10 Bulletin of the Brooklyn Entomological Society Vol. XV 



Drzewina^^ (pp. 1009-1010) in some experiments on Carcinus 

 mcenas found that moisture given off from the sea was the chief 

 stimulus, under certain conditions, in causing these arthropods to 

 move toward the sea and eventually to reach it. Moisture, dif- 

 fusing through the atmosphere, may play some role in these mi- 

 gration responses of water-striders, but, if so, I believe that it is 

 true only when there is a large body of water very near to the 

 location of the pool on which they formerly had lived. It is pos- 

 sible that in such instances the gerrids may respond positively to 

 the moisture diffusing from such a body of water, and by means 

 of such responses eventually reach it. However, it must be re- 

 called that, in the prairie regions' of the Middle West, where 

 many of these observations were recorded, during a severe and 

 extended drought, in a treeless region, evaporation is excessive 

 and moisture from such bodies of water, unless of extensive area, 

 would be dissipated very quickly by the high temperature. It 

 is very improbable that hydrotropism plays much of a role in the 

 responses of these apterous water-striders, in connection with 

 small isolated pools in intermittent brook channels, under such 

 drought conditions as I have described. 



I am strongly of the opinion that.- during severe droughts, in cJ^ 

 rapid, intermittent brook habitats, in such regions as I have men- 

 tioned, after the small isolated pools on which water-striders have 

 been living become dry, other bodies of water are found by these 

 gerrids by a crude method of trial and error. Sometimes, water- 

 striders may reach other pools, if these are large and close by, 

 through responses to moisture, or through the sense of sight. 

 But I believe that the greater part of the process of overland mi- 

 gration is one of repeated trial and error, until at length some of 

 the gerrids may reach another water-strider habitat. Many of the 

 gerrids are unsuccessful in their attempts to reach water — not 

 that there is any voluntary attempt on the part of the gerrids to 

 find another body of water, in the sense of awareness of the 

 end to be achieved, — and therefor, large numbers of apterous -.''^, 

 individuals must perish during periods of severe and extended 

 droughts. 



^9 Drzewina, A., " De I'hydrotropisme chez les Crabes," Comptes rendus 

 hebdomadaires des seances, Societe de biologie, T. LXIV (1908), pp. 

 lOOCHioii. 



