Feb., 1920 Bulletin of the Brooklyn Entomological Society 9 



The whole process of moving away from the site of a former 

 pool to another body of water seems to be a hit and miss proposi- 

 tion. The gerrids first try one direction and then they try an- 

 other. The proceeding appears to be one of trial and error, the 

 errors predominating, a process of repeated attempt and failure, 

 on the part of the water-striders. Some of the apterous gerrids 

 may be successful in reaching water elsewhere, but/ usually/ there 

 does not seem to be any direct response to another body of water 

 per se. Indeed, a casual observer would say that the entire pro- 

 ceeding was one of chance. 



It must be recalled that in the brook channels of the character 

 under consideration there are many obstacles, such as rocks, drift- 

 wood, clumps of dead leaves, and pieces of dried mud. Further, 

 the surfaces of the beds of these brooks are of baked mud and 

 they are very uneven. Partly because of obstacles of this kind, it 

 is very improbable that the sense of sight plays much of a role 

 in assisting these gerrids to find water, for it is evident that the 

 obstacles mentioned must obstruct the view of the gerrids very 

 materially. This is appreciated more definitely when it is re- 

 called that the water-striders are small in size and also that their 

 eyes are close to the surface of the ground. Frequently, it has 

 happened that the nearest pool of water was situated around a 

 bend in the brook, away from the water-striders, so that it was 

 impossible for them to see it from the location of the pool on 

 which they formerly had lived. 



If it is necessary for the gerrids to travel only a distance of 

 two, three, or four yards, or such a matter before reaching an- 

 other pool of water, under such circumstances, vision may prove 

 to be an important factor in locating it. Pools of water are good 

 reflecting surfaces and it is very probable, at such short distances, 

 that the reflections of the rays of sunlight materially aid the ger- 

 rids in finding them. However, for vision to become a factor of 

 importance to the water-striders in reaching other pools, after 

 the one on which they had been living has become dry, it is neces- 

 sary that there shall be few if any obstacles in the brook channel 

 and that the surface of the ground shall be smooth and flat. Such 

 conditions, according to my observations, are seldom found in 

 brooks of the character under discussion. 



