Which characteristic defines an insect?

Insects are specifically defined by having three distinct body parts: the head, thorax, and abdomen. This characteristic sets them apart from other arthropods, which may have different body structures. The head contains sensory organs and mouthparts, the thorax is responsible for movement and typically has legs and wings (if present), and the abdomen is involved in digestion and reproduction.

The presence of three body segments is a fundamental trait of the class Insecta in the phylum Arthropoda, making it crucial in identifying and classifying these organisms. While some insects may have two pairs of wings or may possess a soft-bodied structure, those traits are not universally applicable to all insects. Additionally, insects are characterized by their six legs, so a lack of legs does not define them, but rather disqualifies an organism from being classified as an insect. Therefore, the three distinct body parts are the defining characteristic of insects.