Under natural conditions certain lepidop-terous larvae developed a melanic form which breeds true. The spp. affected are: (Geometridae, Boarmiinae), Abraxas grossulariata, Opisthograptis luteolata, Tephrosia bistortata, T. crepuscularia, Boarmia repandata, B. gemmaria, Hybernia aurantiaria, H. defoliaria, H. marginaria, Phigalia pedaria, Amphidasys betularia, Ellopia prosapiaria, Selenia bilunaria, S. lunaria, Hygrochroma syringaria, Himera pennaria, Ennomos alniaria, Gonodontis bidentata, and Crocallis elinguaria; and (Noctuidae) Miselia oxyacanthae. Many, if not all, of the same series of spp. possess larvae with the power of adapting their coloration and even structure to their immediate environment. Almost without exception the larvae discussed feed on trees and shrubs and rest by day on the twigs and bark. As determined by the extermination of lichens, many of the melanic forms were first detected within the sphere of influence of smoke from Tyneside factories. In Selenia bilunaria the melanism is inherited as a Mendelian recessive.