252 ME. E. B. POULTON ON THE EXTEENAL 



Noctuas) and among the butterflies that the powers of flight in the two sexes are not 

 very unequal. 



The same conclusions are supported even more strongly by looking a little further 

 into details. Among the Sphingina (adopting the arrangement of Stainton's ' Manual ') 

 there is no marked difference between the flight of the sexes in the flower-haunting, 

 day-flying Zygcenidce, Sesiidce, and JEgeriidce. In the Sphingidce, which feed, both sexes 

 fly actively ; while in the species of the genus Smerinthus, which do not feed, both sexes 

 are sluggish, but the females more so than the males. In the Bombycina very few of the 

 moths feed, and the males are nearly always extremely active fliers and the females very 

 sluggish : the exceptions are especially interesting. The Lithosiidce feed eagerly and are 

 commonly taken at sugar, and the females fly actively with the males ; the same facts are 

 true of a few flower-haunting genera among the Cheloniidae, in which both sexes fly by day. 

 I do not know of any other Bombyx which feeds except Cossus ligniperda, which, I 

 believe, has been occasionally seen at sugar ; and in this species both sexes are sluggish. 

 In the great majority of the rest of this group flight among the females is almost or 

 entirely subservient to oviposition, while that of the males is far more active and in 

 many species has been rendered extremely rapid by competition in the struggle to reach 

 the females. The female moth is nearly always at a disadvantage in flight as compared 

 with the male because of her relatively large and heavy abdomen filled with eggs. This 

 cause, always present to some extent, tends to produce important results as soon as a 

 species ceases to feed in the perfect state, and can therefore dispense with rapid flight 

 in the females. Increase in the size of the abdomen will then be an advantage, enabling 

 the female to lay more eggs or larger ones — in either case benefiting the species in the 

 larval state — while greater sluggishness of flight only affects the rate of oviposition. 

 Furthermore, the danger resulting from slow oviposition may be met in special ways. 



By the working of this cause, rendered effective by the cessation of feeding, we meet 

 the first strong indications of degeneration in those females which fly sluggishly to 

 deposit their eggs, but for no other purpose. In such cases it is not uncommon for the 

 females to fly at a different time from the males. Thus the males of Lasiocampa rubi 

 fly with great rapidity in bright daylight, while the females sit perfectly motionless, but 

 in the evening they fly slowly and deposit their eggs. I know this to be the case from 

 my own experience, and I have no doubt that it is the same with Endromis versicolor, 

 Saturnia carpini, and others *. The only doubt is as to the evening flight of the females ; 

 for in other respects these two species are well known to behave in the same manner as 

 L. rubi. The flight of such females, although sluggish, is still necessary, except under 

 certain conditions. The Bombycince are generally large moths, and there would be a 

 two-fold danger in depositing all the eggs in one place — that of insufficiency of food and 

 easy detection by enemies (unless indeed the larvae were well concealed or defended by a 

 disagreeable taste or smell), and the latter, the chief difficulty, applies to small as well 

 as large moths. But while further degeneration is thus rigidly prevented in most cases, 

 there are certain moths which escape from the limiting conditions. 



* Weismann states that the females of Aglia tan deposit all their eggs in one spot, being unable to fly on 

 account of the weight of the abdomen. The males, on the other hand, fly swiftly, seeking for the females. (' Essays 

 on Heredity,' Clarendon Press, 1889, pp. 17, 18.) 



