Caterpillar is the common name for the larvae of members of the order Lepidoptera, which is the insect order comprising butterflies and moths. Most of them are herbivorous, but some caterpillar species can also be insectivorous, even cannibalistic, while some feed on other animal products, such as clothes moths feeding on wool. These insects are known to be voracious feeders, being considered agricultural pests for a good reason. Many moth species damage fruits and other crops in their caterpillar stage. Some species of caterpillar are also highly poisonous, particularly those species that live in the tropical rain forests. Other species are only poisonous in their caterpillar form, meaning when they turn into a moth or butterfly, they no longer have venom. Caterpillars difference in size, color and appearance depending on their species. Some are very brightly colored where other caterpillar species are quite dull-looking in comparison. Some species are very hairy, where others are very smooth, but the main aim of their appearance is to intimidate their predators and to deter them from eating them.
Description and distribution of caterpillars
As mentioned above, caterpillars vary in appearance depending on their species. Many are found to be cryptically colored, in order to resemble the plants they feed on. Caterpillars can also mimic the plant’s appearance, by looking as though they have thorns. They can also mimic objects in their surrounding environment, such as bird droppings, as their size varies from as little as 1mm, up to 75 mm.
Their color can vary from yellow, to black, orange, red, green, blue, white or brown, however, the brightly colored ones are usually poisonous. Beside the color they have, caterpillars use other defensive mechanisms against their predators, such as poison, blending into their surroundings, emitting bad smells and being able to trick their predators into thinking they are actually seeing the head of a snake.
When it comes to their anatomy, the body of a caterpillar consists of a segmented body. This is split into three parts: a head, a thorax, where three pairs of jointed legs with hooks can be found, and an abdomen, where five pairs of stumpy prolegs are usually located. Its primary function is to eat and grow in preparation for pupating, as the caterpillar is the larval stage of butterflies and moths. Many species only eat the leaves of a single type of plant and they will spend most of their time eating, as they have powerful jaws that are meant to serve this purpose. These well-developed jaws have very sharp cutting surfaces that easily chop leaves, as the caterpillar bites off plant material and tears it into small, easily digestible pieces. Underneath the mandible are the two maxillae, smaller mouth parts that guide the food into the mouth. The caterpillar holds onto the food with its thoracic legs. These legs will become the legs of the adult, while the prolegs will disappear in the adult stage. On their head, they also have the oceli, which are simple eyes that detect light. Their touch sense is well developed, as their full body is covered by setae, which are sensory hairs, giving the insect a very developed sense of touch.
The distribution of caterpillars covers all continents, except for Antarctica, as the Lepidoptera are among the most successful groups of insects. They can be found in habitats ranging from desert to rainforest, from lowland grasslands to montane plateaus, but almost always associated with higher plants. With roughly 174,250 lepidopteran species described, the vast majority can be found in the tropics, but they are also spread on the rest of the continents, with North America having over 700 species of butterflies and over 11,000 species of moths, and Australia about 400 species of butterflies and 14,000 species of moths. They can be mostly found in orchards, and will feed off trees like: apple trees, cherry trees, pear trees, but even on hardwood trees, like oak, beech, linden, elm, birch, hawthorn, hazel and horn-beam. they even attack currants and gooseberries.
Behavior and defense mechanisms
A caterpillar’s vast food intake makes it an attractive meal, so many species use camouflage to protect themselves from predators. As many animals feed on caterpillars, due to the fact that they are rich in protein, they have developed throughout the years several defense mechanisms. Besides those meant to protect them from predators, they have also evolved defenses against physical conditions such as cold, hot or dry environmental conditions.
- Some caterpillars are poisonous to predators. They get their toxicity from the plants they eat. Generally, the brightly colored larva are poisonous; their color is a reminder to predators about their toxicity. Some poisonous caterpillars include the Monarch and the Pipevine Swallowtail.
