How are spiders and mites different from insects?

Spiders and mites are classified as arachnids, which distinctly differ from insects in several key anatomical features. One of the main differences is the number of legs. Arachnids have four pairs of legs, which totals eight legs, while insects possess three pairs, amounting to six legs. This fundamental structural difference is a critical characteristic used to differentiate arachnids from insects.

The other options highlight traits that do not differentiate spiders and mites from insects in the same way. For instance, both insects and arachnids have an exoskeleton, which serves as a protective outer layer. Additionally, insects possess three body regions (head, thorax, and abdomen) compared to the two body segments of arachnids (cephalothorax and abdomen). While wings are specific features of many insect species, they are not applicable to spiders or mites. Therefore, the unique presence of four pairs of legs is the defining feature that sets arachnids apart.