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 
Fic. 26.—The pupal case of one of the caddis-worms (Rhyacophila) from the 
rapids of the temporary stream at Glencoe; enlarged as indicated (original). 
Fic. 27.—The larva of the black fly (Simulium); about 15 times natural size 
(after Lugger). 
Fic. 28.—Pupa of the same (after Lugger). 
Fic. 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. 
