The Distribution of Melanism in the Pale Brindled Beauty Moth, Phigalia pedaria, in Great Britain

Abstract

Melanism in moths is the phenomenon in which some individuals of a species are darker than the typical form, due to an increased deposition of melanin in their epidermal scales. For over a century this has received much attention from entomologists and more recently geneticists. In Britain early attempts to account for the occurrence of melanism in many different ecological situations stressed the importance of environmental factors acting directly. For example, White (1876) cites climatic factors as being the ‘exciting cause’ of melanism in the Highlands of Scotland. Tutt (1890, 1891) began by having similar views, correlating melanism with high rainfall and humidity. Later (1899) his emphasis changed and he argued that climatic factors were an indirect cause of melanism; in areas of high rainfall, particularly where smoke was also present in the atmosphere (e.g. Lancashire and Yorkshire) such surfaces as rocks and trees were darkened by the action of rain, or rain and soot. On such surfaces melanic moths were better concealed and ‘natural selection’ augmented by ‘hereditary tendency’ favoured these forms.