A caterpillar is the larval stage of insects in the order Lepidoptera, which includes all butterflies and moths. This form is dedicated almost entirely to consuming food and increasing body mass. This intense feeding enables the rapid accumulation of energy and nutrients required for the dramatic transformation into an adult winged insect. The vast majority of caterpillars are herbivores, meaning their diet consists of plant matter.
The Primary Food Source
The diet of nearly all caterpillars centers on the soft tissues of plants, with leaves being the most common source. Due to their remarkable ability to consume food relative to their size, they possess tough, sharp mouthparts called mandibles. These mandibles are perfectly adapted for shearing and grinding plant material.
The caterpillar’s primary goal is to grow exponentially before entering the pupa stage. A single larva, such as the tobacco hornworm, can increase its body weight by up to ten thousand times in less than twenty days. This intense feeding ensures the caterpillar stores enough reserves to fuel the complex process of metamorphosis, during which the insect does not eat.
The Importance of Host Plant Specialization
While many caterpillars are generalist feeders, consuming a wide variety of plants, a significant number of species rely on specific “host plants.” This specialization is driven by an evolutionary arms race between plants and insects, focused on plant chemical defenses. Plants protect themselves by synthesizing toxic compounds, such as alkaloids or cardiac glycosides, that are poisonous or unpalatable to most herbivores.
Specialized caterpillars have developed biochemical mechanisms to either tolerate or neutralize these toxins. The Monarch caterpillar, for example, feeds exclusively on milkweed plants, which contain potent cardiac glycosides. The Monarch larva resists this poison and sequesters the toxins within its body tissues. This makes the caterpillar chemically defended and bitter-tasting to predators like birds.
Caterpillars must also contend with physical defenses, such as the latex that oozes from a milkweed plant when bitten, or dense, hair-like structures called trichomes on leaf surfaces. Some larvae overcome these physical barriers by employing specific behavioral tactics. For instance, they may bite through the leaf’s main vein before feeding to drain the latex. This intimate and specific relationship between a caterpillar and its host plant dictates where the adult butterfly or moth will lay its eggs, ensuring the newly hatched larva has immediate access to its required food source.
Surprising Dietary Exceptions
Although herbivory is the rule, a minority of caterpillars have evolved to consume diets far removed from leaves and stems. These exceptions include carnivory and detritivory. Some species are ambush predators, like the Hawaiian inchworm caterpillars, which use specialized front legs to capture small insects that brush past them.
Other carnivorous caterpillars live a more hidden existence, such as the Harvester butterfly larva. It camouflages itself among colonies of woolly aphids, preying directly on them and using their waxy secretions for disguise. Some moth caterpillars are detritivores, feeding on decaying organic matter. Examples include clothes moth larvae that consume wool and horn moths that feed on the keratin found in the horns and hooves of dead animals.