What distinguishes insects in the classification of arthropods?

Insects are classified as a distinct group within arthropods based on several key characteristics that are fundamental for their identification. One of the primary distinctions is that insects possess three pairs of legs and a body that is segmented into three distinct parts: the head, thorax, and abdomen. This structural organization is unique to insects among arthropods, which also include arachnids (like spiders), crustaceans (like crabs), and others, each possessing different anatomical features.

The presence of three pairs of legs is significant, as it distinguishes insects from other arthropods that do not conform to this characteristic. For instance, arachnids typically have four pairs of legs, while myriapods (like centipedes) have many legs but not in the organized pairs found in insects. Additionally, the segmentation of the body into three parts further emphasizes the unique classification of insects, differentiating them from other arthropods that may have more or fewer body segments.

In summary, the defining traits of three pairs of legs and a three-segment body structure are what specifically categorize insects within the broader class of arthropods, making the second option the accurate choice in this context.