Most arachnids are predators, feeding on insects and other animals without backbones (invertebrates). Special mouthparts are adapted to catch, eat, and digest prey. On each side of an arachnid's head is a pedipalp, a jointed appendage that commonly looks like a mini leg. A pedipalp helps an arachnid to feel prey (It is also used for sperm transfer during mating by spiders). Because most arachnid eyes seem to be low-resolution light and dark sensors, an arachnid relies more on its sense of touch to feed. A pair of chelicerae on the front of an arachnid's body serve as jaws. Packed with muscles, they move from side to side or up and down to impale and chew prey. Hollow fangs at the tips of spider chelicerae are used to inject the prey with digestive juices or venom. Some arachnids also make silk to catch and immobilize prey.

Arthropods are the most successful group of organisms on Earth in terms of numbers, including almost half of all known species. The hallmark of their success is a body plan that makes them durable and adaptable. All arthropods have external skeletons made of hard material (chitin embedded in a protein matrix). The exoskeleton protects them from predators, weather conditions, and other threats. As they grow, arthropods typically shed the exoskeleton to reveal a bigger, fresh one underneath. The two lengthwise halves of an arthropod body are a mirror image of each other (bilateral symmetry) and typically divided into segments. Each segment has appendages that are specialized for the many activities of the arthropod. An arthropod may use appendages to feed (mouthparts), to breathe (gills, tracheae, book lungs), to reproduce (genitalia), and to move around (walking, swimming, flying). Having a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.