Caddis-flies 



215 



The larvae of the caddis-flies mostly live in portable 

 cases, which they drag about with them as they crawl 

 or climb; but a few having cases 

 of lighter construction, swim 

 freely about in them. Such is 

 Tri&nodes, whose spirally wound 

 case made from bits of slender 

 stems is shown in the accompany- 

 ing figure. 



The cases are wonderful in 

 their diversity of form , of materials 

 and of construction. They are 

 usually cylindric tubes, open at 

 both ends, but they may be 

 sharply quadrangular or trian- 

 gular in cross section, and the 

 tube may be curved or even coiled 

 into a close spiral*. 



Almost any solid materials that 

 may be available in the water in 

 pieces of suitable size may be used in their case build- 

 ing: sticks, pebbles, sand-grains and shells are the 



staple materials. Sticks may be 

 placed parallel and lengthwise, 

 either irregularly, or in a con- 

 tinuous spiral. They may be 

 placed crosswise with ends over- 



FIG. 122. The larva 



spongilla fly, Sisyra 'after 

 Maude H. Anthony) . 



FIG. 123. The case of the free- lapping like the elements of a 

 larvae of Triae- s ti c k chimney, making thick 

 walls and rather cumbrous cases. 

 However built, the case is always lined with the secre- 

 tion from the silk glands of the larva. This substance 

 is indeed the basis of all case construction. The larva 



*As in Helicopsyche, (see fig. 221, on page 370) whose case of finely textured 

 sand grains was originally described as a new species of snail shell. 



