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Show 1895;] COLOUR-VARIATIONS OP A BEETLE. 855 case of the yellow, which is a distinct colour, there must be an alteration in the nature of the pigment. Specimens are occasionally found having the elytron of one side red and that of the other side yellow (fig. 31). Not very rarely also there are more or less distinct patches of yellowish colour on the red ground, as in fig. 32, where they happen to be nearly symmetrical. These specimens are included in the Tables under the head of " unconformable." In addition to the varieties already mentioned, there is also a series of melanic forms. W e have seen that the black pigment of the elytra may either take the form of stripes or of spots. From tbe latter group (fig. 1) a noticeable series of variations leads to a . form totally black above and below. Such a series is illustrated by the figures 7-12 and 15-18. The first step in the progressive pigmentation consists in the appearance of black in the positions of the stripes, which is gradually extended. These specimens are ,thus both spotted and striped. The parts last invaded are the apices, the shoulders, and the borders of the elytra1. The spread of the black is perhaps never quite symmetrical on the two sides and is not rarely noticeably asymmetrical to the degree shown in the figures. The series of progressive pigmentation is closely parallel to that seen in Coccinella bipunctata, the common Lady-bird. Though the invasion of the black pigment proceeds along tolerably regular lines, darkening the parts of the elytra in a fairly constant order, yet as regards quantity of pigment variation towards the black form proceeds continuously, the states becoming successively rarer as the full black is approached. From the fact that the progress is so even it is not easy to give numerical expression to this ; but on sorting the specimens which have more black than fig. 1, it is found that while there are many which approximate to figs. 7 and 8, there are fewer which resemble figs. 9 and 10; those with only a few specks of red, like fig. 11, are still rarer, while tbe totally black state is rarer than any of the others. The darkening of the head and thorax proceeds more or less evenly with those of the elytra, but the correlation is not strict. These melanic forms are, as has been said, an offshoot of the spotted kind and not of the striped. They have red as the groundcolour almost without exception. Fig. 26 represents the darkest specimen I have seen with greenish ground-colour. A few specimens are found without any black markings on the elytra at all. These have the undersides testaceous. In such specimens the thorax has generally very little pigment and is occasionally entirely without any. 1 Curiously enough, the two specimens figured by Olivier, Hist. Nat. des Insectes, pi. viii. fig. 127, c and d, are both of these very melanic forms. The locality is not given, and perhaps the frequency of the varieties m a y differ with locality. |
