This species is single-brooded, with a protracted emergence. Adults are on the wing from May into August. Females lay their eggs in large batches, but the newly hatched caterpillars soon disperse. They spin silk threads and float off downwind until they land again by chance. Fortunately they can eat a very wide range of deciduous trees and shrubs. Caterpillars feed only at night, and are full-grown in September. The pupal stage overwinters in the soil and adults emerge the following spring (6).
The Peppered Moth has been widely used as a textbook example of evolution by natural selection. During the industrial revolution, sooty deposits darkened much of the habitat. The melanic form of the moth was first recorded in Manchester in 1848. Within 50 years it had almost replaced the typical form both there and in other industrial areas (5) Classic experiments carried out by Ketterwell during the 1950s suggested that bird predation was the crucial factor. The melanic form was better camouflaged when resting on sooty tree trunks and branches than was the paler typical form, hence it survived better. Conversely, the typical form was at an advantage in unpolluted areas where the tree bark was covered in lichens.
In recent years, various doubts have been cast on this simplistic explanation, and less fairly on the quality of Keterwell’s work (5) (7). While his pioneering experiments were undoubtedly flawed when judged by modern standards, his basic premise is still accepted by most evolutionary biologists. Although the full picture may well have been more complicated.
Following the Clean Air Acts, introduced from 1964 onwards, smoke pollution and soot deposition have been greatly reduced. The melanic form of the Peppered Moth has now lost its advantage and undergone a dramatic decline in frequency. It is now scarce in areas where it previously dominated and may soon disappear completely. Whatever the finer details of its rise and fall, the carbonaria story is a fascinating one.