XX ii INTRODUCTION, 



peculiar physiological conditions of the sexes, and the necessity in one 

 rather than the other, of utilising all the material for actual nutritive 

 and vegetative processes in the female, leaving a surplus in the males, 

 which, applied to the more actively reproductive organism (if we may 

 so term the males compared with the more passive females), results in 

 the production of distinctly secondary sexual characters. We find, 

 however, as previously mentioned, examples where the smaller male 

 is better scaled and more brilliantly coloured although paler as in 

 Bombyx neustria ; and in cases where the female is smaller and darker, 

 the male, having expended more energy in the formation of extra 

 wing membrane, has less for pigment, but, at the same time, the pig- 

 ment of the males in these cases is usually richer than that 

 of the females, e.g. Agrotis cinerea, Pachnobia leucographa etc. 

 With regard to colour as a secondary sexual character, Scudder, after 

 quoting a large number of cases in which the sexes differ, says : " It 

 is not a little remarkable that in all these examples, and, indeed, in 

 very nearly all that have come under my notice, this sexual diversity 

 is displayed only upon the upper surface of the wings and almost in- 

 variably upon the fore wings. We might perhaps anticipate the 

 restriction of the characteristics to the fore wings, and upon the upper 

 surface the complication of colorational design is greater in butterflies 

 on these than on the hind wings ; yet this same reasoning makes their 

 restriction to the upper surface the more striking, since the under 

 surface of the hind wings of butterflies is usually more variegated than 

 any other part" (p. 531). One hardly likes to suggest that Scudder 

 only made a superficial examination of the undersides but it seems 

 probable. Where sexual colour dimorphism exists on the upper sides, 

 it is so palpable that one cannot fail to note it, but on the undersides, 

 the nice gradations of tint and shades of a hue responding to environ- 

 ment are exhibited in almost every species. In our few British butter- 

 flies we have many illustrations, and, where the undersides present no 

 variation in the sexes, we find, as a rule, that the females are as active 

 as the males, and that both are equally well protected by their coloration 

 responding to their surroundings, e.g. Theda rubi, T. w-album &c. 

 On the other hand Melanargia galatea, Erebia blandina, Theda betulce, 

 Lyccena corydon, L. icarus, L. bettargus etc., exhibit strong and striking 

 sexual colour dimorphism on the under surfaces, and it would not be 

 difficult to make up a very long list of species thus varying. But such 

 cases as these appear to me to be to a much greater extent the direct 

 result of " natural selection " than does the colour variation of the upper 

 sides. There is one particular phase of colour as a " secondary sexual 

 character " that cannot be passed over. I refer to Heplalus humuli. 

 Normally, the female is yellow or orange with red markings, the male 

 pure glossy white, which has gained for it its popular name of " ghost." 

 The males hover in the early evening and are remarkably conspicuous, 

 frequenting open spots where they may be readily seen. This hover- 

 ing and its connection with its white colour was explained by Dr. 

 T. A. Chapman, who observed that the female flew against the male 

 and then dropped, the male immediately following her down and 

 pairing with her. I cannot help again remarking that here " sexual 

 selection " comes into play, and the case is the more striking consider- 

 ing how rarely moths show it. 



