An adult insect has three main body regions: the head, thorax, and abdomen.
The head contains the brain, eyes, antennae, and mouthparts, so it is chiefly responsible for sensing the environment and taking in food. The thorax is divided into three segments and carries the insect’s three pairs of legs; in winged insects, it also bears one or two pairs of wings. The abdomen houses much of the digestive, respiratory, excretory, and reproductive systems.
This three-part plan is called a tagmatic body division. It remains recognizable even when the regions are heavily modified. For example, a beetle’s hardened wing covers, a grasshopper’s enlarged jumping legs, and a bee’s narrow waist all fit within the same basic arrangement.
A common mix-up is confusing body regions with segments. The abdomen may contain many segments, and the thorax itself has three segments, but the major regions remain three. Spiders are not insects: their bodies are organized into two main regions and they have eight legs rather than six.