Invertebrates
Caterpillar
Lepidoptera · also called larva
A caterpillar is a butterfly or moth in its eating phase. Its entire purpose is to grow, storing up the energy the adult will spend on flight and reproduction, and it does this so single-mindedly that it can end up thousands of times heavier than when it hatched before sealing itself into a pupa and rebuilding its body.
Overview
A caterpillar is the larval stage of an insect in the order Lepidoptera, the butterflies and moths, so there are well over 150,000 kinds. It has a hard head with chewing jaws, three pairs of true legs, up to five pairs of gripping prolegs, breathing holes down the sides, and a silk gland by the mouth. Almost all caterpillars eat plant leaves, and most are restricted to one or a few food plants.
Nothing but growing
A caterpillar does little but feed, moulting its skin four to six times as it outgrows it, each stage called an instar. When it is fully grown it moults into a pupa, inside which its body is largely dissolved and rebuilt into a winged adult.
Defences, and value
Caterpillars hide as twigs or droppings, wear warning colours backed by borrowed toxins, carry stinging hairs, evert foul glands, or drop on a thread. As a group they are the engine of the food web, turning leaves into the protein-rich food that nesting songbirds depend on.
Status
Caterpillars are Not Evaluated as a stage, but butterflies and moths have declined sharply in many regions through habitat loss, pesticides, and light pollution, meaning fewer caterpillars and fewer of the birds that eat them.
Caterpillar: common questions
Is a caterpillar a worm?
No, though people often call them worms, as in inchworm, army worm, or silkworm. A worm, in the strict sense, is a soft-bodied animal in a completely different group, with no legs and no jointed body. A caterpillar is an insect larva, with a hard head capsule, chewing jaws, three pairs of jointed true legs, a body of clearly defined segments, breathing holes down the sides, and the fleshy prolegs that grip. Most importantly, a caterpillar is a temporary form, the growing stage of a butterfly or moth, whereas a worm stays a worm for life. The confusion is old and the informal names have stuck, but a caterpillar and an earthworm are only very distantly related.
How does a caterpillar turn into a butterfly?
When a caterpillar is fully grown it stops feeding, finds a safe spot, and moults one last time into a pupa, called a chrysalis in butterflies. Inside the pupal case the caterpillar's body is largely broken down, with many of its tissues digested into a nutrient soup, while small clusters of cells that were set aside during larval life, called imaginal discs, grow rapidly and rebuild the animal into an adult with wings, long legs, compound eyes, and a coiled feeding tube instead of chewing jaws. The nervous system is remodelled but retains enough continuity that some memories from the caterpillar can carry through. After days to months the adult splits the case, pumps up its wings, and flies.
Which caterpillars can sting or hurt you?
A minority, but the ones that can are worth knowing. Many caterpillars carry hairs or spines that are simply irritating, causing an itchy rash if they touch skin or are inhaled, and in Europe the oak processionary and browntail moth caterpillars are a genuine public-health nuisance for this reason. A smaller number have true venom delivered through hollow spines, and among these the puss caterpillar, or asp, of the southern United States is often rated one of the most painful insect stings in North America, and the saddleback, io moth, and buck moth caterpillars also sting sharply. Some tropical species, notably certain Lonomia moth caterpillars in South America, have caused deaths through a venom that disrupts blood clotting. The safe rule is to avoid touching any hairy or spiny caterpillar with bare skin.
Are caterpillars good or bad for the garden?
Both, depending on which and how many. A few species are serious pests, such as the cabbage white caterpillars that skeletonise brassicas, cutworms that fell seedlings at ground level, and the caterpillars of codling and other moths in fruit and vegetables. But the great majority of caterpillars cause little noticeable damage, and as a group they are one of the most valuable things in a garden, because they are the main food that lets birds raise their young. A garden managed to tolerate some leaf damage, with a range of native plants, no broad-spectrum insecticides, and some longer grass and shrubby cover, will support far more birds through the breeding season than a spotless one.