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THE AMERICAN NATUBALIST [Vol. LIII 



11. Physical Coxditioxs oi- axd Behavior in Beook 

 Habitat Durixg Si:\ i;i;i: DnoriiiiT at White Heath 

 7. Desfripfion of rhysicdl Conditions. — Frequently 

 f'(M-taiii pliysical conditions were found to exist in broolv 

 linbilats in tlic early stages of droughts that had a direct 

 hearing- n))on the very existence of the water-striders, 

 Gerris remigis, but which will he niontioned here only 

 very briefly. I wish to refer iiartieniai-l\- to a brook, 

 flowing partly through a forested n -ion, near White 

 Heath. This brook is situated about eighteen miles 

 southwest of Urbana, and the physical conditions to be 

 considered are such as existed during a drought, in the 

 summer of 1911. During the earlier periods of this 

 drought, I often found, in the drying brook bed, small 

 pools of water not entirely isolated from each other. (In 

 Figs. 2 and 3 are shown drought stages in the bed of the 

 l>rook near White Heath that are very similar to those 

 under consideration. The only difference here, of im- 

 portance, is that the pools are in somewhat earlier drought 

 stages than are those to which I have referred.) Such 

 pools wei-e connected by means of riffles not more than 

 6-12 inches wide ( Fi-s'2, :;). When food hecame scarce 

 in pools of this eliai-aeter. oi' when a -euin. often (hie to 

 bacterial growtli ( l-'ig. :'A. foimed on theii- surface^, I have 

 observed that the watei-->t ri(h r- ina(h' their wa\ from 

 one pool to another, hy mean- of the^' -mall rlllles of 

 water, until such connecting llnlo (li-apt'<''>i't'<^ '""I'l the 

 majority of the pools ])ecanie di-y. Kventnally the gerrids 

 were concentrated on the surface^ of the few i-olated 

 pools that remained. Sometimes, the liai-terial growth, 

 which was of a gray color, eaux'd the death of hundred^ 

 of water-striders. 



Such pools as liave he.m mentioned ju'r-i-led hmger 

 in that part of the brook'.- eonr^e tliat extended through 

 the wooded region. In -neli a leglon the pool- were 



larger, with a greatei- volun f water, than l"i'e.|uently 



was the case in more expo.-ed ^itnation-. The-e condi- 

 tions were due primarily to the protection afforded to the 



