The Caterpillar

Jul 28, 2013 Features / Columnists, Interesting Creatures in Guyana

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Caterpillars are the larval form of members of the order Lepidoptera (the insect order comprising butterflies and moths). They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be 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.
Caterpillars have soft bodies that can grow rapidly between moults. Only the head capsule is hardened. The mandibles are tough and sharp for chewing leaves (this contrasts with most adult Lepidoptera, which have highly reduced or soft mandibles). Behind the mandibles of the caterpillar are the spinnerets, for manipulating silk.
Some larvae of the Hymenoptera order (ants, bees and wasps) can appear like the caterpillars of the lepidoptera. Such larvae are mainly seen in the sawfly family. However while these larvae superficially resemble caterpillars, they can be distinguished by the presence of pro-legs on every abdominal segment, an absence of crochets or hooks on the pro-legs (these are present on lepidopteran caterpillars), prominent ocelli on the head capsule, and an absence of the upside-down Y-shaped suture on the front of the head.
Caterpillars can be confused with the larvae of sawflies
Many animals feed on caterpillars as they are rich in protein. As a result caterpillars have evolved various means of defence. 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. Some types of caterpillars are indeed poisonous.
Caterpillars have evolved defences against physical conditions such as cold, hot or dry environmental conditions. Some Arctic species like Gynaephora groenlandica have special basking and aggregation behaviours apart from physiological adaptations to remain in a dormant state.
Many caterpillars are cryptically coloured and resemble the plants on which they feed. They may even have parts that mimic plant parts such as thorns. Their size varies from as little as one mm to about 75 millimetres (3.0 in). 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. Caterpillars of Nemoria arizonaria that grow in spring feed on oak catkins and appear green. The summer brood appear like oak twigs. The differential development is linked to the tannin content in the diet.
More aggressive self-defence measures are taken by some caterpillars. These measures include having 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 (such as cuckoos) will swallow even the hairiest of caterpillars. The most aggressive caterpillar defences are bristles associated with venom glands. These bristles are called urticating hairs. A venom which is among the most potent defensive chemicals in any animal is produced by the South American silk moth genus Lonomia.
Its venom is an anticoagulant powerful enough to cause a human to hemorrhage to death. This chemical is being investigated for potential medical applications. Most urticating hairs range in effect from mild irritation to dermatitis.
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. In addition to being unaffected by the poison, the caterpillars sequester it in their body, making them highly toxic to predators. The 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 the danger colours of red, yellow and black, often in bright stripes. 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.
Some caterpillars have long “whip-like” organs attached to the ends of their body. The caterpillar wiggles these organs to frighten away flies.
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 are gregarious; large aggregations are believed to help in reducing the levels of parasitisation and predation. Clusters amplify the signal of aposematic coloration, and individuals may participate in group regurgitation or displays.
The caterpillar suffers predation from a number of species. The European pied flycatcher is one species that preys upon caterpillars. The flycatcher typically finds caterpillars amongst oak foliage.
Caterpillars have been called “eating machines”, and eat leaves voraciously. Most species shed their skin four or five times as their bodies grow, and they eventually pupate into an adult form.
They grow very quickly; for instance, a tobacco hornworm will increase its weight ten-thousand-fold in less than 20 days.  An adaptation that enables them to eat so much is a mechanism in a specialized midgut that quickly transports ions to the lumen (midgut cavity), to keep the potassium level higher in the midgut cavity than in the blood.
Most caterpillars are solely herbivorous. Many are restricted to one species of plant, while others are polyphagous. 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 predatory, and others prey on caterpillars of other species. A few are parasitic on cicadas or leaf hoppers. Some Hawai’ian caterpillars (Hyposmocoma molluscivora) use silk traps to capture snails.
Many caterpillars are nocturnal. For example, the “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.
Plants evolve mechanisms of resistance to being eaten 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 used in industry. The silk industry is based on the silkworm caterpillar.
Caterpillar hairs have also been known to cause kerato-conjunctivitis. The sharp barbs on the end of caterpillar hairs can get lodged in soft tissues and mucus 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. The hairs easily enter buildings through ventilation systems and accumulate in indoor environments because of their small size, which makes it difficult for them to be vented out. This accumulation increases the risk of human contact in indoor environments.
(Source: Wikipedia – The Free Online Encyclopedia)

 

 

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