A caterpillar is the larval form of a lepidopteran (a member of the insect order comprising butterflies and moths).

Caterpillars are characterized by their long segmented bodies and many sets of “legs”. They eat voracious leaf eaters and grow rapidly, During growth the caterpillar will shed its skin four to five times before pupating into its adult form.

Caterpillars have six true legs (being hexapods) on the thorax, up to four pairs of prolegs on the middle segments of the abdomen, and sometimes a single pair of prolegs on the last abdominal segment. The sawfly larva (Hymenoptera) superficially resembles a caterpillar, but can usually be distinguished because the caterpillar has a gap between true legs and prolegs, whereas the sawfly does not. Another difference is that lepidopteran caterpillars have crochets or hooks on the prolegs. The gap between the prolegs and the true legs can vary from a slight gap in some species to a large gap in families such as the geometridae. The geometrids, also known as inchworms or loopers, are so named because of the way they locomote, appearing to measure the earth (the word ‘geometrid’ means ‘earth-measurer’ in Greek).

Caterpillars do not breathe through their mouths. Instead, air enters their bodies through a series of small tubules along the sides of their thorax and abdomen. These tubules are called ‘spiracles’, and inside the body they connect together into a network of air tubes or ‘tracheae’.

Caterpillars do not have very good eyesight or senses. Instead of fully-developed eyes they have a series of six tiny eyelets or ‘ocelli’ on the lower portion of their head. They rely on their antennae to help them locate food.

Caterpillars are a favorite food source for many species of birds and animals. Because of this the caterpillar has evolved several methods of protecting and/or camouflaging itself. These methods can be either passive, aggressive, or both in some instances. Some caterpillars have large ‘false eyes’ towards the rear of their abdomen. This is an attempt to convince predators that their back is actually their front, giving them an opportunity to escape to the ‘rear’ when attacked. Others have a body coloration that closely resembles their food plant.

More aggressive self-defense measures are taken by the spitfires and hairy caterpillars. These caterpillars have spiny bristles or long fine hairs that will irritate anything that brushes against them, or spit acidic digestive juices at potential enemies. However, some birds, like cuckoos, will swallow the hairiest of caterpillars.

Some caterpillars eat the leaves of plants that are toxic to other animals. They are unaffected by the poison themselves, but it builds up in their system, making them highly toxic to anything that eats one of them. These toxic species, such as the Cinnabar moth (Tyria jacobaeae) caterpillars, are brightly striped or colored in red and yellow – the ‘danger’ colors.

All of these aggressive defense measures are meant to assure that any predator will not be in a hurry to repeat the experience of attempting to eat a caterpillar.

Some caterpillars obtain protection by associating themselves with ants. The Lycaenid butterflies are particularly well known. Recent findings have shown that they communicate with their ant protectors by means of vibrations as well as chemical means.

Some caterpillars are considered serious pests of agriculture or forestry. They include the Small White butterfly (brassicas), the Pine Butterfly, and the Codling Moth (apples).

“Tiny, snail-eating caterpillars found in Hawaiian rain forests tie up their prey with sticky silk and snack on them at leisure. […] It is the first time that caterpillars that eat snails or any other mollusk have been found.” July 22, 2005

Other carnivorous species of caterpillars are also known, but still represent a tiny fraction of all known representatives of these insect larvae.