Spiders - Teacher Background Information

Spiders are not insects as many people believe. Students will learn the physical differences between insects and arachnids in this unit. Students will also study other distinct characteristics of spiders, such as spiders' contribution to other organisms in nature. Spiders' need for food will be explained as will the concepts of "prey" and "natural enemies."

Spiders belong to a class of animals called arachnids. They have four pairs of segmented legs, and can grow a new leg if they lose one. Most spiders have eight eyes, and they do not have antennae or wings. A spider's body is divided into two sections, the abdomen and the cephalothorax. The legs, eyes, and mouthparts are all in the cephalothorax. Most spiders have poison glands and fangs in their jaws, which they use to inject poison into insects. The venom paralyzes or kills their prey.

Spiders usually have six fingerlike silk glands called spinnerets located beneath their abdomen. The silk comes from inside the spider's body as a liquid, thicker than water. When a spider wants to make a web, it squeezes the silk out of the two small holes at the back of its body called spinnerets. The moment it hits the air, the silk dries into a line that looks like a long strand of hair. Many spiders use their sticky silk webs to catch food, which consists of tiny animals. Some spiders use silk as draglines, which are long lines of silk the spider hangs onto as the wind blows it through the air. The spider can always crawl back up the silk line if it is blown some place it doesn't want to be! Some spiders spin silk webs, and others line their burrows with silk. Many spiders lay their eggs in silken sacs. All young spiders, and some adult males, release long silken threads to float or ride the wind to new areas. This is called ballooning.

Although spiders can live almost anywhere in the world, some like it where it is very humid, and some like it where it is very dry. Some spiders live underground and catch their prey by jumping out at them. Others live in trees and capture their prey in their webs. Others live in our houses. Have you seen them hanging from the ceiling? Many times a spider's common name tells something about the spider. Where do you think the garden spider lives? What about the water spider? A wolf spider? A banana spider?

The tarantula is probably the most feared of spiders. It is very big and can stretch itself almost to the size of a one-foot ruler - 10 inches. It is furry, unlike other spiders. It is a nocturnal animal and comes out at night to find food. It is large enough to eat many animals that smaller spiders can't catch. It can eat big beetles, toads and frogs. It can even eat small birds, snakes and lizards. Most spiders live one or two years, but the tarantula takes eight to 10 years to become an adult, and then lives a few more years. Tarantulas can become pets in our homes because they can be tamed.

Spiders are considered humankind's friend because they help keep the insect population in check. Humans use spiders' silk to make threadlike lines for microscopes, telescopes and other scientific instruments.

All animals have natural enemies. Birds, insects such as wasps, snakes, lizards, frogs and fish eat spiders. Sometimes spiders eat each other. Humans try to destroy them because we do not understand how useful they are. Spiders try to protect and defend themselves from their enemies. Ask students to consider if they were spiders what they would do to protect and defend themselves. All of us know how to protect ourselves using methods that are very similar to those of the spider.

1. Escape! How? (Use the dragline to drop into space and wait; move along the web to a safe place; use powerful jaws on smaller animals; use venom.) Usually spiders are frightened of people and try to escape from us because we appear like giants to them. Humans also try to escape from things that we think are dangerous.

2. Hide! A spider can hide by using its colors and patterns for camouflage to blend in with colors and patterns. What colors do spiders have? What are the colors of dirt, trees, leaves and grass? Humans will also hide if they sense a danger.

3. Frighten the enemy! Many people think that all spiders are poisonous, but in general, very few spider bites will be harmful to humans. Tarantulas look frightening, but they are not poisonous. They just scare their enemies and people silly!

4. Use a weapon - venom! Most spiders are not poisonous, but some are: the black widow and the brown recluse are poisonous and make people very ill with their bites. Many people do not understand that spiders very rarely attack their enemies. If a spider sees an enemy, it will usually try to get away. But all spiders use their poison in self defense, when escape is impossible.

At times, spiders will not attack their prey unless it is moving. Many insects have learned that if they do not move, the spider will not detect their motion vibrations. Spiders that ensnare their prey in their webs do not use their poison. The spiders that hunt for their prey, or hide on flowers and capture insects by grasping them with their fangs when the insects come close by, kill their victims with poison.