246 NATURAL HISTORY OF AQUATIC INSECTS CH. 



careful to select such as have a suitable specific 

 gravity. 



" The larvae cannot swim, or can only swim badly, 

 and they usually creep about. In the act of creeping, 

 the head and fore-part of the body are thrust out of 

 the sheath, and the larva drags itself along by its six 

 legs. This operation is often greatly facilitated by 

 the buoyancy of the vegetable fragments attached to 

 the sheath. [Some young larvae can swim very 

 fairly. In spring and early summer a small Caddis- 

 worm, which is common in ponds, may be seen 

 to swim rather rapidly through the water with the 

 help of its long, fringed hind-legs. The movement 

 is a little jerky, but the larva, carrying its case, can 

 move through the water in any direction in a straight 

 line.] 



" Caddis-worms have six hard and jointed legs. 

 They have no false abdominal feet, like those of 

 caterpillars. The head is protected by armour of a 

 brown colour. The first ring of the thorax is of the 

 same texture and colour, and bears a short pair of 

 legs. The second ring is also brown and hard, and 

 carries the second pair of legs ; the third ring is 

 yellowish with brown spots, and carries the third 

 pair. The rest of the body is made up of nine rings, 

 which are white, soft and transparent. On the first 

 of these nine abdominal rings are three fleshy 

 prominences more or less conspicuous in the different 

 species. One of these projects from the upper surface 

 of the ring, while the other two are lateral ; all can be 

 retracted at pleasure. They are hollow at the tip, 

 and when the Insect is taken out of the water, they 



