Anatomy

Most caterpillars have tubular, segmented bodies. They have three pairs of true legs on the three thoracic segments, up to four pairs of prolegs on the middle segments of the abdomen, and often a single pair of prolegs on the last abdominal segment. There are ten abdominal segments. The families of lepidoptera differ in the numbers and positioning of the prolegs.

Caterpillars grow through a series of moults; each intermediate stage is called an instar.

Like all insects, caterpillars breathe through a series of small openings along the sides of their thorax and abdomen called spiracles. These branch into the body cavity into a network of tracheae. A few caterpillars of the family Pyralidae are aquatic and have gills that let them breathe underwater.Berg, Clifford O. 1950. Biology of Certain Aquatic Caterpillars (Pyralididae: ''Nymphula'' spp.) Which Feed on ''Potamogeton''. Transactions of the American Microscopical Society. 69(3):254-266

Caterpillars have 4,000 muscles (compare humans, with 629). They move through contraction of the muscles in the rear segments pushing the blood forward into the front segments elongating the torso. The average caterpillar has 248 muscles in the head segment alone.

Senses

to help them locate food.

Some caterpillars are able to detect vibrations, usually at a highly specific frequency. Caterpillars of the common hook-tip moth, ''Drepana arcuata'' (Drepanoidea) produce sounds to defend their silk nests from members of their own species,Yack JE, Smith ML, and Weatherhead PJ. 2001. Caterpillar talk: Acoustically mediated territoriality in larval Lepidoptera. Proceedings of the National Academy of Sciences of the United States of America 98 (20): 11371–11375. by scraping against the leaf in a ritualized acoustic duel. They detect the vibrations conducted by the plant and not air-borne sounds. Similarly, cherry leaf rollers ''Caloptilia serotinella'' defend their rolls.Fletcher LE, Yack JE, Fitzgerald TD, Hoy RR . 2006. Vibrational communication in the cherry leaf roller caterpillar ''Caloptilia serotinella'' (Gracillarioidea : Gracillariidae). Journal of Insect Behavior 19 (1): 1–18. Tent caterpillars can also detect vibrations at the frequency of wing beats of one of their natural enemies. Fitgerald, TD. 1995. The tent caterpillars. Cornell Univ. Press.

Classification

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); 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, like balloons, between moults. Only the head capsule is hardened. In caterpillars, the mandibles are tough and sharp for chewing leaves; in most adult Lepidoptera, the mandibles are highly reduced, or soft. Behind the mandibles of the caterpillar are the spinnerets, for manipulating silk.

Some larvae of the Hymenoptera (ants, bees and wasps) order can appear like the caterpillars of the lepidoptera. These are mainly seen in the Sawfly family and while the larvae superficially resemble caterpillars, they can be distinguished by the presence of prolegs on every abdominal segment. Another difference is that lepidopteran caterpillars have crochets or hooks on the prolegs while these are absent on the sawfly larvae. Also in lepidopteran caterpillars is the upside down Y shaped suture on the front of the head.Scoble, MJ. 1995. The Lepidoptera: Form, function and diversity. Oxford Univ. Press. The larvae of sawflies differ also in having prominent ocelli on the head capsule.

Appearance

Many caterpillars are cryptically coloured and resemble the plants on which they feed on and may even have parts that mimic plant parts such as thorns. Some look like objects in the environment such as bird droppings. Many feed enclosed inside silk galleries, rolled leaves or by mining between the leaf surfaces.

Some caterpillars have large false eyes towards the rear of their abdomen. This helps convince predators that their back is actually their front, giving them an opportunity to escape when attacked. Others imitate the appearance and behaviour of snakes by raising their heads and startling potential predators by displaying eye spots.

More aggressive self-defense measures are taken by hairy caterpillars. These caterpillars have spiny bristles or long fine hair-like setae with detachable tips that will irritate by lodging in the skin or mucous membranes. However, some birds, like cuckoos, will swallow even the hairiest of caterpillars. The most aggressive defenses are bristles associated with venom glands, called urticating hairs; a venom among the most potent defensive chemicals in any animals is produced by the South American silk moth genus ''Lonomia''. It is an anticoagulant powerful enough to cause a human to hemorrhage to death (See Lonomiasis).Malaque, Ceila M. S., Lúcia Andrade, Geraldine Madalosso, Sandra Tomy, Flávio L. Tavares, And Antonio C. Seguro. 2006. A case of hemolysis resulting from contact with a ''Lonomia'' caterpillar in southern Brazil. Am. J. Trop. Med. Hyg., 74(5): 807-809 This chemical is being investigated for potential medical applications. Most urticating hairs however range in effect from mild irritation to dermatitis.

