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 

 Tricenodes, 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- 

 The case of the free- lapping like the elements of a 

 larvae of Trias- 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 



FIG. 122. The larva of a 

 spongilla fly, Sisyra (after 

 Maude H. Anthony). 



FIG. 123. 



swimming 

 nodes. 



*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. 



