902 FRESH-WATER BIOLOGY 



lakes into deep water. The gills of the caddisfly larvae are always 

 of the filamentous type, never lamelliform. They are wanting in 

 members of several families, and are variously disposed about the 

 body, singly or in clusters, in many others; their number, form, 

 and arrangement furnish group recognition characters. The more 

 typical caddisworms, having their gill filaments along the sides of 

 the abdomen completely inclosed within the case, keep water flow- 

 ing through by means of continual undulating motion of the abdo- 

 men; three tubercles at the base of the abdomen and a pair of 

 stout prolegs at its apex serve to keep the walls properly spaced 

 for the admission and the flow of the water. The case is always 

 large enough so the larva can entirely withdraw itself inside. 

 By this means it doubtless escapes from many enemies. But 

 some of the larger fishes, as, for example, brook trout, eat case 

 and all. 



The pupa of caddisflies is peculiar in that it also is aquatic. 

 It is formed within the larval case or tube, the larva closing the 

 apertures with a perforate web of silk before its final moulting; 

 this web admits water for respiration, but keeps out enemies. 

 True tracheal gills, of the same type as those possessed by the larvae, 

 are present on the pupae of many caddisflies. All the pupae are 

 more or less active; some maintain constant undulating move- 

 ments of the abdomen to keep the water circulating, and at the 

 close of the pupal stage all work their way out of the larval case, 

 and swim to the surface of the water to undergo their final trans- 

 formation. In the case of species that inhabit swiit waters and 

 transform in the current, this takes place very quickly, the adult 



emerging instantly on reaching the sur- 

 face and flying away at once. Although 

 the adults have jaws of the most rudi- 

 mentary sort, the mandibles of the pupa 

 are often large and conspicuous ; they are 

 supposed to be of use in cutting a way 



Fig. 1373. An egg ring of PAryganea. OUt of the larVal CaSC. 



The eggs of caddisflies are laid in 

 various ways and places. Some are dropped in the surface of 

 still pools while in flight, The females of some of the Hydro- 



