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Show 528 MR. L. DONCASTER ON COLOUR-VARIATION [Dec. 12, 5. On the Colour-Variation of the Beetle Gonioctena variabilis. Bv L. D o n c a s t e r , M .A ., F.Z.S. [Received July 7, 1905.] In 1895 (P. Z. S. 1895, p. 850) an account was given by Mr. Bateson of the colour-variation of Gonioctena variabilis, a Chrysomelid beetle. His material was collected almost entirely at Granada in the months of March and April; and he found that although the insect is extraordinarily variable, yet when a large collection is made the beetles could be classified into two chief groups with very few intermediates between them. The ground-colour of the elytra varies from a brilliant red through orange and buff to a greyish green; and although the intermediate colours (orange and buff) are comparatively rare, no sharp line between the red and green can be drawn. There is also a great diversity in the markings: some individuals, chiefly those with red elytra, have two black spots on each elytron (spotted type), others (almost exclusively greens) are without these spots but have rows of minute black dots (striped form), and a third class has both spots and stripes. A large series of figures is given in the paper referred to. Bateson further found that the spots and stripes have a definite relation to the sculpturing of the elytra; the spots having their centres on certain of the longitudinal rows of punctulations, while the stripes lie between them. The spotted or striped type may be associated with either the red or green colour, but Bateson observed that almost invariably the spotted elytra were associated with black pigmentation of the ventral surface of the abdomen, and that specimens with no spots had no black pigment in this position. The colour of the underside therefore provided a means of dividing a population into two classes with exceedingly few intermediates; some had dark undersides and spotted elytra (with or without stripes in addition), the remainder had light undersides and were without spots. When classified in this way, it is found that most of the dark-spotted specimens have red or reddish elytra, and most of the unspotted light are green ; and further, that about 80 per cent, of the first class are males, and about 70 per cent, cf the second class females. The males are easily distinguished from the females by the presence of a small rounded depression in the last uncovered abdominal plate ; this is absent in the female. In addition to the variations mentioned, there may be more or less suffusion of the elytra with black pigment, until a totally black form is reached. The specimens in which this melanic variation is not very pronounced show that they belong to the class which is both spotted and striped; it occurs much more frequently in red than in green individuals. |
