Feb., 1920 Bulletin of the Brooklyn Entomological Society 3 



During periods of drought in summer, it is a very common oc- 

 currence to find certain of the physical conditions of intermittent 

 brooks decidedly modified from what they are in periods of nor- 

 mal rainfall. The most evident modification is that due to the 

 loss in volume of water. If a drought period should be pro- 

 longed, such brooks may be so changed as to form merely a series 

 of isolated pools, along their otherwise dry channels. In the 

 earlier drought stages, these pools are connected by means of 

 _riffles of water, which become more and more contracted as the 

 drought develops. 



While droughts are in their earlier stages, and the brook pools 

 are still connected by means of narrow riffles of water, I fre- 

 quently have observed that the apterous individuals of both species 

 of gerrids travel from one pool to another by means of these 

 small riffles. Eventually, if the drought continues until the riffles 

 disappear, these water-striders are found on the surface of the 

 isolated pools that are distributed along the beds of the brooks. 

 Specimens of Gerris remigis frequently are present in such sit- 

 uations in large numbers, while individuals of Gerris marginatus 

 are comparatively few. That some of the alate specimens of 

 both species, also, stride along the surface of these riffles^ from 

 one pool to another is a well-authenticated fact, as I often have 

 observed such responses. Of course it is quite possible that some 

 of them make the journey by flight, but I have not seen such 

 flights. Certainly the alate gerrids are found intermingled with 

 the apterous individuals that are congregated on the surface-film 

 of the isolated pools of water, after the riffles have become dry. 



As the volume of water continues to decrease, in intermittent 

 brooks, owing to the prolongation of the drought period, it is 

 evident that the alate gerrids, Gerris marginatus, become re- 

 duced in numbers, from day to day. This probably is due to 

 their migration by flight. But when the brooks have reached the 

 isolated pool stage, it is found that some members of this species 

 of water-strider are still present, for they are congregated on the 

 surface of these pools. Not only is this true with respect to the 

 apterous forms, but it is true, also, with respect to the alate indi- 

 viduals, as I previously have indicated. At this stage in the proc- 

 ess of the drying up of these brooks, gerrids belonging to the 



