Every insect hatches from an egg and ends up an adult with wings, or at least the ancestors of wings. What happens in between is one of three stories, and knowing which one a species follows tells you what its young look like, where to find them, and why the caterpillar eating your cabbages and the butterfly on your buddleia are the same animal.
Complete metamorphosis: four stages
Egg → larva → pupa → adult. The larva is a feeding machine that looks nothing like the adult: a caterpillar, a grub, a maggot. When it has eaten enough it becomes a pupa, and inside the pupa most of the body is dissolved and rebuilt. The adult that emerges does not grow any more; a small fly is a fly that was a small maggot, not a young one.
Why it works: the larva and the adult eat different things in different places, so they never compete with each other. A caterpillar eats leaves; the butterfly drinks nectar. A grub eats roots; the beetle eats pollen.
Who does it: beetles, butterflies and moths, bees, wasps and ants, flies, lacewings, caddisflies, fleas. Roughly four out of five named insect species.
Find the stages: the monarch caterpillar on milkweed and the monarch itself; the woolly bear crossing a path in autumn is a caterpillar that spends the winter as a caterpillar, surviving being frozen, and grows up into the Isabella tiger moth; a seven-spot ladybird larva looks like a tiny grey alligator and eats even more aphids than its parents. Card pages for these species show “Complete metamorphosis” with the four stages.
Incomplete metamorphosis: three stages
Egg → nymph → adult. The nymph looks like a small, wingless version of the adult, usually eats the same food in the same place, and grows by moulting its skin several times. Wing buds appear on the outside and get bigger with each moult until the final one produces a flying adult. No pupa, no rebuild.
Some of these young live underwater and are called naiads: dragonfly, damselfly and mayfly nymphs hunt in ponds and streams for months or years, then climb a stem, split their skin and fly away. The empty skins left on reeds in June are the giveaway.
Who does it: true bugs (stink bugs, aphids, cicadas, water striders), grasshoppers, crickets and katydids, dragonflies and damselflies, mantises, stick insects, cockroaches, termites, earwigs, mayflies.
Find the stages: green stink bug nymphs are round, black-and-red and cluster on the leaf where they hatched; a common field grasshopper nymph is the same shape as the adult with stubby wing pads; the periodical cicada spends 13 or 17 years underground as a nymph before its single summer as an adult. Card pages show “Incomplete metamorphosis” with three stages.
No metamorphosis: just growing
Egg → young → adult. The young are miniatures of the adult and simply moult bigger. A few primitive insects do this (silverfish, bristletails), and so do all the relatives that are not insects: spiders, scorpions, ticks, harvestmen, centipedes, millipedes, woodlice, snails and slugs.
Find the stages: a baby wolf spider rides on its mother’s back looking exactly like her; a young common pill-bug is a pale, smaller pill-bug; silverfish hatch as tiny silverfish and keep moulting all their lives. Card pages show “No metamorphosis”.
How to tell which type you are looking at
| You found | It is probably | Life cycle |
|---|---|---|
| A caterpillar, grub or maggot | Larva of a butterfly, beetle or fly | Complete |
| A hard case with something inside | A pupa or chrysalis | Complete |
| A small wingless version of a familiar insect | A nymph | Incomplete |
| An empty split skin on a reed | Shed nymph skin of a dragonfly or mayfly | Incomplete |
| A tiny spider, woodlouse or centipede | Just a young one | None |
Why it matters in practice
- Pest control is about the stage, not the species. Caterpillars, not butterflies, eat the cabbages. Grubs, not beetles, eat the lawn roots. Aphid nymphs and adults both suck sap, which is why aphids explode in numbers: there is no non-feeding stage.
- Timing. A species with complete metamorphosis has a window when it is invisible (the pupa). The seasonal activity strip on each species page is built from adult sightings, so a gap in the strip is often the pupal weeks, not an absence.
- Where to look for the young. For incomplete metamorphosis, look where the adults are. For complete metamorphosis, look where the food is: the host plant, the soil, the pond, the compost.
Front desk note: we print the type of life cycle on every card because it comes straight from the order the species belongs to, which is settled science. We do not print how long each stage takes, because that changes with temperature and food, and a number that is right in Florida is wrong in Scotland.
The Field Guide sorted by life cycle
Of the 181 species in the guide, 101 grow by complete metamorphosis, 43 by incomplete metamorphosis and 27 by none; the remaining 10 are cards we do not print a life cycle for: three caterpillar cards, and groups such as springtails, mites and earthworms. Browse them by group: beetles, butterflies and moths, bees, wasps and ants and flies for complete; true bugs, grasshoppers and crickets, dragonflies and mantids for incomplete; spiders and crawlers for none.
Not sure what stage the thing on your windowsill is in? A photo in the identifier gives the species, and the species page gives the life cycle.
Questions
What is the difference between complete and incomplete metamorphosis?
Complete metamorphosis has four stages: egg, larva, pupa, adult. The larva (caterpillar, grub, maggot) looks nothing like the adult and rebuilds itself inside the pupa. Incomplete metamorphosis has three stages: egg, nymph, adult. The nymph looks like a small wingless adult and grows by moulting, with wings appearing gradually.
Which insects have a 3-stage life cycle?
True bugs (stink bugs, aphids, cicadas), grasshoppers and crickets, dragonflies and damselflies, mantises, stick insects, cockroaches, termites, earwigs and mayflies. Their young are called nymphs, or naiads when they live in water.
Which insects go through complete metamorphosis?
Beetles, butterflies and moths, bees, wasps and ants, flies, lacewings, caddisflies and fleas. That is most insect species: the four-stage life cycle is the majority pattern.
Do spiders go through metamorphosis?
No. Spiders, scorpions, centipedes, millipedes, woodlice and snails hatch as small versions of the adult and simply moult or grow larger. Only insects do true metamorphosis.
How long does an insect life cycle take?
Anything from a couple of weeks (fruit flies, aphids in summer) to seventeen years (periodical cicadas). Temperature and food change it enormously, which is why the same species can have one generation a year in the north and three in the south.