Plants have evolved poisons to protect themselves from herbivores and some caterpillars have evolved countermeasures and eat the leaves of these toxic plants. In addition to being unaffected by the poison, they sequester it in their body, making them highly toxic to predators. These chemicals are also carried on into the adult stages. These toxic species, such as the Cinnabar moth (''Tyria jacobaeae'') and monarch (''Danaus plexippus'') caterpillars, usually advertise themselves with brightly striped or coloured in black, red and yellow—the danger colours (see aposematism). Any predator that attempts to eat a caterpillar with an aggressive defence mechanism will learn and avoid future attempts.

Some caterpillars regurgitate acidic digestive juices at attacking enemies. Many papilionid larvae produce bad smells from extrudable glands called osmeteria.

Caterpillars can evade predators by using a silk line and dropping off from branches when disturbed.

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 Travassos, MA, and NE Pierce. 2000. Acoustics, context and function of vibrational signalling in a lycaenid butterfly–ant mutualism. Animal Behaviour 60: 13-26 and typically provide food rewards.

Some caterpillars are gregarious; large aggregations are believed to help in reducing the levels of parasitization and predation.Entry, Grant L. G., Lee A. Dyer. 2002. On the Conditional Nature Of Neotropical Caterpillar Defenses against their Natural Enemies. Ecology, 83(11):3108–3119 Clusters amplify the signal of aposematic coloration, and individuals may participate in group regurgitation or displays.

Behavior

Caterpillars have been called "eating machines". They eat leaves voraciously, most species shed their skins four or five times as their bodies grow larger, and they eventually pupate into an adult form. Caterpillars grow very fast; for instance, a tobacco hornworm will increase its weight ten-thousand-fold in less than twenty days. An adaptation that enables them to eat so much is a mechanism in a specialized midgut which transports ions at a very high rate to the lumen (midgut cavity), to keep the potassium level higher in the midgut cavity than in the blood.Chamberlin, M.E. and M.E. King (1998). Changes in midgut active ion transport and metabolism during the fifth instar of the tobacco hornworm (Manduca sexta). J.Exp.Zool. 280:135-141.

99% of caterpillars are solely herbivorous. A few, including the clothes moth, feed on detritus. Most predatory caterpillars feed on eggs of other insects, aphids, scale insects, or ant larvae. Some are cannibals, and others prey on caterpillars of other species (e.g. Hawai'ian ''Eupithecia'' ). A few are parasitic on cicadas or leaf hoppers.Pierce, N.E. 1995. Predatory and parasitic Lepidoptera: Carnivores living on plants. Journal of the Lepidopterist's Society 49 (4): 412-453 Some Hawai'ian caterpillars (''Hyposmocoma molluscivora'') use silk traps to capture snails.Rubinoff, Daniel and William P. Haines. 2005. Web-Spinning Caterpillar Stalks Snails. Science 309(5734): 575. DOI: 10.1126/science.1110397

Many caterpillars are nocturnal. For example, the notorious "cutworms" (of the Noctuidae family) hide at the base of plants during the day and only feed at night. Others, such as gypsy moth (Lymantria dispar) larvae, change their activity patterns depending on density and larval stage, with more diurnal feeding in early instars and high densities.

Economic effects

Caterpillar cause much damage, mainly by destroying leaves. The cotton bollworm causes enormous losses. Other species eat food crops. Caterpillars have been the target of pest control through the use of pesticides, biological control and agronomic practices. Many species have become resistant to pesticides. Bacterial toxins such as those from ''Bacillus thuringiensis'' which are evolved to affect the gut of lepidoptera have been used in sprays of bacterial spores, toxin extracts and also by incorporating genes to produce them within the host plants. These approaches are defeated over time by the evolution of resistance mechanisms in caterpillars.

Plants evolve mechanisms of resistance to herbivory by caterpillars, including the evolution of chemical toxins and physical barriers such as hairs. Incorporating host plant resistance (HPR) through plant breeding is another approach used in reducing the impact of caterpillars on crop plants.

Some caterpillars are beneficial. The silk industry is due to the silkworm, which is a caterpillar.