Crab, scorpion, and spider all belong to the class Arachnida. They are characterized by having two main body parts, four pairs of legs, and no antennae.
Mites, on the other hand, are a diverse group of arthropods that belong to the subclass Acari, which includes ticks and chiggers. Mites are characterized by having a single body part and only two pairs of legs. They can be parasitic or non-parasitic and have a wide range of lifestyles and ecological roles.