How are members of the Class Insecta distinguished from other Arthropods like arachnids?

By having three distinct body parts: head, thorax, and abdomen

While all insects belong to the Phylum Arthropoda and share features like a segmented body and exoskeleton, the Class Insecta is defined by a specific division of its body plan. Insects are characterized by having three clearly demarcated tagmata, or body sections: the head, which bears the mouthparts and primary sensory organs; the thorax, which serves as the locomotive center, bearing the legs and wings; and the abdomen, which houses most of the digestive and reproductive organs. This tripartite body structure (head-thorax-abdomen) fundamentally separates insects from other major arthropod classes, such as Arachnida (spiders), which typically only exhibit two main body parts (cephalothorax and abdomen).