SEX RECOGNITION AMONG AMPHIPODS. 2QI 



contact with other individuals is the factor which effects the union 

 of the males with the females. Each reacts to the reactions of 

 the other. The male has a strong instinci to seize and carry 

 other individuals of the same species. The female has the instinct 

 to lie quiet when another individual comes into contact with her, 

 especially if she is seized. The instinctive reactions of the two 

 sexes are complementary and cooperate to bring about and main- 

 tain the peculiar sexual association characteristic of the Gamma- 

 ridea. If the association of the sexes is brought about by their 

 peculiar modes of reaction to certain contact stimuli, it would 

 seem probable that the only reason why males do not carry other 

 males as well as females is that they are prevented from so doing 

 by the active resistance of their intended mates. I was accord- 

 ingly led to try the experiment of mutilating some male speci- 

 mens so that they could no longer make effective resistance to 

 seizure. The large second gnathopods (the principal means of 

 defense) of several males were cut off and the mutilated indi- 

 viduals were placed in a dish with several males which were re- 

 cently torn from females. The mutilated males were soon seized 

 and carried about as if they were members of the other sex. In 

 one case a mutilated male was carried about for over five hours. 

 The mutilated males were more active than females are under 

 the same conditions, and did not assume the same bodily atti- 

 tude, but nevertheless their captors carried them without any 

 manifest awareness of the deception to which they were subjected. 



Male Hyalellas, however, will not carry dead specimens of 

 either sex, at least for more than a short time. I have observed 

 males of both Hyalclla and Gammarus struggling for a time 

 with a dead specimen, but their efforts to carry it were soon dis- 

 continued. The failure to carry dead individuals may be due to 

 odor or some sort of chemical stimulation from the object seized, 

 or to the lack of an occasional movement causing a struggle on 

 the part of the male to retain his hold. Stimuli of the latter 

 kind may be necessary to cause the instinctive reaction of the 

 male to continue. 



There can be little doubt that the origin of the instinct of the 

 male amphipod to seize and carry the female is to be sought in 

 a modification of the act of copulation. The lower Crustacea af- 



