FIELD. — INDIVIDUAL VARIATION IN THE LEPIDOPTERA. 395 



Comparing the results obtained from the males with those from the 

 females, we find the greater variability in the latter sex, except in the 

 case of the sinuous margin. This does not accord with the generally 

 accepted theory of the variability of the sexes, but if there were sufficient 

 statistical data it would probably appear as a rather common phenome- 

 non among the Lepidoptera ; for dimorphic and trimorphic females are 

 found in very many species. 



IV. Conclusions. 



In the moth Thyreus abhotii, the most aberrant dimension of the fore 

 wing is likewise the most variable. This accords with Darwin's law. 



The females show, in general, a greater degree of variability than the 

 males ; but in the one markedly aberrant feature under discussion, their 

 variability is less than that of the males. I submit the following hy- 

 pothesis to explain the smaller variability of the sinuous margin in the 

 females. The fact that the wing broadens so greatly toward the tip, and 

 is provided with an irregular edge, suggests that this form of margin 

 may be of advantage in rendering the insect less distinctly visible when 

 its wings are in rapid motion. This idea is upheld by the fact that 

 a related species, Amphion nessus Cramer, has the same effect produced 

 in color where irregularity of form is wanting. If this hypothesis be 

 admitted, then we must look for a better development of the protective 

 feature in the female than in the male, since the female, when burdened 

 with eggs, cannot fly as rapidly as her mate. The sinuous margin being 

 of less importance, comparatively, for the male, we can understand its 

 greater variability in that sex, where it is less rigidly selected. 



The work which I have here described was performed in connection 

 with my studies under Dr. Charles B. Davenport, at Harvard University, 

 in 1896-97. I am indebted to him for valuable suggestions and advice. 



