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Plants & Animals: Introduction to Spiders


 

 What is a Spider?

Spiders are different from insects, but they have many things in common. They do not have backbones, they are invertebrate animals, they have jointed legs, and they have exoskeletons. Spiders are arthropods. They have legs with many joints and a hard outer body covering called an exoskeleton. Spiders form the second largest group of arthropods with insects making up the largest group.

How do you tell a spider from an insect?

Spiders have 4 pairs of legs, and two body segments. They do not have wings or antennae. Spiders belong to the class Arachnida while insects belong to the class Insecta. There are other arachnids beside spiders that make up the class Arachnida. Scorpions, ticks, mites and harvestmen belong to the Arachnida class.

There are more than 37,000 species of spiders that have been identified. Scientists believe that there are as many species not yet discovered. There are about 3000 species of spiders found in North America and approximately 700 species found in Florida.

Spiders vary in size from the 10’’ bird-eating tarantulas of the rain forest to as small as the point on a safety pin. They are found everywhere on earth.

The spider’s body has two main parts, a combined head and chest called the cephalothorax. This is where the eyes, mouth, fangs, jaws and pedipalps are located. The 4 pairs of legs are also attached to the cephalothorax. The second part of the body is called the abdomen. The spinnerets are located at the end of the abdomen. The word spider means maker of silk and all spiders make silk. The silk is made in the abdomen and is squirted out the spinnerets. Silk is liquid until it hits the air and then hardens into strands of silken line.

Spiders are carnivores, they eat other animals. A spider bites its prey with the fang at the tip of the jaw. The jaws are called chelicerae. The venom is injected into the prey, paralyzing or killing it. The spider may use its pedipalps or feelers to sense what is around it and to hold its prey down while it bites with its fangs. The venom and digestive juices dissolve and liquify the prey. The spider then sucks up the liquid using its stomach’s pumping action. Spiders can only eat liquid food, so the chelicerae strain the liquid. The exoskeleton of prey is discarded after the meal.

Spiders have six spinnerets that make up to seven different kinds of silk: thin strands, thick strands, sticky strands, smooth strands or combinations. Silk is made up of a protein fiber which is the strongest natural made substance in the world. Spiders use silk in many ways: for safety lines, draglines, for signal lines, for webs, for egg cases, for holding prey, for building nurseries, for transportation, as in ballooning, and for snaring prey.

Spiders are divided into 2 general groups, web builders and non-web building spiders. All spiders make silk, but all do not make webs. They can be divided into wandering spiders who search out prey, or waiting spiders, who sit and wait for prey to come close to them and then they ambush the prey and water spiders, who trap a bubble of air under their abdomen and take their air-chamber below the water and get their prey.

Web-weaving spiders build webs to catch prey and they live in or near the web. The web-builders are arranged by the shape of their web. The orb weavers design circle webs, the sheet web spiders form flat horizontal webs, funnel webs are cone-shaped webs, dome shaped webs are upside down bowl shaped, and tangle webs are irregular and haphazard in appearance. In some webs the spider has a personal resting spot which may be obvious, or it may be hidden in or near the web. It is usually the female spider that is observed in a web. The male spider only makes a web when he is very young. At maturity he leaves his web and searches for a female.

Spiders lay many egg cases which they construct with their silk. Some spiders place these eggs in a well hidden location and some spiders carry the egg case with them until it hatches or is close to hatching and then the female constructs a nursery web for the young spiderlings. Baby spiders are called spiderlings. They remain in and around the egg case until they have molted and then they balloon away.

Ballooning helps spiders to avoid over-crowding and competition for food. Spiderlings climb to the highest point they can find. They tilt their abdomen and spinnerets upward as they release silk. The strand of silk lengthens and is picked up by the breeze and the spiderlings are pulled upward and travel to a new location. They manage to find the appropriate environment for their survival, in buildings, shrubs, forests, leaf litter and lawns. Many spiders only live for one or two years, but some kinds of tarantulas live for 20 years.

While all spiders in the United States are poisonous, most spiders are not dangerous because they cannot harm people. Their fangs are not strong enough to break the skin or their venom is too weak to affect us. There are some spiders which could be dangerous. These are the sac spiders because their venom is strong and some people have an allergy to it. Sac spiders wrap themselves up in a sac-like tubular resting site inside a rolled leaf. The widow spiders are comb-footed spiders. In Florida are Black, Brown and Red Widow spiders. The most dangerous of these is the Black widow spider. The female black widow spider is less than 1/2 inch long, has a round, spherical, shiny black abdomen. A bright red hourglass pattern is on the underside of the abdomen. Black widow spiders are secretive and hide in their irregular tangle webs built in dark places or under fallen branches. The brown recluse spider is called the "violin" spider because of the pattern of a violin on its cephalothorax. Her venom is harmful to humans. The brown recluse spider is about 1/2 inch long, brown with the violin pattern on her back. The brown recluse is shy and secretive. It hides in a loose web built under furniture, or in dark places. This spider may hide in closets or in folded clothes which are not used often. These spiders, the widows and the recluse spider, rarely bite people.

Spiders are very helpful. They control many insects by preying on them. They are the most important predator of insects in the world. As you learn more about spiders you may become fascinated with them. As you become fascinated you may become less fearful of them. Follow safety rules when watching them and remember that spiders go out of their way to stay out of people’s way.