What anatomical feature is common to all arachnids, including spiders and mites?

The anatomical feature that is common to all arachnids, including spiders and mites, is the presence of four pairs of legs. This characteristic is fundamental to the classification of arachnids within the broader category of arthropods. Unlike insects, which have three pairs of legs, arachnids possess eight legs, allowing for a variety of locomotion and predatory behaviors. This distinguishes them significantly from other segments of the arthropod family such as insects, which can only have six legs.

In addition to having four pairs of legs, arachnids typically exhibit two primary body regions: the cephalothorax and the abdomen. While all arachnids share an exoskeleton and the general structure of two body regions, it is the four pairs of legs that is a defining feature across this entire class of organisms. This unique anatomical trait is one of the key identifiers that aid in differentiating arachnids from other arthropods.