Invertebrates
Moth
Lepidoptera · also called moths
Butterflies get the attention, but they are a rounding error next to the moths. About nine in every ten scaled-winged insects are moths, and the butterflies are just the one lineage of moths that took to flying by day. Many adult moths never eat — they hatch, mate, lay, and die within a week, running entirely on fuel their caterpillar stored.
Overview
Moths make up most of the order Lepidoptera, which has roughly 160,000 named species and probably over half a million in total, on every continent except Antarctica. Butterflies are one branch within them, so a butterfly is technically a kind of moth. All Lepidoptera have wings and bodies covered in tiny overlapping scales — the name means “scale-wing” — and a life cycle of egg, caterpillar, pupa, and winged adult.
Living on a caterpillar’s savings
Caterpillars are eating machines; many adult moths do not eat at all. The giant silk moths — luna, atlas — emerge with no working mouthparts and live only long enough to breed. Others feed on nectar, sap, or rotting fruit through a coiled tongue and last for weeks.
Night flight
Many moths have simple ultrasonic ears tuned to hunting bats and take evasive dives when they hear one; tiger moths even click back to jam the bat’s sonar. Moths seem to steer by holding a distant light at a fixed angle, which is why a nearby bulb sends them spiralling in — a behaviour now recognised as a light-pollution problem.
Useful and harmful
The domestic silk moth has been farmed for thread for ~5,000 years, hawkmoths and yucca moths pollinate pale night-scented flowers, and a few species — clothes moths, the codling moth, armyworms — are serious pests.
Status
Moths as an order are Not Evaluated, but the trend is decreasing: long-running surveys in Britain, Europe, and North America show the abundance and biomass of moths falling by a quarter to a half in some regions, with consequences for the birds and bats that depend on them.
Moth: common questions
What is the difference between a moth and a butterfly?
There is no clean scientific line, because butterflies are simply one branch of moths that took up day flying, so all butterflies are moths but most moths are not butterflies. The everyday rules of thumb work fairly well in temperate countries. Butterflies usually have clubbed, thread-like antennae, hold their wings closed vertically over the back at rest, fly by day, and often have bright colours, while moths more often have feathery or tapered antennae, rest with wings held flat or folded like a tent, fly at night, and are drab. But there are day-flying moths, brightly coloured moths, and moths that rest like butterflies, so the guides fail often enough that only the antennae and wing-coupling details are reliable.
Why do moths fly toward lights?
The leading explanation is that many moths navigate at night by keeping a far-off light source, historically the moon or bright stars, at a constant angle to their flight path, which keeps them travelling in a straight line. An artificial light is close, not effectively infinite, so holding it at a fixed angle makes the moth curve continuously toward it and spiral in. Newer high-speed video work suggests the moth may instead be trying to keep its back to the brightest part of the sky, a reflex for staying upright, and that a bright bulb below or beside it scrambles that response. Either way, artificial light at night disrupts moth behaviour and is now considered a real conservation problem.
Do adult moths eat, and how long do they live?
It varies enormously. Some adult moths feed actively on nectar, sap, or rotting fruit through a coiled tongue and can live for weeks or, in a few that overwinter, months. But a large fraction of moth species, including the giant silk moths such as the luna and atlas moths, emerge from the pupa with no functional mouthparts at all. These adults cannot feed and live only on the reserves built up by the caterpillar, giving them roughly a week, sometimes only a few days, in which their entire job is to find a mate and, for the female, lay eggs before they die.
Are moths really declining, and does it matter?
Yes on both counts, based on some of the longest-running insect datasets available. Standardised light-trap networks in Britain and the Netherlands, and studies in Germany and North America, show the total abundance and weight of moths falling markedly over the last 40 to 50 years, with many individual species contracting in range. The drivers are the familiar mix of habitat loss, agricultural intensification, pesticides, nitrogen deposition, artificial light, and climate change. It matters because caterpillars and adult moths are a primary food for nesting birds, for bats, and for many other predators, and because night-flying moths are important and under-appreciated pollinators.