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THE AMERICAN NATURALIST [Vol. LIII 



are to move toward the nearest one. If vision were the 

 main factor in assisting the hemipterons in finding pools 

 of water, they would be expected to go to the nearest one 

 first. Another fact against the idea of vision being the 

 chief influence in guiding these insects to water is that the 

 dry channels of the streams, where I have made my ob- 

 servations, frequently have very rough and uneven sur- 

 faces, with small boulders, stones, lumps of baked mud, 

 pieces of driftwood, and clumps of dead leaves scattered 

 along them. When the small size of these insects and the 

 nearness of tlieir eyes to the surface of the ground are 

 botli taken into (•(uisidcratioit. it becomes very evident 

 that tli<' vari(»u> object- tliat liave been enumerated must 

 obstruct the view of tlie water-striders in a very serious 

 fashion. Tlien again, sometimes the nearest pool was 

 around a bend in the stream, away from the gerrids, thus 

 making it impossible to be seen by them at a distance. 



With reference to the exi)eriments of Weiss (1914, p. 

 33) it is probable that siglit was an iin])ortant factor in 

 directing the gerrids to tlie water. esi)ecially over the 

 shorter distances, one to six \ anl- lnchi.>ive. On a bright, 

 sunny day. it is evident tliat tlie uli>tening mu\ reflective 

 qualities of a lnuly ot' wattM- nm>t l)e taclor> of importance 

 in attracting- tlioe a<iiiatic lieniiitteron.-. It must l)e re- 

 called that the ]H)n(l to which Weiss refers was a body of 

 water extending over an area of 3,000 square feet in ex- 

 tent while the pools to which I have referred were pro- 

 portionately insignificant in size. If there was a gradual 

 slo]ie to the shore of this pond and if the ground, where 

 tile experiments of Weiss were performed, had a smooth 

 -u if ace free of obstructions to the view, all this should be 

 in favor of tlie idea that vision was the important factor 

 in ilirectiiiii- tlie water sti'iders back again to the pond. 

 However, the local i.liy>i<-al conditions are not described. 



Certain experiments of Drzewina (1908) are, perhaps, 

 wmthy of mention in this general connection. This 

 writer fonml. in lier ol>servations on the hydrotropism of 

 eial.~. that the-e animals responded positively, and with 



