What defining feature do spiders have that distinguishes them from insects?

Spiders are distinct from insects primarily because they possess two body regions, which are the cephalothorax and the abdomen. This characteristic is fundamental to the class Arachnida, to which spiders belong, and it differentiates them from insects that have three body regions: the head, thorax, and abdomen.

The presence of only two body regions in spiders is significant because it plays a role in their overall morphology and physiology. For example, the cephalothorax houses the eyes, mouth, and legs, while the abdomen contains the digestive and reproductive organs.

Insects, on the other hand, exhibit a more complex body structure with three distinct regions, contributing to their diverse adaptations and life cycles. This structural difference is a key aspect of the classification within the animal kingdom. Other features like the number of legs or the absence of wings further illustrate the distinctions, but the two body regions are the defining characteristic that clearly separates spiders from insects.