its 

 legs are unusually long, the hind legs being fringed with 

 hairs. 



The genus Limnophilus varies very much in its case-making 

 taste. The species L. rhmvhicus constructs the very common 

 spiky case made of tiny sticks or green stems placed trans- 

 versely (Fig. 242, L.r.). L. flavicornis specially favours the 

 shells of small bivalves and snails, and is even regardless of 

 whether or no the shells are still inhabited by their rightful 

 owners, who may be carried off and remain for the rest of 

 their lives fixed to the case of the caddis. Other species of 



