Moth

For every butterfly species on Earth there are about eight moths. Around 160,000 of them, against roughly 18,500 butterflies, and most people could not name a single one.

Some are spectacular. The atlas moth has a wingspan approaching a foot. The hummingbird hawk-moth hovers at flowers and is constantly mistaken for a bird. The Madagascan sunset moth is as brilliantly colored as anything with wings.

And the famous thing moths do, flying into lights, still has no settled explanation. The best current answer is not the one most people have heard.

Moths have no scientific name of their own, and they cannot have one, for an interesting reason.

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Lepidoptera

Moths and butterflies together make up the order Lepidoptera, meaning “scale wing”. Butterflies form one branch within it. Moth is simply the word for everything else.

So moths are not a single branch of the family tree. Some moths are more closely related to butterflies than they are to other moths. The old name Heterocera was used for them, but it describes a convenience rather than a real group, much like the word “monkey”.

In other words, butterflies are a kind of moth that took up flying by day.

Plenty of moths fly by day and are brightly colored, and the furry body is a real adaptation: fur traps heat, which matters for an insect flying on cold nights.

The old explanation is that moths navigate by the moon, keeping it at a fixed angle, and a nearby lamp confuses that into a spiral. It is a neat story, and the evidence for it is weak.

Research using high-speed cameras found something different. Moths are not flying toward the light at all. They are turning their backs to it.

Flying insects keep themselves upright by assuming the brightest thing around is the sky, and tilting their backs toward it. For millions of years that worked perfectly, because the brightest thing above really was the sky.

An artificial light breaks it. The moth tips its back toward the bulb, which means it banks, circles and ends up trapped in orbit around it, unable to fly level. It is not drawn to the lamp. It has lost its sense of which way is up.

That makes artificial light a genuine problem. Moths caught circling a lamp all night do not feed, mate or lay eggs, and light pollution is now considered one of the causes of insect decline.

Moths range from the pygmy sorrel moth at about 3 millimeters across to the atlas moth and white witch, which approach a 12-inch wingspan.

• Antennae. Often feathery, and in males packed with scent receptors. A male emperor moth can detect a single female from several miles downwind.
• Wings. Covered in tiny scales, like butterflies. Many have intricate patterns that disappear completely against bark.
• Body. Thick and furry, trapping heat so the moth can fly in cold air.
• Proboscis. A coiled tube for drinking nectar. Some adult moths have none at all and never eat.
• Ears. Many moths have simple ears tuned to bat echolocation, and will drop out of the air when they hear one.

Some moths go further. Tiger moths answer a hunting bat with their own ultrasonic clicks, which jam the bat’s sonar or warn it that the moth tastes foul.

Moths live on every continent except Antarctica, in rainforest, desert, mountain, grassland, wetland and city.

They occupy niches butterflies never touch. Some caterpillars live underwater. Some eat lichen on bare rock. Some live inside a single leaf, tunneling between its surfaces. A few feed on wool and feathers, which is how clothes moths got their reputation, and it is only a couple of species doing it.

Many migrate. Bogong moths in Australia travel hundreds of miles to spend summer in cool mountain caves, and they navigate at night using the stars and Earth’s magnetic field.

Caterpillars do the eating. They feed on leaves, stems, roots, seeds, wood, fungi and occasionally other insects, and they are built to grow as fast as possible.

Adult moths mostly drink nectar, and are omnivores across the group as a whole, with some taking sap, fruit juice or the moisture around animals’ eyes.

Many large moths never eat at all. The atlas moth and luna moth emerge without a working mouth, live a week or two on fat stored as a caterpillar, mate, and die. The entire adult stage exists only to reproduce.

Most moths fly at night, which shapes everything about them.

They find each other by scent rather than sight. A female releases pheromones, and a male detects them with his feathery antennae from an extraordinary distance, then flies upwind along the trail.

Defense is mostly about not being seen. Many moths are superbly camouflaged against bark or lichen and simply sit still all day. Others startle predators by flicking open their forewings to reveal huge eyespots underneath. Some mimic wasps, bird droppings or dead leaves.

The arms race with bats is the most sophisticated part. Bats hunt moths by echolocation, so moths evolved ears to hear them, evasive dives, ultrasonic jamming, and in some species long trailing hindwing tails that scatter the bat’s sonar and make it strike the wrong place.

Moths go through complete metamorphosis: egg, caterpillar, pupa, adult.

The pupal stage is where moths usually differ from butterflies. Most moth caterpillars spin a silk cocoon around themselves, and others pupate underground or in leaf litter.

That silk has been enormously important to people. Silk comes from the cocoon of the domesticated silkmoth, Bombyx mori, an insect so thoroughly bred by humans that it can no longer fly or survive without us, and no longer exists in the wild at all.

Adult lifespans are short, usually a week to a few months, though a caterpillar may feed for far longer. One Arctic moth caterpillar freezes and thaws for up to fourteen years before it finally pupates.

Bats are the great moth predator, along with birds, spiders, frogs, lizards and parasitic wasps that lay eggs inside caterpillars.

Human pressures are habitat loss, pesticides, and light pollution, which disrupts the navigation described above.

Moth numbers have fallen sharply in several well-studied countries, and because moths are counted more carefully than most insects, those declines are some of the clearest evidence we have that insects in general are disappearing.

There is no single status across 160,000 species, and most have never been assessed.

Where long-term counts exist the picture is worrying. Britain has lost a large share of its larger moths since the 1960s, and several species have disappeared entirely.

Simple things help: planting night-scented flowers, leaving some grass and leaf litter undisturbed, avoiding pesticides, and turning off outdoor lights or fitting warmer-colored bulbs, which confuse moths far less than cold white ones.

Moths are major pollinators, and this is badly underappreciated. They work the night shift, visiting flowers that open after dark, and studies tracking pollen have found moths carry it between plants more widely than daytime pollinators do.

Some partnerships are exact. The yucca plant and the yucca moth depend entirely on each other: the moth deliberately packs pollen into the flower and lays eggs there, and neither species could survive alone.

Caterpillars are also one of the most important foods in nature. Most songbirds cannot raise a brood without large numbers of them, and a single pair of blue tits may collect thousands to feed one nest of chicks.

So moths feed the birds, pollinate the plants, and almost nobody notices, because it all happens after dark.