302 



VICTOR E. SHELFORD. 



In the 10-12 cm. per sec. current positive orientations (Figs. 

 22-24) are increased at the expense of the negative and indefinite 

 while the same difference is further emphasized in the 16-20 cm. 

 per sec. current and this without any material increase in the 

 number of individuals resting on the screen. 



16-20 cm. per sec. represents the flow to which the swift 

 stream species appear best adapted. In this current the per 



STRAIGHT CURRENT 



10-12 CM. PER SEC 



rtR CENT 

 tTHEOSTOMA 



CAMBARUS 



GQNIOBASIS 



HYDROPSYCHE 



ARGIA 



PERLA 



HEPTAGENINAE 



PSEPHENUS 



PER CENT 

 ETHEOSTOMA. 



CAMBARUS 



GONIOBASIS 



HYDRUPSYCUE 



ARGIA 



PERLA 



HEPTAGENINAE 



PSEPHENUS 



ORIENTATION 



HYDROPSYCHE OR RAPIDS COMMUNITY 

 10 20 30 40 50 60 70 80 90 100 



SAND-GRAVEL BOTTOM OR POOL COMMUNITY 

 10 20 30 40 51) fiO 70 8(1 90 100 



10 20 30 40 50 60 70 80 90 100 



10 20 30 40 50 60 70 80 90 100 



NOTROPIS 



HYBOPSIS 



AMBLOPLITES 



CALOPTERYX 



CAMPELOMA 



MACROMIA 



ANODONTOIDES 



SPHAERIUM 



NUTROPIS 



HYBOPSIS 



AMBLOPLITES 



CALOPTERYX 



CAMPELOMA 



MACROMIA 



ANODONTOIDES 



SPHAERIUM 



POSITIVE 



25 



NO ACTIVITY | | 



FIGS. 22-25. Showing reactions to 10-12 cm. current in per cent, positive, 

 negative, indifferent, and inactive (22-23) and per cent, of active individuals posi- 

 tive, negative, and indefinite (24-25). Here there is more activity among the 

 rapids animals and less among the pool animals than in the 4-6 current. The 

 percentage of active positive is much higher in the case of the rapids animals. 

 Temperature as in Figs. 18-21. 



cent, of active is more nearly the same for the different species 

 (compare Fig. 28 and 24 with 20). These figures show a re- 

 markable uniformity of positive reaction, over 93 per cent, for all 

 but Psephenus and Hydropsyche, which are most efficient in 

 clinging and less active otherwise. They are thus ecologically 

 equivalent to the rest of the animals living under stones. 



