200 



Aquatic Organisms 



Then ensues a quiescent period of a fortnight or more 

 during which great changes of form, both external and 

 internal, take place. The stuffs that the larva accumu- 

 lated and built into its body during its days of foraging, 

 and that now lie inert in the soft white body of the pupa 

 are being rapidly made over into the form in which 

 they will shortly appear in the body of the dainty aerial 

 caddis-fly. However, the pupa is not wholly inactive. 

 By gentle undulations of its body it keeps the water 

 flowing about its gills; and when, at the approach of 

 final transformation, its new muscles 

 have grown strong enough, it is siezed 

 with a sudden fit of activity. It 

 breaks through the barred door of 

 the case, pushes out, swims away, 

 and then walks on the surface of the 

 water, seeking some emergent plant 

 stem, up which to climb to a suitable 

 place for its final transformation. 

 There the caddis-fly emerges, at first 

 limp and pale, but soon becoming 

 daintily tinted with yellow and brown, 

 full-fledged and capable of meeting the 

 exigencies of life in a new and wholly 

 different environment. 



It is a marvelous change of form 

 and habits that insects undergo in 

 metamorphosis especially in com- 

 plete metamorphosis. Such trans- 

 formations as occur in other groups are hardly com- 

 parable with it. The change from a tadpole to a frog, 

 or from a nauplius to an adult copepod, is slight by 

 comparison; for there is no cessation of activity, and no 

 considerable part of the body is even temporarily put 

 out of use. But in all the higher insects an extra- 

 ordinary reversal of development occurs at the close of 



FIG. 1 08. Pupal skins 

 of Limnophilus, left 

 at final molting at- 

 tached to a reed 

 above the surface of 

 the water. 



