OCR Text |
Show 1895.] COLOUR-VARIATIONS OP A BEETLE. 853 black undersides (as in fig. 1, &c.) consist of patches of pigment usually placed so as to cross respectively the one the 2nd, 3rd, and 4th, the other the 7th and 8th double rows of punctulations. The extent of these spots varies greatly and in its condition of least development the central of the two spots coincides very nearly with the 3rd double row of punctulations, and the lateral spot similarly is very nearly on the 7th double row. All these arrangements are liable to irregularity, but in the main they are as described, and essentially in the spotted form pigment crosses certain of the double rows of punctulations, while in the striped form it is almost entirely in the intervals between them. The association of the black spots on the elytra with a dark underside is exceedingly close, and it scarcely ever happens that a specimen which has black pigment in the position of the spots (fig. 1) has a light underside. This principle is obeyed whatever the ground-colour of the elytra may be. Figs. 25 and 26, 28-30 are given as illustrations of specimens in which the rule is not followed, the undersides not being black though there is black pigment on the elytra in the situation of the spots. This division into spotted forms with dark undersides and striped forms with light undersides is the most conspicuous feature in the variation of the insect. In great measure the distinction is sexual, most of the males belonging to the spotted division and most of the females to the striped. Curiously the number of each sex which belongs to the opposite group is about the same, for on sorting it appears that Of males about 80 per cent, are of the spotted class. „ „ 18 „ „ striped „ Oi females „ 25 „ „ spotted „ „ „ 71 „ „ striped „ This reckoning is made without reference to ground-colour, and specimens that have striping as well as spots are of course included in the spotted class, to which they belong also by reason of their dark undersides. Of the spotted form the great majority (73 per cent, of all males and 19*6 per cent, of all females) have red a8 the ground-colour of the elytra. Of the striped form, on the other hand, the majority (19 per cent, of all males and 65*7 per cent, of all females) have greenish grey as the ground-colour. In fact the great majority of males agree pretty closely with fig. 1, while the great majority of females resemble fig. 13 ; but of the males the next commonest form is that shown in fig. 13, and of females the second commonest is that of fig. 1. It thus appears that the form which may be called normal for the female is the commonest variety in the male, and the converse is true though to a less degree. The thorax of the striped forms usually has black pigment as |