FIELD BOOK OF PONDS AND STREAMS 



have cemented together with their own saliva (Fig. 198). 

 When a caddis worm outgrows its case it occasionally squirms 

 out and makes a new one, selecting the same materials and 

 building by the same pattern, but it generally enlarges its 

 case by simply building onto the front edge. It holds itself 

 in the case by draghooks at the hind end of the body but its 

 head and thorax are protruded during its continual clamber- 

 ing. Their gills are filamentous, in many being attached 

 along the sides of the abdomen, completely protected by the 

 case. Three tubercles (Fig. 188, i) keep the body from press- 

 ing against the edge of the case and give the water a free 

 entrance into it. It is kept circulating over the gills by un- 

 dulating movements of the body which the larvae will con- 

 tinue even after they are removed from the cases. 



The pupse (Fig. 188, 2) live in cases and some larvas never 

 have any cases till they are nearly ready to pupate. They 

 are as truly aquatic as the larvae, and they have gills like them. 

 There are no other aquatic insects whose pupje have gills 

 except a few groups of flies, such as Simulium and Chironomus. 

 The larvae keep up a continuous foraging and eating until 

 they are fullgrown, when they spin silken screens across the 

 open ends of their cases, or fasten little pebbles over them, 

 and within the cases they begin their pupation or change to 

 the adult form. Water is let in, other things are shut out. 

 Such pupal cases are often fastened to a stone and all through 

 the early summer dozens of them belonging to little Heli- 

 copsyche pupas (Fig. 199, i) can be found glued fast to the 

 edges of the rocks. All caddis fly pupae are more or less 

 active and many continue their undulating motions to keep 

 the water flowing over their gills. By the end of the pupal 

 period the heavy- jawed, hungry larvae are changed into dainty 

 adults which do not eat at all (Fig. 188, 3). In swift water 

 species the pupas after leaving their cases swim to the surface, 

 shed their pupal skins, and instantly fly into the air; quiet 

 water pupae clamber out on the shore or upon projecting stones. 



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