Caddis-flies 



215 



The larvce of the caddis-flies niosth' hve in portable 



cases, which they drag about with them as the^' crawl 



or climb ; but a few having cases 



of lighter construction, swim 



freely about in them. Such is 



Tricenodes, whose spirally wound 



case inade from bits of slender 



stems is shown in the accompany- 

 ing figure. 



The cases arc 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 btiild- 



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- 

 lapping like the elements of a 

 stick 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 



The larva of a 

 spongilla fly, Sisyra (after 

 Alaude H. Anthony). 



Fig. 123. The case of the free- 

 swimming larvae of Trise 

 nodes. 



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

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



