SELECTIVE COUPLING OF GAMMARIDS. 



101 



0.014; f r D. fasciatus, r - = 0.690 0.042. Figs. 3 

 and 4 give the fitted lines of regression for mean lengths of the 

 females associated with males of the corresponding length classes. 

 The formation of breeding pairs, according to Holmes (1903) 

 and others who have studied the question of " sex-recognition " in 

 gammarids and among other crustaceans, 1 is brought about in a 

 purely mechanical way. The initial encounter of male and female 

 is by accident. Males tend to clasp objects with which they come 

 into contact. A male seized by another male struggles until freed. 

 A female, on the contrary, is passive when clasped, with abdomen 

 and thoracic legs flexed, the whole body compact. This account 



13 



12 



11 

 10 



9 



8 



7 



16.5 



1Q5 11.5 12.5 13.5 14.5 15.5 



FIG. 4. Regression of mean length of females upon length of males, pairs of 

 D. fasciatus, lengths in nuns., class centers of males, abscissas; ordinates, 

 mean lengths of associated females. 



we can confirm for the species dealt with in our measurements. 

 There is clearly a good mechanical reason for the formation of 

 couples in which large males carry large females, and small males 

 carry small females. Holmes (1903) observed that a small male 



1 Holmes (1903, 1909); Pearse (1909); Andrews (1910); Chidester (1911). 



