No. 628] HABITAT RESPONSES OF WATER-STRIDER 409 



other bodies of water, after their o^\^l pool became dry. I 

 will give a few examples : In one instance, only two of the 

 trapped water-st riders out of eight reached the surface of 

 another pool, situated about eleven yards upstream. This 

 was not accomplished until the gerrids had consumed 

 much time in random movements, stops and deviations 

 from the straight path. At another time, fifteen gerrids 

 were trapped in a pool along the bed of a brook that had 

 become almost dry, owing to a prolonged drought. Five 

 water-striders out of the tifteen, or one tliird of the total 

 number, only, were nble to find another ]h)o1 of water 

 seven yards upstream from the poiiil wliere they had been 

 isolated. These gerr'nU iii;i(h- m;iii\ t i-i;tN. riT..r^. .h'l;iys. 

 and turnings before tli("\ it'iu'lied the surfncc-filni. On 

 still another occasion forty watei'-st riders were eiUrai)]-)ed 

 on the surface of a stream pool. After the water had dis- 

 appeared entirely, the hemipterons left tlieir haunts, and 

 stuml)led along the di'y bed of the stream. After much 

 wandering about, first in one dii-ection and then in an- 

 othei-, six of the gerrids found a pool fourteen yards down- 

 streiiii! from their former al)ode. The method of reaching 

 the \vat(M- scMMtied to l)e due to a ('rud(> foi-m of trial and 



I am inclined to hclicvc thai in iHM'iod- of severe 

 drou-ht, large numbers of apterous indivi<]uals die. If 

 the drought is a short one, and there i> a certain minimum 

 of dampness under logs, stones, and clumps of dead 

 leaves, some of the water-striders may sur\i\e in such 

 situations. In regard to physical conditions of a some- 

 what comparable character, Kirkaldy (189U, pp. 109, 

 110) makes the following statement with reference to 



