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.

Insects are by far the most diverse and abundant group of organisms on Earth. More than one million insect species have been identified, and estimates of how many species exist range into the tens of millions. Insects got their start way back, in the Paleozoic (about 500 million years ago) and have evolved in tandem with flowers (coevolution). Insects have become specialized on particular flowers, leading to complex sets of adaptations that couple them in a feeding and pollination relationship (a mutualism). Specialization allows for many types of insects to live in the same habitat, accommodating their exceptional biodiversity. In a backyard in the temperate zone, one might find several thousand species of insects. Specialization can occur within one plant species as well, with different insects using different parts of the plant. In just one species of tropical tree, Dr. Terry Erwin of Smithsonian's National Museum of Natural History found about a thousand species of beetles.

Insects are the only animals without backbones (invertebrates) that can fly. Wings of insects are flat and paper-thin, supported by a network of veins. They flap their wings at incredible rates, up to about two hundred times per second. Most insects have two sets of wings that work in tandem because they are coupled by a fold, a hook, or other structure that catches the back wing as the front wing beats. Wings are made of two layers of cuticle for strength. The front wing is often hardened to serve as a protective cover, leaving the back wing to provide most of the flying power. Many insects walk around instead of, or in addition to, flying. The good walkers tend to touch three legs to the ground at a time, alternating with other sets of three (tripedal gait). The stable triangles formed by the legs allows them to move quickly without falling over. Some insects "walk" on water or swim.