A caterpillar is the larval form of a lepidopteran (a member of the insect order comprising butterflies and moths).
Caterpillars have long segmented bodies and many sets of "legs". They eat leaves voraciously, grow rapidly, shed their skins generally four or five times, and eventually pupate into an adult form.
A brightly-coloured Monarch caterpillar
Caterpillars have six true legs (being hexapods) on the thorax and eight prolegs on the abdomen. The sawfly larva (Hymenoptera) resembles a caterpillar, but they can be distinguished because the caterpillar has a gap between true legs and prolegs, whereas the sawfly does not.
Caterpillars do not breathe through their mouths. 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 airtubes or 'tracheae'.
Caterpillars do not have very good eyesight or senses. Rather than having 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.
Many species of birds and animals consider caterpillars to be a tasty protein snack, so the caterpillars have evolved several methods of protecting and/or camouflaging themselves. These methods can be either passive, aggressive, or both. 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-defence 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.
A final grouping of 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 Cinnabar (Tyria jacobaeae) caterpillars, are brightly striped or coloured in red and yellow - the 'danger' colours.
The aim of all these aggressive defense measures is to assure that any predator that eats (or tries to eat) one of them won't be in a hurry to repeat the experience.