CADDIS-WORMS AND CADDIS-FLIES 167 



have numerous slender and transparent gill-filaments, arranged 

 in regular rows on the back and sides of the abdomen. The last 

 segment bears a pair of strong hooked feet, which are useful 

 for holding on to the case. The first abdominal segment 

 bears three rather long prominences, one dorsal and two 

 lateral, which can be protruded or retracted at pleasure. 

 These prominences are long enough to reach the inside of 

 the case ; the lateral ones are roughened at the tip by spines 

 or hooks, while the median one, which is particularly mobile, 

 seems to regulate the position of the body within the case. 

 Some such adjustment is the more necessary because the 

 caddis-worm, like the Chironomus-larva, maintains a steady 

 flow of water through its tube. This is set up by an un- 

 dulating, up-and-down movement of the abdomen ; it is no 

 doubt important as a means of supplying the gill-filaments 

 with a perpetually renewed supply of aerated water. Caddis- 

 worms feed largely on vegetable matter, but they do not refuse 

 animal food. The larger ones, in particular, often devour 

 blood-worms and other ill-defended animals of the waters. 

 They will also attack other caddis-worms, breaking open the 

 tubes in the middle where the inmate has neither jaws nor 

 hooked feet to resist the attack. 



When the larva is full-fed, it spins a cocoon within its case, 

 and enters upon its resting-stage. Before pupating, it is careful 

 to make up the ends of its tube with strainers or gratings, which 

 exclude dangerous or irritating objects, but allow water to enter 

 freely. Then the larval skin is cast, and the body assumes 

 nearly the form of the fly, though it is temporarily shrouded 

 within a thin pupa-skin. The legs and wings are not glued 

 down as in Lepidopterous pupae, but still retain some degree 

 of mobility. Gill-filaments are usually found on the abdomen 

 of the pupa, as of the larva. At the head-end of the pupa 

 there may be seen a large pair of compound eyes, and also two 

 long, curved, crossed mandibles, which look like weapons of 

 attack or defence. They are neither one nor the other, but 

 are used solely to break open the case, when the fly is ready to 

 become free. At this time the pupa opens its case, creeps, 

 swims, or floats to the surface, gains some place of safety out of 

 the water, and then divests itself of the pupa-skin. Some 

 small caddis-flies emerge from a floating pupa-case after the 

 fashion of a gnat or harlequin-fly. 



