Why Do Luna Moths Have No Mouth? A Short Adult Life

Do luna moths really have no mouth?

Strictly speaking, a luna moth has the remains of a mouth, but nothing that works. Most moths and butterflies have a proboscis, a long feeding tube that they uncoil to sip nectar, sap or juice. In the luna moth and its relatives, the giant silk moths (family Saturniidae), the mouthparts are vestigial: small, reduced leftovers of structures that were useful to their ancestors.

The adult cannot uncoil a proboscis, cannot drink and cannot chew. Its digestive system is also reduced and does not process food. If you look closely at a luna moth’s head, you see furry white scales and two big feathery antennae, but no feeding tube coiled underneath like a watch spring.

This is only true of the winged adult. The caterpillar of the luna moth has powerful jaws and eats tree leaves almost nonstop for about six weeks.

How long do luna moths live without eating?

A luna moth lives about 7 to 10 days after it emerges from its cocoon. Most of the life cycle happens before that:

So the luna moth is not short-lived as an animal. Counting a winter spent as a pupa, an individual can live close to a year. It is only the flying, mating stage that is brief.

Living on stored fat: the caterpillar does the eating

Biologists sometimes describe animals like the luna moth as “capital breeders”. They pay for reproduction with savings, energy collected earlier and stored in the body, instead of with “income” from feeding as adults. For a luna moth, the savings are the fat body: tissue packed with fat, proteins and sugars that the caterpillar built up from leaves.

The caterpillar grows from a hatchling a few millimeters long to about 7–9 cm (2.8–3.5 in) in five stages. Those reserves carry through the pupa, and when the moth emerges, a female’s abdomen already holds 200 to 400 eggs. She does not need extra food to mature them. She only needs enough fuel to call males with her scent, mate and fly from leaf to leaf laying her eggs.

Males use their reserves to fly, often for long distances, following the female’s pheromone with their large feathery antennae. Once the fuel runs out, the moth dies, usually within a week or so.

Why would evolution remove the mouth?

Losing the ability to eat sounds like a handicap, but it fits the luna moth’s way of life. Several factors work together:

• Food is not worth the risk. Searching for nectar takes time and exposes the moth to bats, birds and other predators. A moth that does not feed can spend its few nights only on breeding and stay hidden the rest of the time.
• Everything is prepared in advance. Because the eggs are largely ready when the female emerges, adult feeding would add little to the number of eggs she can lay.
• Unused parts fade away. Once adults stopped feeding, there was no advantage in keeping a costly proboscis and working gut, and these structures were reduced over many generations.

A short adult life also means fewer nights exposed to predators. The luna moth’s other defenses, its leaf-green color, its eyespots and its bat-fooling hindwing tails, help it survive just long enough to breed.

Other moths that never eat as adults

The luna moth is not unusual among its relatives. All giant silk moths have reduced mouthparts and do not feed as adults. In North America this includes the cecropia moth, the polyphemus moth, the promethea moth and the io moth. In Asia it includes the Atlas moth, one of the largest moths on Earth, whose adults also live only one to two weeks.

The domestic silk moth (Bombyx mori), whose caterpillar is the silkworm used to make silk, is another non-feeding adult. It belongs to a different family but has the same strategy: the caterpillar eats, and the adult only breeds.

Many other moths feed normally. Hawk moths have a long proboscis and hover at flowers like hummingbirds, and some adults live for weeks. Many butterflies also feed for weeks or months; you can compare them in what do butterflies eat?

Other insects with mouthless adults

The same idea appears in other insect groups:

• Mayflies spend months or even years as aquatic nymphs, then emerge as winged adults with non-working mouthparts. Most adult mayflies live from a few hours to a couple of days.
• Bot flies (family Oestridae) develop as larvae inside large mammals and emerge as adults that do not feed and live only briefly.

In each case, the young stage is built for eating and growing, and the adult is built for breeding and spreading the next generation.

Common misunderstandings

It is sometimes said that a luna moth “lives only a day”. In fact most adults live about a week, and the full life cycle lasts months. People also wonder whether the moth “starves to death”. Its body is not designed to eat as an adult, and its life is timed so that its reserves last through mating and egg laying. Because adults cannot drink, luna moths also do not come to the sweet fruit or sugar baits that attract many other moths at night, although they do come to lights.