A résumé of investigations on the evolution of melanism in the Lepidoptera

Abstract

Apart from flight, the wings in the Lepidoptera are used for developing various pattern and colour devices enabling them to pass the daytime protected from predators. In the cryptic species, complicated patterns have been built up suitable for concealment on lichened tree trunks, boughs, rocks, reeds or posts, and the phenomenon of ‘industrial melanism’ is found among this group only. Of the 780 species of Macro-lepidoptera which occur in the British Isles, about seventy are in the process of replacing their populations with dark or black individuals. In the majority of cases the black mutant is inherited as a simple Mendelian dominant. The position of the melanics occurring in other groups, however, is completely different. In those insects which gain protection from mechanisms other than crypsis (warning, threat and flash coloration), or even in those cryptic moths which benefit from resembling dead leaves, the melanics are rare, recessive, sub-lethal and are probably maintained by recurrent mutation. A third group of melanics can be referred to as ‘geographic melanics’, and they are confined to the Highlands of Scotland, the west coast of Great Britain and Ireland. Their inheritance in the few cases known is dominant (complete or incomplete) or in others multi-factorial; so far never recessive. It will be noted that these areas are comprised of primeval forest, ‘unspoilt’ countryside, or environments little affected by civilization. In recent years we have been attempting to analyze the relative advantages and disadvantages of the melanics and their light forms and, to date, these studies have been confined to investigations on the so-called industrial melanics. These have, for the most part, been conducted on Biston betularia L., the Peppered Moth (Selidosemidae), and one of its melanics carbonaria Jordan; its other melanic, insularia Th.-Mieg, is complicated and probably determined by a series of alleles and is, except for population frequencies, omitted from this paper.