As with most common names, the application of the word is arbitrary, since the larvae of sawflies commonly are called caterpillars as well. Both lepidopteran and symphytan larvae have eruciform body shapes.
Caterpillars of most species are herbivorous (folivorous), but not all; some (about 1%) are insectivorous, even cannibalistic. Some feed on other animal products; for example, clothes moths feed on wool, and horn moths feed on the hooves and horns of dead ungulates.
Caterpillars are typically voracious feeders and many of them are among the most serious of agricultural pests. In fact many moth species are best known in their caterpillar stages because of the damage they cause to fruits and other agricultural produce, whereas the moths are obscure and do no direct harm. Conversely, various species of caterpillar are valued as sources of silk, as human or animal food, or for biological control of pest plants.
The origins of the word "caterpillar" date from the early 16th century.
The inchworm, or looper caterpillars from the family Geometridae are so named because of the way they move, appearing to measure the earth (the word geometrid means earth-measurer in Greek); the primary reason for this unusual locomotion is the elimination of nearly all the prolegs except the clasper on the terminal segment.
Caterpillars have soft bodies that can grow rapidly between moults.
Lepidopteran caterpillars can be differentiated from sawfly larvae by:
- the numbers of pairs of pro-legs; sawfly larvae have 6 or more pairs while caterpillars have a maximum of 5 pairs.
- the number of stemmata (simple eyes); the sawfly larvae have only two, while caterpillars usually have six.
- the presence of crochets on the prolegs; these are absent in the sawflies.
- sawfly larvae have an invariably smooth head capsule with no cleavage lines, while lepidopterous caterpillars bear an inverted "Y" or "V" (adfrontal suture).
Many animals feed on caterpillars as they are rich in protein.
Caterpillars have evolved defenses against physical conditions such as cold, hot or dry environmental conditions.
The appearance of a caterpillar can often repel a predator: its markings and certain body parts can make it seem poisonous, or bigger in size and thus threatening, or non-edible.
More aggressive self-defense measures are taken by some caterpillars.
Plants contain toxins which protect them from herbivores, but some caterpillars have evolved countermeasures which enable them to eat the leaves of such toxic plants.
Some caterpillars regurgitate acidic digestive juices at attacking enemies.
Many caterpillars display feeding behaviors which allow the caterpillar to remain hidden from potential predators.
Some caterpillars, like early instars of the tomato hornworm and tobacco hornworm, have long "whip-like" organs attached to the ends of their body. The caterpillar wiggles these organs to frighten away flies and predatory wasps. Some caterpillars can evade predators by using a silk line and dropping off from branches when disturbed. Many species thrash about violently when disturbed to scare away potential predators. One species (Amorpha juglandis) even makes high pitched whistles that can scare away birds.
Some caterpillars obtain protection by associating themselves with ants. The Lycaenid butterflies are particularly well known for this. They communicate with their ant protectors by vibrations as well as chemical means and typically provide food rewards.
Some caterpillars are gregarious; large aggregations are believed to help in reducing the levels of parasitization and predation. Clusters amplify the signal of aposematic coloration, and individuals may participate in group regurgitation or displays. Pine processionary (Thaumetopoea pityocampa) caterpillars often link into a long train to move through trees and over the ground. The head of the lead caterpillar is visible, but the other heads can appear hidden. Forest tent caterpillars cluster during periods of cold weather.
Caterpillars suffer predation from many animals.
Caterpillars have been called "eating machines", and eat leaves voraciously.
Most caterpillars are solely herbivorous. Many are restricted to feeding on one species of plant, while others are polyphagous. Some, including the clothes moth, feed on detritus. Some are predatory, and may prey on other species of caterpillars (e.g. Hawaiian Eupithecia). Others feed on eggs of other insects, aphids, scale insects, or ant larvae. A few are parasitic on cicadas or leaf hoppers (Epipyropidae). Some Hawaiian caterpillars (Hyposmocoma molluscivora) use silk traps to capture snails.
Many caterpillars are nocturnal.
Caterpillars cause much damage, mainly by eating leaves.
Plants evolve mechanisms of resistance to being eaten by caterpillars, including the evolution of chemical toxins and physical barriers such as hairs.
Some caterpillars are used in industry.
Caterpillar hair can be a cause of human health problems.
Caterpillar hair has also been known to cause kerato-conjunctivitis. The sharp barbs on the end of caterpillar hairs can get lodged in soft tissues and mucous membranes such as the eyes. Once they enter such tissues, they can be difficult to extract, often exacerbating the problem as they migrate across the membrane.
This becomes a particular problem in an indoor setting.
Caterpillars are a food source in some cultures.
In popular culture
In the Old Testament of the Bible caterpillars are feared as pest that devour crops. They are part of the "pestilence, blasting, mildew, locus" because of their association with the locust, thus they are one of the plagues of Egypt. Jeremiah names them as one of the inhabitants of Babylon. The English word caterpillar derives from the old French catepelose (hairy cat) but merged with the piller (pillager). Caterpillars became a symbol for social dependents. Shakespeare's Bolingbroke described King Richard's friends as "The caterpillars of the commonwealth, Which I have sworn to weed and pluck away". In 1790 William Blake referenced this popular image in The Marriage of Heaven and Hell when he attacked priests: "as the caterpillar chooses the fairest leaves to lay her eggs on, so the priest lay his curse on the fairest joys".
The role of caterpillars in the life stages of butterflies was badly understood.
Butterflies were regarded as symbol for the human soul since ancient time, and also in the Christian tradition. Goedart thus located his empirical observations on the transformation of caterpillars into butterflies in the Christian tradition.
Since then the metamorphoses of the caterpillar into a butterfly has in Western societies been associated with countless human transformations in folktales and literature.
Famously, in Lewis Carroll's Alice's Adventures in Wonderland a caterpillar asks Alice "Who are you?". When Alice comments on the caterpillar's inevitable transformation into a butterfly, the caterpillar champions the position that in spite of changes it is still possible to know something, and that Alice is the same Alice at the beginning and end of a considerable interval. When the Caterpillar asks Alice to clarify a point, the child replies "I'm afraid I can't put it more clearly... for I can't but understand it myself, to begin with, and being so many different sizes in a day is very confusing". Here Carroll satirizes René Descartes, the founder of Cartesian philosophy, and his theory on innate ideas. Descartes argued that we are distracted by urgent bodily stimuli that swamp the human mind in childhood. Descartes also theorised that inherited preconceived opinions obstruct the human perception of the truth.
More recent symbolic references to caterpillars in popular media include the Mad Men season 3season 3]] The Sopranos season 5 The Test Dream Tony Soprano Ralph Cifaretto has a caterpillar on his bald head that changes into a butterfly.