14 



ised in the male by the possession of a bright patch of orange at the 

 tip of the forewing. This is often reduced in the female; or may 

 be entirely absent, the two sexes then looking quite dissimilar ; as 

 may be seen in Teracohix casta, a species from East Africa. Here 

 the male, except for the apical patch of orange, is mainly a white 

 butterfly; the female is without the orange tip, and is almost com- 

 pletely suffused on the upper surface with dark fuscous, the white 

 only appearing in isolated patches. The same kind of description 

 applies to many species of the genus Liias : the large form known 

 as I.rias t'fippe, for example, being in the male a bright yellow 

 butterfly with a conspicuous orange tip, while a common form of 

 the female is a dark smokj'-brown insect with a few whitish patches, 

 and often with no trace of the apical orange. Cases of this kind 

 are numerous, and even where the difference between the sexes is 

 very much less than in the examples we have just had under notice, 

 the tendency in the female to assume a duller or darker appearance 

 than the male is very frequently to be recognised. I suppose that 

 most of us are familiar with the tawny butterflies spotted with black 

 on the upper surface, and showing streaks or patches of silvery 

 lustre on the hindwing beneath, that frequent our woods and clear- 

 ings in July and August. In Dri/as (Anii/nnis) paphia, one of the 

 handsomest of these Fritillaries, as they are called, the ground colour 

 of the female is always duller, and the black spots larger, than in 

 the male. But besides the ordinary form of the female, in which 

 the difference from the male is not markedly conspicuous, there is 

 another form, in this country perhaps most often met with in the 

 New Forest, in which the ground colour is of a deep olive-brown, 

 contrasting strongly with the bright fulvous hue of the male. In 

 another species of Fritillary, An/yunis saijana, which is a native of 

 •China and Japan, the ground colour of the female is of a sage-green 

 so deep that the black spots are scarcely visible upon it, while a few 

 patches upon the forewings are of a creamy white, which tells out 

 conspicuously against the dark green background. The male, both 

 in markings and colour, is verj^ like that of our British Dryas 

 {Anjyjinis) paphia, and of course very unlike its own female. With- 

 out going further in search of instances, we can say then, that the 

 difference between the sexes, in at any rate a large number of cases, 

 manifests itself by an increase of dark pigment in the female. This 

 may prevail to a greater or less extent, and may have the general 

 effect of rendering the female comparatively dull and inconspicuous, 

 or in some instances (as in the female of Arqijitnis sa(fo)ia just 

 spoken of) may constitute a feature in a pattern which becomes 

 conspicuous by virtue of its contrasts of colour. 



The kind of sexual dimorphism that we have been considering is 

 chiefly concerned with the colouring of the upper surface ; we will 

 now turn to another type of difference, in which it is the under and 

 not the upper side at which we must look for the marks of sexual 

 differentiation. 



