8 Bulletin of the Brooklyn Entomological Society Vol. XV 



convinced that large numbers of apterous water-striders do die. 

 I have observed that the gerrids are, apparently, far more nunier- . 

 ous at the beginning of a severe droughty extending over a con- 6^- 

 ih siderable area, than they are when the brooks again contain their 

 normal volume of water, after such a drought has run its course. 

 From this point to the close of the paper, the discussion treats, 

 mainly, of the responses of apterous gerrids of the species Gerris 

 remigis. Attention already has been directed to the fact that dur- 

 ing the later stages of severe droughts,- which extend over a long ^ 

 period of time, the isolated brook pools decrease rapidly in size, 

 until, eventually/ they become dry. At this point in the drought 

 stage, just prior to the total evaporation of all water in these 

 small pools, it might be expected that the apterous water-striders, 

 trapped on their surfaces, promptly would make their way over- 

 land to other pools of water in the immediate vicinity,, but actual 

 observation proves that this, usually, is not the case. The gerrids 

 do not leave the pools until after all the water has dried up. The 

 finding of another pool of water, if farther away than six or 

 eight yards or such a matter from the original pool on which the 

 gerrids had lived is a very precarious proceeding. In whatever 

 direction the water-striders venture, after leaving the site of the 

 former pool, the strong probabilities are that they will continue 

 to move in that general direction until some external stimulation 

 results in a response that turns them from it. The commonest 

 form of stimulation is that of contact, which is due to the gerrids 

 encountering certain obstacles in their pathway, such as stones, 

 driftwood, pieces of dry mud, and clumps of dead leaves. After 

 coming in contact with such obstacles, the water-striders turn 

 either to the right or to the left, thus being directed along a new 

 line of progress. Sometimes, they remain near these objects 

 with their bodies closely applied to them. After remaining in 

 such positions for varying lengths of time, they then move for- 

 ward again. It is not always possible to tell what the stimulus 

 is that results in the change of direction. It is quite within rea- 

 son to believe that, in certain instances, such a response may be 

 due to some internal stimulation that causes a change in the in- 

 ternal activities of the body, which change evinces itself in such 

 a response as has been mentioned. 



