88 



ANIMAL COMMUNITIES OF STREAMS 



of May-flies. Such streams have, however, no permanent aquatic resi- 

 dents. These aquatic forms are not aquatic during their entire lives. 

 They require water only during their early stages. If the water is 

 running at the time the female is ready to deposit eggs and if she is 

 properly stimulated by the conditions, she deposits them without regard 

 to future conditions. If the wet weather continues long enough, the 

 larvae will mature and the other adults will appear, otherwise they die. 

 This type of animals continues after the stream becomes large enough 



Stream Communities 



Fig. 26. — The pupal case of one of the caddis-worms (RhyacophUa) from the 

 rapids of the temporary stream at Glencoe; enlarged as indicated (original). 



Fig. 27. — The larva of the black fly (Simulium); about 15 times natural size 

 (after Lugger) . 



Fig. 28. — Pupa of the same (after Lugger). 



Fig. 29. — Pupa of the same in the pupal case (original). 



to have permanent pools. At such a stage the number of species is 

 increased, but no two collections are alike (see Table XVII). Clinging 

 to the upper surface of the stones are black-fly larvae, caddis-worms 

 (Rhyacophilidae) (Fig. 26); under stones, May-fly nymphs, those col- 

 lected as different times often belonging to different species. On some 

 occasions there are great numbers of unidentifiable dipterous larvae 

 and caddis-worms without gills or cases. Such a stream may possess 

 any or all of these on one occasion, and none or only a few of them on 

 another. 



