Caterpillar

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A caterpillar is the larval form of a member of the order Lepidoptera (the insect order comprising butterflies and moths). They are mostly phytophagous in food habit, with some species being entomophagous. They grow through a series of moults and each intermediate stage is called an instar. They are voracious feeders and many of them are considered pests in agriculture. Many moth species are better known in their caterpillar stages because of the damage they cause to fruits and other agricultural produce.

[edit] Morphology

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.

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.^[1] The larvae of sawflies differ also in having prominent ocelli on the head capsule.

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.

Caterpillars have 4,000 muscles (the human being has only 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.

Caterpillars do not have good vision. They have a series of six tiny eyelets or 'stemmata' on each side of the lower portion of their head. These can probably form well focused, but poorly resolved images.^[1] They move their heads from side to side probably as a means of judging distance of objects, particularly plants. They rely on their short antennae 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,^[2] 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.^[3] Tent caterpillars can also detect vibrations at the frequency of wing beats of one of their natural enemies. ^[4]

[edit] Defense

Many species of animals feed on caterpillars as they are protein rich, and caterpillars in turn have evolved various defenses. 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 have a body coloration that closely resembles their food plant, or common objects in the environment, such as bird droppings or twigs. Many feed in leaf mines, ties, rolls or silk galleries. Others will use startle or intimidation, with the sudden presentation of eyespots or snake mimicry. Some species use a silk line and drop off the branches when disturbed.

Some caterpillars will regurgitate acidic digestive juices at attacking enemies. Many papilionid larvae produce bad smells from extrudable glands called 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.^[1] 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 is produced by the South American silk moth genus Lonomia. The defensive chemical is an anticoagulant of such potency that it can kill a human, causing them to hemorrhage to death (See Lonomiasis).^[5] This chemical is being investigated for potential medical applications. Most urticating hairs however 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 is sequestered in their body, making them highly toxic. 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, are usually advertised with 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 learn and 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 vibrations as well as chemical means ^[6] and typically provide food rewards.

Some caterpillars are gregarious and such aggregations are believed to help in reducing the levels of parasitization and predation.^[7] Clusters amplify the signal of aposematic coloration, and individuals may participate in group regurgitation or displays.

[edit] Behavior

Caterpillars have rightly been called eating machines. They eat leaves voraciously, shed their skins generally four or five times, and eventually pupate into an adult form. Measured by weight increase, caterpillars have a very fast growth rate. 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.^[8]

Caterpillars are predominantly herbivores: 99% of species feed on plants. 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.^[9]

Some Hawai'ian caterpillars (Hyposmocoma molluscivora) use silk traps to capture snails.^[10]

Many caterpillars are nocturnal. For example, the notorious "cutworms" (of the Noctuidae family) hide at the base of the plant 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.

[edit] Economic importance

Caterpillars are of immense economic importance. They can both cause damage and help in production. The silkworm is a caterpillar which has spawned an entire industry. Species like the Cotton bollworm are known to cause enormous losses to farmers. They 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 the caterpillar targets.

Host plants themselves evolve mechanisms of resistance to herbivory by caterpillars and these include 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.