402 



THE AMEBIC AN NATURALIST 



[Vol. LIII 



was I able to discover the water-striders the followmg 

 day. 



It is possible that there was a trifle of moisture in such 

 situations, and that it was this, acting as a stimulus, which 

 kept the gerrids there. But during these periods of 

 drought — accompanied, as they are, in the prairie regions 

 of Illinois with excessive temperature and glaring sun- 

 light — I have failed, positively, to find moisture in sucli 

 places, except under large objects, which were not present 

 in the bed of the brook. Frequently, however, I have 

 noticed that under these objects, in the brook bed, the tem- 

 perature was slightly lower and of course there also was 

 the shade. It is probable, also, that evaporation is re- 

 duced in such protected places. That certain arthropods 

 are sensitive to the ('va])oi'ating ]iower of air is known 

 from the work of SlK'ir.ud (I'.M.'l i)p. 85-102), who has 

 demonstrated that llic > dlnw ma igined millipede, Fon- 

 taria corrugate Wood, and the ground beetles, Ptero- 

 stichus adoxiis and Pterostichus pennsylvanicus, respond 

 negatively to the increased evaporating power of air. 

 Therefore, lower tem])erature, shaded surroundings and 

 reduced ovai)orati(m may ho the three factors that ac- 

 counted foi- flic \va1iT-<tri(lcrs iTniai iiiiig in -ituatidiis of 

 the charact.'i' that have Immmi uumiI i.nicd. Of (m.uix' if it 

 was proved that thcv stavrd llinv dav after day, this 

 wculd iiidicalc the |M.»ihilitv of their tiding over a short 

 dn.uuht in such protected phices. 1 have noticed on a 

 feu- occasions, at h-ast, that th.' gen-ids have died, if 



high temperature, during a drought. I also have ob- 

 served, when the water in in>- lahoi-atory a(|uaria was 

 allowed to evaporate gradually, tintil it entirely dis- 

 a]>i>eared. that, sometimes in a i\"\v houi-< and at other 

 tinie-^ in a few dax >, the ucrrid- died. Tld. occurred, not 



strider^ reniai.nnu-.inietly in one , ,1; 'ce 'for a' long period 

 of tinu'. r.ut it niu^t 1m- n.-alh-.] that the l.-mperature is 



