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.

Crustaceans have a full toolkit for feeding. On the front of a crustacean’s head are two pairs of sensitive antennae for feeling food. Picture a lobster’s long antennae swinging up and down. Two appendages, often claws, are used to seize food and break it up into pieces. Crustaceans typically have three pairs of biting mouthparts for chewing. The configuration of their mouthparts varies, depending on whether they are predators, scavengers, or filter feeders. While crustaceans have many adaptations for eating, they are also a common prey for other organisms. Humans eat millions of tons of crabs, lobsters, and shrimp every year. Tiny crustaceans, including krill and copepods, are part of the plankton that tend to congregate in the top few hundred meters of the ocean and are are vital to the marine food web. As krill and copepods feed, they concentrate ocean nutrients into their bodies, which then become available to the fish and filter-feeding organisms that prey on them.