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

Anatomy

Caterpillars have long segmented bodies. They have six true legs on three thoracic segments, and 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. Sawfly larvae (Hymenoptera) superficially resemble caterpillars, but can be distinguished because they have prolegs on every abdominal segment, whereas the true caterpillar does not. Another difference is that lepidopteran caterpillars have crochets or hooks on the prolegs. Thus, true caterpillars always have a few abdominal segments lacking prolegs, in some cases several, or even all segments may lack them. 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.

Both caterpillars and sawfly larvae have soft bodies that can grow rapidly between molts, like a balloon. Only the head capsule is hardened. In caterpillars, the mandibles are tough and sharp for chewing leaves; in most adult Lepidoptera mandibles are highly reduced or soft. Behind the mandibles are the spinnerets, for manipulating silk. Another distinctive characteristic of the caterpillar, is that the head has an upside down Y on the front (Scoble, 1995).

Like all insects, 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.

Caterpillars do not have very good eyesight. Rather than having fully-developed eyes they have a series of six tiny eyelets or 'stemmata' on each side of the lower portion of their head. They can probably form well focused, but poorly resolved images (Scoble, 1995). They move their heads from side to side probably as a means of aiding the resolution of objects, particularly plants. They rely on their short antennae to help them locate food. The larvae of sawflies differ from true caterpillars in this respect, as well, as they have prominent ocelli on the head capsule.

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 et al., 2001) by scraping against the leaf in a 'ritualized acoustic duel.' They detect the vibration of the leaf, not air-borne sounds. Similarly, cherry leaf rollers Caloptilia serotinella defend their rolls (Fletcher et al., 2006). Tent caterpillars can also detect vibrations at the frequency of wing beats of one of their natural enemies (Fitzgerald, 1995).

Defense

Many species of birds and animals consider caterpillars as protein rich food, and caterpillars have evolved various defenses. 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, or common objects in the environment, such as bird droppings. Many feed in leaf mines, ties, or rolls. Others will use startle or intimidation, with the sudden presentation of eyespots or snake mimicry.

Some caterpillars will regurgitate acidic digestive juices at attacking enemies. Others use the skunk-like defence of producing bad smells from extrudible glands: osmeteria.

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 (Scoble, 1995). However, some birds, like cuckoos, will swallow even the hairiest of caterpillars. The most aggressive defenses are those of the many caterpillars that have bristles associated with venom glands, called urticating hairs, and one of these is among the most potent defensive chemicals in any animals; e.g., the defensive chemical produced by the South American silk moth genus Lonomia is an anticoagulant of such potency that it can kill a human, causing them to hemorrhage to death (see Lonomiasis; Malaque et al. 2006) This chemical is being investigated for potential medical applications (i.e., to prevent blood clotting). Most urticating hairs range in effect from mild irritation to dermatitis.

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 coloured in black, red and yellow—the danger colours (see aposematism).

The aim of all these aggressive defense measures is to assure that any predator that eats (or tries to eat) one of them will avoid future attempts.

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 means of vibrations as well as chemical means (Travassos and Pierce, 2000).

Behaviour

Caterpillars have rightfully been called eating machines. They eat leaves voraciously, shed their skins generally four or five times, and eventually pupate into an adult form. Caterpillars have the fastest growth rate of any animal in the world.^{citation needed]} For instance, a tobacco hornworm will increase its own weight ten thousand times in less than twenty days. One of their adaptations that enables them to eat this 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.^{citation needed]} This mechanism is not found in any vertebrates.

Caterpillars are predominantly herbivores: 99% of species feed on plants (Pierce, 1995). A few feed on detritus including the clothes moth. Of those that are predatory, most 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, 1995). Some Hawai'ian caterpillars use techniques similar to spiders to hunt their prey of snails (Rubinoff and Haines, 2005).

External links

Larva Locomotion: A Closer Look —- video clips show how monarch larvae walk. More video clips from nature

3D animation Papilio polyxenes larvae walking.

Bevan Brownhill

References

• Fitgerald, TD. 1995. The tent caterpillars. Cornell Univ. Press.

• 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.

• 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

• Pierce, N.E. 1995. Predatory and parasitic Lepidoptera: Carnivores living on plants. Journal of the Lepidopterist's Society 49 (4): 412-453

• Rubinoff, Daniel and William P. Haines. 2005. Web-Spinning Caterpillar Stalks Snails. Science 309(5734): 575. DOI: 10.1126/science.1110397

• Scoble, MJ. 1995. The Lepidoptera: Form, function and diversity. Oxford Univ. Press.

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

• 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.

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