Distribution, Food and reproductive Capacity of some fresh-water Amphipods. 17 



females already carrying eggs or young, leads one to believe that males 

 in the proper physiological condition for mating are at all times abun- 

 dant enough to supply each female with a consort at a time when 

 the latter is capable of oviposition. Thus we have only to consider the 

 rate of periodic maturation of ova in order to understand the rate of 

 recurrence of the reproductive activities. 



It can be easily observed that at a time just prior to moulting 

 the ovaries become enlarged and very dark in color. This has been 

 observed to occur periodically throughout the yearly reproductive period 

 in aquarium specimens and would thus favor the view that oviposition 

 may take place after every moult, of the female. 



The attempt was made to verify it experimentally but only par- 

 tial success was attained. In experiments performed with pairs of the 

 two species, G. fasciatus and Hyalella, three successive sets of eggs 

 were secured from a single female in each case, after the same num- 

 ber of moults. In the case of Gr. limnaeus only two sets were ob- 

 tained in as many moults. No success resulted from a like treatment 

 of pairs of E. gracilis. 



The method of procedure was usually as follows; — several mated 

 pairs of each species were at first placed in as many small glass dishes 

 containing aquatic plants and pond water. These were kept covered 

 with glass except when the daily observations were to be made. Every 

 other day the amphipods were fed pieces of freshly killed Asellus. 

 After the first moult and subsequent deposition of eggs, in every case 

 the male was removed to a separate dish. All dishes were numbered 

 and proper data kept on separate numbered sheets. A moderate sized 

 aquarium was stocked plentifully with extra males in order that a 

 supply of breeding males might be available at all times. 



Whenever the eggs of one female hatched her former mate was 

 again placed with her. If mating did not take placed within a few 

 minutes another male was substituted. This treatment was continued 

 until the female was usually mated a second and sometimes a third 

 time. In the case of one female Hyalella, the right male was not 

 found until the second day after the female had moulted. The two 

 mated and on the morning of the third day another set of eggs was 

 present which eventually hatched quite normally. 



The following tabulated data are representative of those secured in 

 the above described manner. 



From this table one can see that in the case of two forms at 

 least, three sucessive sets of eggs may be extruded immediately follow- 



Embody, Amphipods, Biol. Suppl., m. Serie, 1911. ^ 



