DEPARTMENT OF EXPERIMENTAL EVOLUTION. 



123 



There are 8 easily recognized secondary sex-characters in this species : 

 (1) Body size — the males are smaller than the females; (2) outline 

 of the head — in the male the head-outUne does not form a definite beak 

 as it does in the female ; (3) and (4) character of the first pair of antennce — 

 these in the male are considerably developed and distinctly different 

 from the very rudimentary structure in the female; (5) and (6) outline 

 and hairiness of the ventral anterior margin of the carapace — in the 

 male this margin forms almost a right angle and is quite covered with 

 hairs, in the female the margin is gently rounded and hairless; (7) and 

 (8) character of the first pair of thoracic appendages — in the male these are 

 relatively simple structures armed with a hook-shaped, finger-like 

 projection; in the female they are branched into many long terminal 

 filaments, while there is nothing resembling the hook-like structure 

 of the male. These 8 secondary sex-characters, together with the 

 character of the gonads (which is readily determined by microscopical 

 examination of the living animal), make it possible to state definitely 

 the degree of intersexuality of each individual intergrade (fig. 3). 



Fig. 3a. 



Fig. 3b. 



a, antenna; br, breast (ventral anterior margin of carapace); bk, beak; ap, first thoracic appendage. 



FiQ. 3a. Normal male from sex-intergrade-producing strain of Daphnia longispina. 



Fig. 3b. Normal female from sex-intergrade-producing strain of Daphnia longispina. 



Sex intergrades may have any combination of these significant male and female secondary sex 

 characters. Further, these characters are often present in intermediate condition. Obvi- 

 ously with such a sex difference as that between the female and male first thoracic appendage 

 there is room for a wide variety of intermediate conditions. Such a great range actually 

 occurs. 



