What characteristic defines insects?

The defining characteristic of insects is that they possess an exoskeleton, three pairs of jointed legs, and their body is divided into three distinct regions: the head, thorax, and abdomen. This structure is foundational to the classification of insects within the broader category of arthropods. The exoskeleton provides support and protection, while the three pairs of jointed legs allow for a range of movements and adaptations that have made insects one of the most successful groups in the animal kingdom.

Insect body regions serve specific functions; the head houses sensory organs and mouthparts, the thorax is the locomotive center with legs and wings (when present), and the abdomen typically contains vital organs for digestion and reproduction. This unique combination of physical traits distinguishes insects clearly from other arthropods, such as arachnids which have eight legs and two body regions, or other classes that may not have the same structural features as insects. Recognizing these characteristics is crucial for understanding insect biology and the role they play in ecosystems and agriculture.