Caterpillar

A caterpillar is the larval stage of insects in the order Lepidoptera. This order includes all butterflies and moths.

While most people use the term for these specific larvae, the name is somewhat arbitrary. Some larvae from the suborder Symphyta, known as sawflies, are also called caterpillars. Both groups have an eruciform body shape, which means they have a worm-like body. These creatures are vital to ecosystems and human economies. They can be serious agricultural pests, but they are also sources of silk and food.

Caterpillars are often described as eating machines because they grow so rapidly. Most species are herbivores that consume plant material like leaves. However, about 1% of species are predatory and eat other insects. Some are even cannibalistic. Other caterpillars eat animal products, such as wool or the horns of dead ungulates.

To support this massive growth, caterpillars have a specialized midgut. This organ quickly transports ions to the midgut cavity, or lumen. This process keeps potassium levels higher in the cavity than in the hemolymph, which is the insect's blood. This mechanism allows them to process food at an incredible rate. For example, a tobacco hornworm can increase its weight ten-thousandfold in less than twenty days.

Growth requires a caterpillar to shed its skin, a process called moulting. Because their bodies are soft, they must expand several times to accommodate their increasing size. Most caterpillars shed their skin four or five times before they reach the pupal stage.

Scientists can distinguish Lepidoptera caterpillars from sawfly larvae by looking at specific anatomy. Sawfly larvae have six or more pairs of prolegs on their abdominal segments. In contrast, Lepidopteran caterpillars have a maximum of five pairs. Caterpillars also possess crochets, which are tiny hooks on their prolegs, while sawflies do not. Additionally, caterpillars usually have twelve stemmata, or simple eyes, whereas sawflies have only two.

Evolution has provided caterpillars with diverse ways to survive predators. Many use camouflage, or cryptic coloration, to blend into their surroundings. The species Nemoria arizonaria is a great example of this. If they hatch in the spring to eat oak catkins, they appear green. If they hatch in the summer, they appear dark to match oak twigs.

Other caterpillars use aposematism, which is the use of bright colors to warn predators they are dangerous. Species like the monarch or cinnabar moth use red, yellow, and black stripes to advertise their toxicity. Some caterpillars even acquire toxins from their host plants and sequester them in their own bodies.

Some caterpillars use much more aggressive physical defenses. Many species have spiny bristles or long, fine hairs called setae. Some of these have detachable tips that irritate the skin of a predator. The most extreme version is urticating hairs, which are bristles connected to venom glands. The South American silk moth genus Lonomia produces an anticoagulant venom so potent it can cause humans to hemorrhage.

Other caterpillars use chemical or sound-based defenses. Some produce bad smells from glands called osmeteria, like the giant swallowtail. One species, Amorpha juglandis, can even make high-pitched whistles to frighten birds.

Social behaviors also play a role in caterpillar survival. Some species are gregarious, meaning they live in large groups. This can help reduce the risk of being eaten by a single predator. The pine processionary caterpillars are known for linking together into long trains to move through trees.

Other caterpillars, such as the Lycaenid butterflies, form relationships with ants. They provide food rewards to the ants in exchange for protection. Some caterpillars also hide by living inside silk galleries or by mining between leaf surfaces.

Our understanding of these insects is supported by a long history of study and fossil discovery. The word "caterpillar" appeared in the early 16th century, likely derived from Middle English words meaning "hairy cat."

We can also trace their history through the fossil record. A geometrid moth caterpillar named Eogeometer vadens was found in Baltic amber, dating to the Eocene epoch about 44 million years ago.

Even older fossils have been found in Lebanese amber, dating back approximately 125 million years. These fossils show that these specialized life stages have existed for a very long time.