There are countless insects and animals that live under the desert floor. Deserts are especially populated with a group of invertebrates called arachnids. Arachnids have a hard outer covering called an exoskeleton. Exoskeletons prevent moisture from escaping their bodies. This feature makes them well adapted to the desert. Arachnids include spiders and scorpions that get their water source from the animals they eat. A common arachnid in North American and Australian deserts is the trap-door spider. This clever spider sets traps for its prey. The trap-door spider digs a long, narrow burrow in the ground and then lines the inside and opening of the burrow with webbing. The spider then covers the opening with soil and vegetation and waits in the burrow until an insect stops over the hole. The spider then jumps out, grabs the insect, and drags it forcefully into the hole.

A large spider that lives in the Americas and the Mediterranean is the tarantula. These spiders can grow to be bigger than a person's hand. Tarantulas hunt snakes, insects, baby birds, and lizards. These spiders will bite people, yet they are not poisonous. Some poisonous spiders include the black widow and the brown recluse.

Scorpions are another group of arachnids that have large, and sometimes poisonous, pinchers. They use these pincers to catch their prey. Scorpions eat a diet that consists mostly of insects. They do not always use their stingers against prey; sometimes they are used for defense. There are two other groups of arachnids that are harmless to people. These are whip scorpions and wind scorpions. Both groups do not have stingers or pincers like other scorpions. Wind scorpions burrow under the sand during the day and only come out at night to hunt for small insects.

Some desert animals live only a short time to mate and deposit eggs before they die. Sometimes there may be many years between the time the adults mate and when the eggs actually hatch. A scientific study done in 1955 in the Mojave Desert proved this seemingly impossible delay in hatching true. The study started when a dry lake bed was flooded for the first time in twenty-five years. Shrimp from eggs laid twenty-five years ago, when the lake last held water, hatched. These eggs held enough moisture all those years to survive until conditions were right for them to hatch.

The sidewinder rattle snake travels easily over the sand in an s-shaped loop. The looped part anchors the snake to the sand while the rest of its body pulls it foreword.