- Some caterpillars blend into their surroundings extraordinarily well. Many are a shade of green that matches their host plant. Others look inedible objects, like bird droppings, such as the young Tiger Swallowtail larva.
- Some caterpillars have eyespots that make them look like a bigger, more dangerous animal, like a snake.
- Some caterpillars encase themselves in a folded leaf or other hiding place.
- Some caterpillars can emit very bad smells to ward off predators. They have an osmeterium, an orange, y-shaped gland on their neck which gives off a strong, unpleasant odor when the caterpillar is threatened. This keeps away dangerous wasps and flies that try to lay eggs in the caterpillar.
The chemical defenses are mostly represented by the spiny bristles or long fine hair-like setae with detachable tips, that are meant irritate by lodging in the skin or mucous membranes of the attacker. Some other species use the plants they feed on, by extracting toxins from that render them unpalatable to most of their predators. Even though some plants contain toxins which are meant to protect them from herbivores, 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 most aggressive caterpillar defenses are bristles associated with venom glands. These bristles are called urticating hairs and the venom they produce can act like an anticoagulant powerful enough to cause a human to hemorrhage to death. This can be found in the case of the South American silk moth.
When it comes to their behavior, caterpillars can be considered eating machines, as they constantly feed, in order to prepare themselves for the next stage in their metamorphosis as a butterfly. Most species shed their skin 3-4 times due to the rapid growth of their bodies. They begin their life by eating their way out of their egg shell, and then devouring the plant they are standing on. They go through the leaves of their host plant voraciously, thing which makes them be seen as great agricultural pests. As mentioned before, most of the species are herbivorous, with a diet normally restricted to just one species of plant. But there are also species that feed on eggs of other insects, ant larvae, aphids or scale insects. Larger species are also known to use their silk to form traps and capture snails.
They are mostly nocturnal, hiding at the base of the plants during the day, and feeding at night. A caterpillar spends most of its life crawling on and devouring its food source. But when it’s time to become an adult, most caterpillars start to wander away from what they’ve been eating. They find a sheltered, safe spot in which to pupate, action which takes place inside a protective shell known as a chrysalis.
Caterpillars as pests
Even though most of the butterflies and moths that emerge from the chrysalis are beneficial (except for the clothes moths), the caterpillars on the other hand, can cause much damage through their intensive diet. Especially as they prefer dieting on a single specie of plants, caterpillar can cause a lot of damage to mono-cultural crops. Swarms of gregarious caterpillars can do more harm than good, completely defoliating large swaths of forest and causing the death of trees. The hair-like setae of some species can cause allergies and extreme irritation in animals and humans, sometimes so severe that it requires hospitalization.
On the plants that are attacked, you will be able to see the caterpillars, they crawl and feed off the leaves, buds and fruit. The fruit will have holes in them, that look like spoons, making them inedible, useless for selling and thus a lost product that can economically affect your annual crop.
Unfortunately, many species have become resistant to the pesticides used against them, and even if plants develop mechanisms of resistance to being eaten by caterpillars, including the evolution of chemical toxins and physical barriers such as hairs, these approaches are defeated over time by the evolution of resistance mechanisms in the insects.
They can also pose danger to the human health, as caterpillar hairs containing venom can cause injuries ranging from urticarial dermatitis and atopic asthma, renal failure, and intracerebral hemorrhage. The hair easily enters 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. The most common effects are the skin rashes that appear after a contact with the caterpillar, which is actually a form of allergic reaction.
When dealing with caterpillars in the form of on infestation, you need to be carful and wear gloves and protective gear at all time. Removing tents or nests from plants must be done carefully, as to avoid any irritation or allergic reactions caused by your contact with the insects. The best thing you can do is try and prevent these infestations, by looking for egg masses in the fall and destroying them before they are able to hatch. However, if this is too late, try using insecticides that might help keep the population under control. Even though the larvae are very resistant to the pesticides, you may affect the adults that are preparing to lay the next generation of eggs, thus slowing down the growth and slowly eradicating them.