Spider (arthropod)

I

Introduction

Spider (arthropod), any of a large group of invertebrates (animals without backbones) that have spinning glands used to produce silken threads and webs. There are about 40,000 species of spiders. Spiders are found worldwide, except for in the oceans, and they live in all habitats and at most elevations.

Spiders, along with daddy longlegs, mites, ticks, and scorpions, belong to a group of animals known as arachnids. Spiders are not insects. Spiders have a different body structure than insects. The bodies of spiders divide into two parts (a fused head and thorax and an abdomen), and spiders have eight legs and eight eyes; they lack antennae and wings. In contrast, the bodies of insects form three parts (head, thorax, and abdomen), and insects have six legs, two eyes, two antennae, and, typically, four wings.

All spiders are carnivorous (feeding mainly on the flesh of other animals). Spiders eat insects and sometimes other arthropods (invertebrate animals with jointed limbs, segmented bodies, and hard shells known as exoskeletons), including other spiders. Scientists often divide spiders into two types: web spiders and ground spiders. Web spiders produce webs to capture prey, while ground spiders hunt prey directly without using a web. Almost all spiders use poison glands to kill or paralyze their prey or to defend themselves.

Contrary to popular belief, most spider bites are not dangerous to humans. Of the 40,000 species of spiders, only about 30 species produce bites that may cause illness. Spiders rarely attack humans unless they feel threatened, and if they do bite, the wound is rarely serious. There is absolutely no reason to kill any spider or to call an exterminator if you have spiders in your house. As an old English saying goes, “If you want to live and thrive, let a spider run alive!”

---

---

II

External Body

Spiders range in size from less than 1.0 mm (0.04 in) to more than 10 cm (4 in) in length, with a leg span of up to 20 cm (8 in). A spider’s body is divided into two parts: the front portion, called the prosoma or cephalothorax, and the rear portion, called the opisthosoma or abdomen. A narrow stalk called the pedicel connects these two parts.

A hard shell called an exoskeleton covers the entire body of a spider. The exoskeleton is made of cuticle, a material composed of a combination of protein and tough fibers called chitin. The cuticle forms thin layers stacked on top of one another, an arrangement that improves the strength and elasticity of the exoskeleton. The spider’s cuticle provides attachment sites for many muscles, and it also prevents desiccation (loss of body water). The cephalothorax cuticle is strong and stiff, while the cuticle of the abdomen is soft and extensible. As a spider grows, it sheds or molts its exoskeleton and grows a new one to cover its larger body.

A

Cephalothorax Structures

The cephalothorax contains a number of structures and appendages: one pair of biting mouthparts known as chelicerae; a pair of poison glands; one pair of short, leglike appendages called pedipalps or palps; and four pairs of legs. The spider’s eight eyes are also located on the cephalothorax.

A

1

Mouthparts

When a spider catches prey it uses a pair of jointed appendages known as the chelicerae, located in front of the mouth opening. Chelicerae resemble tiny pocketknives. Each chelicera has a sharp fang that swings out of its resting position to stab into the victim. Near the tip of the fang is a duct opening that comes from a poison gland. The fang acts like a hypodermic needle—it ejects venom from the poison gland and delivers it into the prey.

Spiders also use chelicerae as multipurpose tools. They have been called the “hands” of the spider. Spiders can use their chelicerae to perform tasks such as digging burrows in the soil and transporting small prey.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain