318 



INTRODUCTION TO ZOOLOGY chap, xxi 



which are protected by a brown chitinous skin. The rest of 

 it can only be seen when it has been induced to leave the 



case. This is easily accomplished 

 by cutting the case carefully open, 

 or by prodding it from behind 

 very gently with the head of a 

 pin or some other blunt object; 

 the caddis-worm will then, though 

 apparently with great reluctance, 

 leave its case, and expose the soft 

 defenceless hind end of its body. 

 Its structure is shown in Fig. 241. 

 There are three distinct thoracic 

 segments behind the head with its 

 two simple eyes, and each of these 

 bears a pair of relatively long 

 jointed legs on which the larva 

 crawls actively about. Behind this 

 region are nine abdominal seg- 

 ments, all of which, except the 

 first and the last, bear on each 

 side a bunch of soft, white iila>- 

 ments, some of which often float 

 out at the side, whilst some are 

 laid across the back, crossing those 

 from the other side (see Fig. 241). 

 These are the breathing organs, or 

 tracheal gills, which contain extensions of the 

 ^of Larva"' ^^^'^^^^^! so that the air in them is only separated 

 from the water by a very delicate membrane, and 

 can be readily oxygenated from the oxygen dissolved in the 

 water; for this process, however, it is necessary that there 

 should be a constant stream of water through the case and 

 over the gills. This is caused by the movements of the 

 abdomen, and is facilitated by the structure of its first and 

 last segments. The last bears a pair of strong hooks, which 

 holds the Caddis firmly to the end of its case (Fig. 241, b); the 

 first has on it three white fleshy processes or tubercles (Fig. 

 241, a), which can be retracted or extended, and which ap- 

 parently serve to keep the body fixed at this point by the 

 pressure of the tubercles against the inside of the case. The 



Fig. 241. — Larva of the Caddis- 

 fly, the " Caddis- worm. " (x2i.] 



a, Fleshy protuberances ; 

 &, abdominal books ; c, tracbeal gills. 



