SEXUAL DIFFERENCE IN WING LENGTH 



269 



TABLE 1 

 Average dimen-sions. See text. 



It will be seen from table 1 that the mean wing length of homo- 

 zygous normal females is considerably greater than that of similar 

 males — their brothers. The average length of the middle femur 

 is also greater in the females than in the males but the sexual 

 dimorphism with respect to wing length is relative as well as 

 absolute, as is shown by the fact that the ratio of wing to femur 

 among the females greatly exceeds that among the males, the 

 difference between the averages being nearly fifty times the error 

 of the difference. Even so, this would not prove the existence of 

 a fundamental sexual dimorphism if there were a tendency for 

 generally large flies to have the ratio large. Without entering 

 the maze of spurious correlation caused b}^ using indices, we can 

 see from the regression lines (fig. 2) that in both sexes, but espe- 

 cially in the male, there is a tendency for the wing to get propor- 

 tionately smaller as the general size of the insect, as measured by 

 the size of the middle femora, increases. The dimorphism is 

 therefore real. The sexes are built on different plans. 



It is in all ways probable that there is a large complex of fac- 

 tors concerned in the de^'elopment of a normal wing. It is pos- 

 sible that the abnormal forms considered here are caused by the 

 dropping out of certain of the factors from this complex. It 

 would seem that the normal females get a double dose and the 

 males but a single dose of those factors of the normal complex 

 which are connected with the X-chromosomes. It is well known 

 that many factors do not cause as great a somatic development 

 when in a simplex condition (for example, heterozygous) as these 



