Head to head
Caterpillar vs Earthworm
Caterpillars are often called worms, as in inchworm, armyworm, and silkworm, which makes people ask whether they really are worms. They are not. A caterpillar and an earthworm are only very distantly related animals, they are built completely differently, and only one of them is a permanent form.
| Caterpillar | Earthworm | |
|---|---|---|
| What it is | The larva of a butterfly or moth | A segmented worm (annelid) |
| Permanent or a stage | A temporary stage; becomes a winged adult | Its whole life |
| Head and jaws | Hard head capsule with chewing jaws | No head capsule, no jaws |
| Legs | Three pairs of true legs plus fleshy prolegs | None |
| Eyes | Several simple eyes | None; light-sensitive skin |
| Breathing | Through holes down the sides | Through the moist skin |
| Diet | Plant leaves | Soil and dead plant matter |
| Group | Insects (phylum Arthropoda) | Annelids (phylum Annelida) |
Caterpillar vs Worm: common questions
Is a caterpillar a worm?
No, although many caterpillars have worm in their common names. A worm, in the strict sense, is a soft-bodied animal with no legs, no hard head, and a simple body, and true worms such as earthworms belong to a completely different branch of the animal kingdom from insects. A caterpillar is an insect larva, with a hard head capsule, jointed legs, breathing holes, and a body plan that will be rebuilt into a butterfly or a moth. The old names, inchworm for a looper caterpillar, silkworm for the larva of the silk moth, armyworm for certain crop-pest caterpillars, stuck from a time before the biology was sorted out, and they describe the shape rather than the animal's true identity.
How can I tell a caterpillar from a worm in the garden?
Look for a head and legs. A caterpillar has a clearly defined head at one end, usually a different colour or texture from the body, with mouthparts you can see, and it has legs: three pairs of small jointed legs just behind the head, and several pairs of stubby fleshy legs along the middle and rear of the body. It is also usually found on or near a plant it is eating, often with chewed leaves nearby. An earthworm has no distinct head, no legs at all, and a body made of many identical rings, and it is found in the soil, in compost, or on the surface after rain. If in doubt, a gentle look at one end will settle it: a face and legs mean a caterpillar.
Are caterpillars and earthworms both good for a garden?
Yes, in different ways, and both are worth tolerating. Earthworms are among the most valuable animals in any garden, dragging dead leaves into the soil, mixing organic matter through it, improving drainage and aeration, and building fertile topsoil, which is why gardeners actively encourage them. Caterpillars have a worse reputation because a few species chew holes in prized plants, but as a group they are the single most important food that lets garden birds raise their young, so a garden that supports plenty of caterpillars supports plenty of birds. A patch of nettles or native shrubs left for caterpillars, and a mulched, undug bed for worms, both pay back.