The larvae of butterflies and moths look almost nothing like their parents. Caterpillars are fleshy with helmet-like head coverings made of chitin. In addition to 3 pairs of thoracic legs, 3-5 pairs of fleshy abdominal legs end in retractable hooks (crochets) that allow larvae to grab onto trees or other surfaces. A caterpillar hatches from an egg laid on a host plant chosen by its mother. It spends most of its time munching on the host plant- mainly leaves, but also stems, bark, or flowers. The caterpillar grows over a period of weeks, or in rare cases years, shedding its skin periodically (molting) like a shirt that has gotten too small. While growing, it employs defenses such as spines, irritating body hairs, or startling coloration to repel predators. A mature caterpillar activates the silk-making gland in its head and ejects silken threads through an organ (spinneret) on its lower lip to make a protective shelter. Whether a cocoon, a web, or a chrysalis, the caterpillar is able to use these as refuges while undergoing metamorphosis.

An insect has a brain, connected to bundles of nerves (ganglia) in each of its three body segments. Like us, they have sensory nerves that receive information from their environment and send it to the brain. Information comes from many types of touch receptors. Touch-sensitive hairs all over an insect's body are sensitive enough to detect vibrations in the air, such as from an approaching predator. Insect hairs also do chemical detection (chemoreception). Pores at the end of chemoreceptors on mouthparts or other body parts allow odors to reach the nervous system. Insect antennae may have thousands of chemoreceptors, used to detect chemicals in the air (pheromones) emitted by members of the opposite sex. An insect, like us, has a pair of eyes on its head. But, insect eyes are compound, with hundreds or thousands of lenses, together making an image consisting of spots of light, like pixels. An insect's sound receptors are on its legs.