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, with more-or-less distinct 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 colour between green and brown, and can sense the twig's colour with its skin to match its body colour to the background.

The peppered moth is found across a wide range including China, Russia, Mongolia, Japan, North and South Korea, Nepal, Kazakhstan, Kyrgyzstan, Turkmenistan, Georgia, Azerbaijan, Armenia, and Europe. (North American records refer to the closely related species *Biston cognataria*, not *Biston betularia*.) In Japan, melanic morphs have been recorded, though at lower frequencies than in some European populations.

Reader's Guide

The peppered moth is a key example of population genetics and natural selection, particularly in the context of industrial melanism. During the Industrial Revolution in England, pollution killed lichens and blackened trees with soot, causing light-coloured moths (typica) to become more visible to predators and die off, while dark melanic moths (carbonaria) flourished. This shift is one of the most cited cases of natural selection in action. The moth's colour forms are controlled genetically, with the melanic allele dominant to the non-melanic allele in European populations. Importantly, all morphs interbreed and produce fertile offspring, so they represent a single species, not separate species or subspecies. The peppered moth also demonstrates remarkable camouflage abilities: its caterpillars can sense twig colour with their skin and adjust their body colour, a trait also found in cephalopods, chameleons, and some fish. The moth's resting behaviour and cryptic coloration have been studied extensively, including using ultraviolet-sensitive video cameras to understand how birds perceive the moths. The peppered moth remains a foundational case study in evolutionary biology.

Did You Know?

• The caterpillars can sense the twig's colour with their skin and match their body colour to the background.
• In Great Britain and Ireland the peppered moth is univoltine (one generation per year), while in southeastern North America it is bivoltine (two generations per year).
• The melanic allele producing morpha carbonaria is dominant to the non-melanic allele in European populations.
• In Japan, no melanic morphs have been recorded; all are morpha typica.

Physical Form & Coloration

The peppered moth is a robust member of the Geometridae family, boasting a wingspan that typically falls between 45 and 62 millimeters, with an average near 55 millimeters. Its forewings are notably narrow and elongated, set against a white base scattered with black speckling — the very feature that gives the species its common name. Transverse lines cross the wings in black, though in rare instances these markings shift toward grey or brown, and in exceptionally uncommon cases the spotting blends brown with black or grey. The density of dark maculation varies dramatically across individuals: some specimens carry barely a trace of dark spotting, while others are so heavily peppered that the wings read as black dusted with white. Males can be distinguished by their strongly bipectinate antennae, a feathery structure that aids in detecting female pheromones. The species is strictly nocturnal, found across temperate regions of the northern hemisphere.

The Twig-Mimicking Caterpillar

The larval stage of Biston betularia represents one of nature's most remarkable camouflage strategies. The caterpillar not only replicates the slender form of a twig but also shifts its coloration between green and brown to blend with its surroundings. In 1887, Edward Bagnall Poulton identified this species as one of the first animals known to employ countershading — a gradient of shading that flattens the caterpillar's apparent three-dimensional shape, making it look like a flat stick to predators. More recent research has revealed an even more sophisticated mechanism: the caterpillar can detect the specific color of the twig it rests upon through its skin and then adjust its own body color to match. This active color-matching ability is shared by cephalopods, chameleons, and certain fish, though the caterpillar's transition is considerably slower than those animals'. This dual strategy of shape mimicry and dynamic color adjustment provides layered protection against visual predators throughout the larval instars.

Courtship, Mating & Reproduction

The adult phase of the peppered moth unfolds in a tightly choreographed nocturnal ritual. In Great Britain and Ireland the species produces a single generation each year, while in south-eastern North America it completes two. Pupae overwinter in the soil, and adults emerge between late May and August, with males appearing slightly ahead of females — a pattern consistent with sexual selection pressures. Males take to the air every single night of their adult lives, navigating upwind along pheromone trails carried on the breeze to locate receptive females. Females, by contrast, fly only on their first night; thereafter they remain stationary, releasing pheromones into the wind. During flight, males face the constant threat of bat predation. Once a male locates a female, he guards her from rival suitors until she deposits roughly 2,000 pale-green, ovoid eggs — each about one millimeter long — into crevices in tree bark using her ovipositor.

Daytime Refuge & Ultraviolet Camouflage

By day, peppered moths retreat to concealed positions high in trees, where they are most vulnerable to avian predators. Researcher Michael Majerus documented three primary resting sites: a few inches below a branch-trunk junction in the shadow of the trunk, on the underside of branches, and among foliate twigs. A seven-year field study in Cambridge (2001–2007) examining 135 individuals found that over half rested on tree branches — mostly on the lower half — while 37% chose tree trunks, predominantly the north-facing side, and just 12.6% perched on or beneath twigs. Crucially, Majerus and colleagues used ultraviolet-sensitive video cameras to reveal that the light typica morph reflects UV in a speckled pattern matching crustose lichens on branches, while the dark carbonaria morph blends against plain bark. However, against foliose lichens on trunks, typica moths lose their UV camouflage, exposing a vulnerability invisible to the human eye.