Peppered moth

The peppered moth (Biston betularia) belongs to the Geometridae family. It flies at night and lives mostly in the temperate parts of the northern hemisphere. Its evolution is a well-known case study in population genetics and natural selection.

The caterpillars of this moth mimic both the shape and color of twigs. Recent research shows they can detect a twig’s color through their skin and adjust their own body color to match the background, helping them avoid predators.

**Description** The moth’s wingspan is 45 to 62 mm, with an average of 55 mm. It has a fairly stout body and narrow, elongated forewings. The wings are white with black speckling, plus more or less distinct black cross lines. Rarely, the speckling and lines can be gray or brown, and in very rare cases, a mix of brown and black or gray. The amount of black speckling varies widely—some individuals have almost none, while others are so heavily marked the wings look black with white spots. Males have strongly comb-like antennae. Prout (1912–16) described the various forms and related species.

**Distribution** This moth occurs in China (many provinces including Heilongjiang, Jilin, Inner Mongolia, Beijing, Hebei, Shanxi, Shandong, Henan, Shaanxi, Ningxia, Gansu, Qinghai, Xinjiang, Fujian, Sichuan, Yunnan, Tibet), Russia, Mongolia, Japan, North and South Korea, Nepal, Kazakhstan, Kyrgyzstan, Turkmenistan, Georgia, Azerbaijan, Armenia, Europe, and North America.

**Ecology and Life Cycle** In Great Britain and Ireland, the peppered moth has one generation per year. In southeastern North America, it has two. Its life cycle has four stages: eggs, several caterpillar stages, pupae (which overwinter in the soil), and adults. During the day, the moths rest on trees, where birds prey on them. The caterpillar mimics a twig and can be green or brown. In 1887, Edward Bagnall Poulton noted it as one of the first animals known to use countershading for camouflage, making it appear flat. The caterpillar senses a twig’s color with its skin and changes its own color to match—a slower version of the camouflage seen in cephalopods, chameleons, and some fish. Late in the season, it enters the soil to pupate for winter. Adults emerge from pupae between late May and August, with males emerging slightly before females. They emerge late in the day, dry their wings, and fly that night. Males fly every night searching for females, while females fly only on their first night. After that, females release pheromones, which males follow upwind. During flight, bats prey on them. Males guard females from other males until eggs are laid. Females lay about 2,000 pale-green, oval eggs, each about 1 mm long, into bark crevices using their ovipositor.

**Resting Behavior** A mating pair or a lone moth spends the day hiding from birds. Males stay with females to ensure paternity. Peppered moths usually rest in the upper parts of trees in sheltered spots: a few inches below a branch-trunk joint on a trunk (where it’s shadowed), on the underside of branches, or on leafy twigs. Researcher Michael Majerus noted that creationist critics often cite a 1985 statement by Clarke et al. that in 25 years they found only two moths on tree trunks or walls near traps. Majerus explained that few people look for moths high in trees, where peppered moths actually rest by day. This is supported by experiments with captive moths (Mikkola, 1979, 1984; Liebert and Brakefield, 1987). Majerus et al. (2000) showed that moths are cryptically camouflaged against their backgrounds when resting on tree boughs. In human-visible light, the typical light form (typica) blends with lichens, and the dark form (carbonaria) blends with plain bark. Birds see ultraviolet light, which humans cannot. Using an ultraviolet-sensitive video camera, Majerus et al. found that typica reflects UV light in a speckled pattern, matching crustose lichens on branches in both UV and human light. However, typica is less well camouflaged against foliose lichens on trunks—though it matches in human light, foliose lichens do not reflect UV. In a seven-year Cambridge experiment (2001–2007), Majerus recorded natural resting spots of 135 moths: over half were on tree branches (mostly on the lower half), 37% on trunks (mostly on the north side), and 12.6% on or under twigs.

**Polymorphism** The peppered moth has several melanic and non-melanic forms, controlled genetically. A particular color form is indicated in a standard way by following...