CalUbcBtis 



397 



of quick strokes of its tail and gills, and that clambers 

 freely about over shore vegetation. It is an artful 

 dodger; and it is protectively colored. It feeds on a 

 great variety of vegetable substances living and dead, 

 and hence finds abundant food in every weedy pond. 

 It is eaten by every carnivore in the pond that can 

 catch it ; and doubtless it has many enemies that exceed 

 it in swiftness and many others that lie in ambush and 

 capture it by stealth. Hence, tho nearly always 

 present, it rarely appears very abundantly in old 

 ponds. 



The life cycle of Callibcetis is run in less than six 

 weeks. A single female may lay looo eggs. If all 

 these were to develop and reproduce, the increase from 

 a -single pair during one summer season would be some- 

 thing like this: 



1st brood 1,000 (half females) 



2d brood 500,000. 



3d brood 250,000,000. 



4th brood 125,000,000,000. 



These alluring possibilities of increase in an organism 

 that is choice fish food once led the senior author into a 

 series of experiments that extended through two years 

 and that met with uniform failure because the breeding 

 of the mayflies covild not be controlled. The rearing 

 was easily managed but even with the largest measure 

 of freedom that could be provided, the adults would 

 not mate and lay eggs in captivity. The problem of 

 their successful artificial propagation is still unsolved. 

 However, there has never been a new pond opened at 

 the Cornell University Biological field station, that has 

 not received the eggs of wild females of Callibstis, and 

 that has not raised a good crop of the nymphs ere 

 their slower-breeding carnivorous enemies developed. 



