Distribution, Food and reproductive Capacity of some fresh-water Amphipods. 9 



have been required. A few variations in the use of the two gnatho- 

 pods were noted and two of them are illustrated in figures 2 and 3. 

 In a few cases all four gnathopods were used, doubly securing the 

 union of two mated animals. 



It is seen that in the cases represented by figures 1 and 2 the 

 gnathopods were used in a manner similar to that of a pair of tongs, 

 thereby uniting the mated pair so strongly that is was impossible to 

 separate them without injury to one or both. 



b) Duration of Mating. 



Since mating is a one sided affair for which the male is almost 

 solely responsible, it is evident that the physiological condition of the 

 male determines at what time mating takes place. When does the 

 sexual desire appear in the male and how long may it persist if not 

 satisfied by copulation, are questions upon which the writer can throw 

 only dim light. The observations taken indicate that if the sexual im- 

 pulse appears between any two moults, it may likewise assert itself 

 between the second and third succeeding moults but whether it may occur 

 between every two, is not known. 



A male may exhibit the sexual impulse for a number of days 

 during the period between moults. In the case of G. limnaeus when 

 the period between moults is 36 days, in water having a mean daily 

 temperature of 17,3" C, the impulse may appear as early as the six- 

 teenth, day and may persist until the twenty-seventh day. The possible 

 range is thus 12^) days. With Hy ale 11 a when the period between 

 moults is 28 days at a mean daily temperature of 23,2® C, it may 

 appear at the seventh day and may persist until the twenty-fifth day, 

 a possible range of 18 days. 



The writer was unable to secure complete data with reference to 

 the other two species under consideration but one male G. faciatus 

 was observed to copulate successfully four days before moulting. 



Once mated the pair remain together until the female moults. Then 

 they usually separate for a brief period.'') The male seems to lose his 



*) This range may be greater. The cases given represent the earliest and 

 latest times of mating which resulted in copulation and subsequent appearance of 

 fertilized eggs. 



*) This period varies from a few minutes to several hours. One Hyalella 

 thus remained separated from his mate from about 9 A. M., until 4 P. M., of the 

 same day. In the case of G. fasciatus, one male grasped the female throughout 

 the moulting period. As the old skin split transversely along the suture between 



