Butterfly

There is no clean scientific line, because butterflies are one branch of moths that took up day flying, so all butterflies are moths but the reverse is not true. The common rules of thumb hold reasonably well in temperate countries. Butterflies usually have slender antennae tipped with a small club, rest with their wings held closed above the back, fly by day, and are often brightly coloured, while moths more often have feathery or tapering antennae, rest with wings spread flat or folded like a tent, fly at night, and are drab. But there are day-flying moths, brilliant moths, and moths that rest like butterflies, so the antennae and some wing-hinge details are the only fully reliable guide.

The adult stage, the winged butterfly, is usually short. Many small species live only a week or two as adults, and even large ones often manage just a few weeks, spending that time entirely on finding mates and, for females, laying eggs. There are exceptions. Butterflies that overwinter as adults, such as some tortoiseshells and the mourning cloak, can live nine to eleven months, and the migratory generation of the monarch lives around eight months to make the round trip feasible. The whole life cycle, egg to death, is often a single season, though some species pass the winter as an egg, caterpillar, or chrysalis and a few take two years in cold climates.

Through complete metamorphosis inside the chrysalis, or pupa. When a fully grown caterpillar pupates, most of its internal tissues are dismantled by its own digestive enzymes into a nutrient-rich fluid, and clusters of dormant cells called imaginal discs, which were set aside early in the caterpillar's life, use that fluid to grow the adult structures — wings, long legs, compound eyes, the coiled proboscis, antennae, and reproductive organs. This takes days to weeks depending on species and temperature. The adult then splits the chrysalis, crawls out with crumpled wings, pumps fluid into the wing veins to expand and stiffen them, and flies once they dry.

The pressures are the same ones affecting insects broadly, and butterflies show them clearly because they are well monitored. Flower-rich grassland and woodland edges, where both caterpillars and adults feed, have been lost to intensive farming and development. Herbicides remove the specific host plants many caterpillars must have, milkweed for the monarch being the best-known case. Insecticides, including seed treatments and garden sprays, kill caterpillars and adults directly. Climate change shifts the timing of emergence out of step with host plants and pushes cool-adapted species off the tops of mountains. Nitrogen pollution favours a few rank grasses over diverse wildflowers. The declines are large, on the order of a fifth to a half in several well-studied regions.