Peppered moth

Lore & Background

The peppered moth has a wingspan ranging from 45 to 62 mm, with a median of 55 mm. Its wings are white, 'peppered' with black, and have more-or-less distinct black cross lines. The black speckling varies; in some examples it is almost absent, while in others it is so dense that the wings appear black sprinkled with white. The antennae of males are strongly bipectinate. The caterpillar is a twig mimic, varying in color between green and brown, and can sense the twig's color with its skin to match its body color to the background, an ability also found in cephalopods, chameleons, and some fish. The peppered moth is found across a wide range including China, Russia, Mongolia, Japan, North Korea, South Korea, Nepal, Kazakhstan, Kyrgyzstan, Turkmenistan, Georgia, Azerbaijan, Armenia, Europe, and North America. In Great Britain and Ireland it is univoltine (one generation per year), while in southeastern North America it is bivoltine (two generations per year). The life cycle consists of eggs, several larval instars (caterpillars), pupae that overwinter in the soil, and adults. Adults emerge between late May and August, with males slightly before females. Males fly every night in search of females, while females fly only on the first night and then release pheromones to attract males. The female lays about 2,000 pale-green ovoid eggs about 1 mm in length into crevices in bark. Peppered moths rest by day in the upper part of trees at unexposed positions, such as below branch-trunk joints, on the underside of branches, or on foliate twigs. The moths are cryptically camouflaged against their backgrounds; typica are camouflaged against lichens and carbonaria against plain bark. Birds can see ultraviolet light, and typica reflect ultraviolet light in a speckled fashion, camouflaging them against crustose lichens common on branches.

Reader's Guide

The peppered moth is a key example in the study of population genetics and natural selection. Its evolution over the last two hundred years has been studied in detail, particularly in relation to the Industrial Revolution in England. At the start of this period, most peppered moths had light-colored wing patterns that camouflaged them against light-colored trees and lichens. Due to widespread pollution, many lichens died out and trees became blackened by soot, causing most light-colored moths (typica) to die off due to predation, while dark-colored melanic moths (carbonaria) flourished. This shift illustrates how environmental changes can drive natural selection. The peppered moth exhibits polymorphism, with several melanic and non-melanic morphs controlled genetically. In Europe, there are three morphs: the white typica, the dark melanistic carbonaria, and an intermediate medionigra. In Britain, the intermediate is named insularia. In North America, the melanic black morph is morpha swettaria. In Japan, no melanic morphs have been recorded. The melanic allele is dominant to the non-melanic allele in European and North American populations. Importantly, individuals of each morph interbreed and produce fertile offspring, so there is only one species. The peppered moth's resting behavior and camouflage have been studied using ultraviolet-sensitive video cameras, showing that typica are camouflaged against crustose lichens in both human-visible and ultraviolet wavelengths.

Did You Know?

• The caterpillars can sense the twig's color with their skin and match their body color to the background.
• In Japan, no melanic morphs have been recorded; all are morpha typica.
• The female lays about 2,000 pale-green ovoid eggs about 1 mm in length.