Spiders are invertebrates—they lack a backbone—and they do not have an internal skeleton. Instead, the entire body is enclosed in a hard, protective covering called an exoskeleton. The exoskeleton is made of protein and strengthened by a substance called chitin. Sensitive hairs and spines cover the exoskeleton.

Spiders vary greatly in size and color. In body length they range from 1/50th of an inch (0.5 mm) to 3 1/2 inches (9 cm). Most spiders are gray, black, or brown, but some are such vivid colors as green or red and others have colorful markings on their bodies.

The spider's body has two major regions—the cephalothorax, which consists of a fused head and thorax, and the abdomen. The two regions are separated by a tapered area called the pedicel. At the front of the cephalothorax are the eyes, usually four pairs, and the mouth. The mouth is adapted to sucking—spiders do not chew their prey, but instead suck out the internal juices from their victims.

Six pairs of appendages are attached to the cephalothorax—one pair of chelicerae, one pair of pedipalps, and four pairs of walking legs. The chelicerae terminate in fangs, which contain a poison that paralyzes or kills prey. (Most spiders feed on insects; the larger species feed on small fish, birds, and reptiles.) The pedipalps are used as feelers and to hold the prey during feeding. (In male spiders, the pedipalps are also used to transfer sperm to the female.)

In the abdomen are the respiratory, or breathing, organs. Most spiders have two types—(1) a pair of lunglike organs, called book lungs; and (2) tracheae (air-carrying tubes that branch from the outside to all parts of the body). Near the tip of the abdomen, on the underside, are specialized organs, called spinnerets, from which the spider spins out its thread-like strands of silk. A spider usually has six spinnerets. Some spiders have an additional spinning organ called a cribellum, for spinning out special ribbon-like strands of silk.

Spiders usually lead solitary lives. Most kinds of spiders are aggressive. After mating it is not unusual for the females of some species to kill the male, which is ordinarily quite a bit smaller than the female. (This behavior accounts for the name of the black widow spider.) Mating may or may not be preceded by courtship, depending on the species. After mating, the female retreats to a nest, often made of silk, in which she lays her eggs.

The female may lay from 2 eggs to 3,000 eggs, depending on her size and the species. In general, the larger the female, the more eggs she lays. The eggs are covered by a cocoon made from silk. As the young spiders (spiderlings) grow, they molt their exoskeleton and grow a new one to accommodate their increasing size. Most spiders live for a year or less. Some of the larger female tarantulas, however, may live for 20 years or more.

SpiderlingsSpiderlings stay within the egg sac until their first molt.
Is It a Spider or an Insect?

You see a small creature scurry across the floor. But you aren’t sure if it is a spider or an insect. How do you decide? Do what scientists do. First, they take a close look at the animal. They count its legs. They count its body parts, too. These are the best clues to use when deciding if a creature is a spider or an insect.

Count the legs. Did you count six legs, three on each side? An insect has six legs. Did you count eight legs? A spider has eight legs, not six. If you count the body parts, you see another difference. An insect has three main body parts. A spider has only two main body parts.

Legs and body parts are just two ways in which insects and spiders are different.

How Does a Spider Kill Its Prey?

Most spiders eat insects and other spiders. A few large spiders eat frogs, birds, or small lizards. But all spiders kill their prey with a poisonous bite. A spider bites its prey with its fangs, which are long, pointed teeth. Poison, called venom (VEHN uhm), flows through the fangs. The venom stuns or kills the spider’s prey.

Now the spider has a problem. It has no chewing parts in its mouth. It cannot eat its prey by chewing. Instead, the spider sucks the liquids from its prey. A spider may also spray special juices from its mouth onto the prey. These juices turn the prey’s body into a soupy liquid. Then the spider can slurp up its meal.

But what if the spider isn’t hungry? A spider simply stuns its prey. Then it spins and weaves a silk case around the prey. The spider hangs the prey on its web or puts it in a safe place. When the spider does get hungry, it has a meal waiting.