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With nearly 1 million species recorded, insects are the most diverse group of organisms on the planet. They account for approximately 75 percent of all described animal species, and the vast majority of the species on Earth that are yet to be identified are likely insects and other arthropods. In fact, scientists conservatively estimate that the total number of insect species could exceed 8 million when all are eventually discovered. While the majority of that amazing diversity exists in the tropics, there are well over 150,000 insect species found within the United States and Canada alone and many more if other arthropods, the larger group to which insects, spiders, centipedes, and scorpions all belong.

Insect Anatomy

Insects share several common characteristics. Unlike mammals, birds, reptiles, and amphibians, insects lack an internal skeleton. Instead, insects have a hard exoskeleton on the outside of their bodies; this provides both protection and support. Their body is divided into three distinct regions: the head, thorax, and abdomen.

The Head

An insect’s head has two rather prominent compound eyes, two antennae, and mouthparts. The rounded compound eyes are composed of hundreds of tiny individually lensed eyes. Together, they render a single, somewhat pixilated image and afford insects rather good vision, especially for both distance and motion. Above the eyes are two antennae. They bear sensory structures that help with orientation, smell, and taste. The head also bears mouthparts, which vary considerably across insect groups; some insects, such as grasshoppers, beetles, and caterpillars, have chewing mouthparts. Others, such as true bugs and mosquitoes, have piercing-sucking mouthparts, but there are many other options, and they include mouthparts adapted for sponging (flies), rasping-sucking (thrips, biting flies), and siphoning (butterflies and moths). Some insects have reduced or vestigial mouthparts or may even lack them altogether.

The Thorax

The thorax is an insect’s second body section. It bears the appendages and muscles that enable an insect to move. All insects have three pairs of jointed legs, one pair on each segment. Many insects also have one or two pairs of wings. Besides flight, wings may serve a variety of other functions, including assisting in thermoregulation, sex recognition, sound production, camouflage, mimicry, and self-defense.

The Abdomen

The last section of an insect’s body is its abdomen, which contains the reproductive, digestive, and excretory systems along with a series of small lateral holes, called spiracles, that enable air exchange. In female insects, the tip of the abdomen may have an added structure called an ovipositor, which is used to insert or place eggs. In some insects, such as bees and wasps, the ovipositor is modified into a stinger that can be used for self-defense.