All the curiosities about caterpillars

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Caterpillars are the larval stage of butterflies and moths, both of which belong to the order Lepidoptera, and are simply called lepidopteran larvae in entomology. The term "caterpillar" derives from the Latin "Bruchus," meaning "grasshopper larva," and shares the same root as the verb "to browse," which refers to the act of chewing plants. Their growth symbolizes metamorphosis and renewal, but is also a classic example of holometabolism, a type of complete development where an insect transitions from egg to larva, then pupa, and finally adult. During their growth, caterpillars go through various molting stages, increasing in size: being a larva, the caterpillar spends all its time eating and digesting, to grow and eventually turn into a pupa (many species spend the winter as pupae) and subsequently an adult.

In Europe, you can find many species of caterpillars, from cities to the most untouched areas: indeed, hundreds of butterflies and thousands of moths coexist in Europe. The largest caterpillar is that of the Death's-head hawkmoth (Acherontia atropos), the famous moth from the film "The Silence of the Lambs": it can reach 12 cm! Among the most common is the Large White caterpillar (Pieris brassicae), a pest of crops often seen on plants like cabbages and broccoli, but also on many wild plants. One of the most beautiful species is the Pavonia moth caterpillar Saturnia pavonia, with bright colors and ring patterns meant to scare predators. Special mention goes to Papilio hospiton, a rare and endangered species, endemic to Sardinia and Corsica, and to the Phengaris arion, a rare caterpillar that lives in symbiosis with certain ants.

The most fascinating moment in the life of a caterpillar is its metamorphosis into a chrysalis. During this process, the caterpillar "liquefies" inside the cocoon: almost all its tissues disintegrate to form new organs and structures that previously did not exist. However, this process is governed in an extremely precise manner: the caterpillar's body already contains specialized groups of cells from birth, called imaginal discs, which later transform into adult parts. During the pupal stage, also known as chrysalis, these imaginal discs become structures like wings, compound eyes, or antennae. This sophisticated reconstruction process is common to many insects, including beetles, flies and hoverflies, wasps, and bees.

Caterpillars have a total of six true legs, which are located on the thorax and are jointed, like all insects. However, in addition to these, most caterpillars also have what are called prolegs, which are fleshy and unjointed appendages equipped with small hooks. These work somewhat like suction cups and allow them to anchor themselves to surfaces like leaves and branches. The number of prolegs varies depending on the family: Geometridae moth caterpillars, for example, have only two pairs on the terminal part of the body, while those from other butterfly families can have up to five pairs. The prolegs are essential for the caterpillar's movement and stability.

One of the most fascinating tools available to caterpillars is silk, the same silk for which Bombyx mori is bred, a species selected by humans for the quality of this fiber that becomes valuable when properly processed. These filaments have numerous uses, are composed of proteins, and are produced by specialized glands located near the mouth. Some species build true silk cocoons to protect themselves from predators, often during the chrysalis stage, while others use it to anchor themselves firmly to leaves or branches while feeding. The caterpillars of Lasiocampid moths wrap groups of leaves with silk, creating safe and protected hideouts. This ability to manipulate silk is a key adaptation that increases the survival chances of caterpillars during the vulnerable stages of their life.

Caterpillars have developed numerous strategies of mimicry and defense against predators. Some species are cryptic, meaning they have shapes and colors to help them blend into their environment, making them almost invisible on leaves and branches. Other caterpillars, on the other hand, adopt Batesian mimicry, imitating the appearance of dangerous animals. The Asian butterfly caterpillar Hebomoia glaucippe or even the moth Deilephila elpenor, for example, display colored patches that resemble snake eyes and often adopt poses reminiscent of a snake's head, an effective deterrent against predators. These ingenious defense mechanisms allow caterpillars to survive a large number of threats.

Many caterpillars defend themselves thanks to the toxicity acquired from the plants they feed on, often expressed with conspicuous colors that serve as a signal for predators. One of the most well-known examples is the Monarch butterfly caterpillar (Danaus plexippus), a species widespread throughout much of the American continent and that makes incredible migrations. Through feeding, the caterpillar accumulates toxic glycosides from the plants it consumes, making it toxic to predators. Some species are poisonous and stinging, like the notorious processionary, so dangerous that contact with them can cause severe reactions even in humans. Some tropical species like the Lonomia obliqua caterpillar can even cause death. This chemical defense mechanism is extremely effective against predators, who quickly learn to avoid brightly colored caterpillars, often associated with toxicity.

An interesting fact about caterpillars is that some are effective predators, a unique adaptation evolved by some caterpillars found on the Hawaiian islands. Unlike most caterpillars, which are herbivores, these caterpillars are active hunters and belong to the genus Eupithecia. They are characterized by pointed mandibles and a peculiar behavior; they are ambush predators and seize small arthropods, mainly flies, with their true legs while holding on with their prolegs. Living in an isolated environment with scarce plant resources suitable for their development, they have developed this strategy to survive. This behavior is a perfect example of evolutionary adaptation in response to environmental conditions.

Caterpillars represent one of the most interesting and varied larval stages in the animal kingdom. We often wonder what they really are for, but despite their often modest appearance, they contain an extraordinary diversity of forms, colors, behaviors, and particular evolutionary adaptations, overturning the normal conception we have. From species that skilfully camouflage themselves to those that mimic dangerous animals, from toxic caterpillars to predatory ones, these creatures demonstrate an extraordinary ability to survive. Although they are often considered merely a transitional phase to adulthood or sometimes a problem to be dealt with using insecticides, caterpillars play fundamental roles and the ecosystem services they offer make an important contribution to the stability of ecosystems. For these reasons and their biological complexity, they deserve special attention.