The Caterpillar or Larva

A caterpillar is an eating machine. Its consists of a pair of jaws or mandibles for chewing plant matter followed by a long gut for digestion. It moves using three pairs of true legs (like all insects) and five further pairs of 'prolegs', sucker like structures with hooks on the end for gripping hold of the leaves and stems. Along the side of the larva are small openings, spiracles, nine pairs in all, through which respiration occurs. A modified set of salivary glands, spinnerets, produce silk, which is discussed later on this page. All butterfly larvae are hairy, some quite spectacularly covered with bushes of setae, they may well be off-putting to potential predators.

When first hatched the larva or caterpillar is very small indeed, just a few millimeters long. These first instar larvae look similar regardless of which species they belong to. Usually the caterpillar immediately searches out food and starts to eat, although some species overwinter at this stage.

Polygonia c-album first Instar. First instar shedding skin. Due to the nature of the skeleton of insects they cannot grow in the same way that we do. Every so often the caterpillar sheds its skin so that it can expand and grow to a larger size. This process is known as ecdysis and each time it happens the caterpillar moves on to a new instar. Most European species molt four times and so their final stage is usually the fifth instar.
Caterpillars feed for a large part of their time, consuming an ever increasing amount of foodplant as they get rapidly larger. Some species prefer the cover of night to avoid unwanted attention, the Comma, Polygonia c-album, spends most of its time underneath leaves for the same reason. Their excrement, usually called frass, is dropped all over the place in small lumps. You are what you eat! The larva of the Large white, Pieris brassicae, pictured left demonstrates where the frass gets its colour from.

Some butterfly species actually have a system of catapulting it away to deter predators, for example the Small skipper, Thymelicus sylvestris. The White admiral, Limenitis camilla actually covers itself in its own frass as disruptive camouflage to confuse predators.


Comma larva side view.

Two views of the fully grown Comma larva, Polygonia c-album.

The right view shows the disruptive colouring. Which end would you go for if you were a bird?

Comma larva top view.



Caterpillars produce a silken thread from organs beside their jaws. This is used for a variety of purposes. It gives the caterpillars a good hold on their foodplant and some use it to rest between bouts of feeding

Others, for example Small tortoiseshell (Aglais urticae) and Peacock (Inachis io) build a web in which the young caterpillars live together, presumably for defensive purposes, only dispersing when near fully grown.

Communal Small tortoisehsell larvae.


Silk pad with larva attached. When a caterpillar is fully grown it takes time to wander in search of a suitable pupation site. This stage is sometimes known as the prepupa. The larva will let all frass clear its system before pupation.

Different families pupate in different ways. A Nymphalid (left) spins a silken pad and hangs head down using its anal claspers to grip on. A Pierid (right) however spins a pad then attaches itself with head upwards, spinning a silken girdle for support.

A short while after the larva has attached itself the change to a pupa begins. It is thought a hormone is introduced into the system to begin this process.

Large white girdle

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