Spiders are creatures that spin silken webs and belong to a group of insects called arachnids. They are different from most other insects because they do not have antennae and they have eight legs instead of six. A spider only has two body parts instead of three. These are cephalothorax and the abdomen, which are connected by a slender stalk. There is a groove across the bottom of the abdomen and at the tip of the abdomen , there are two to four pairs of organs called spinnerets. Spiders have a hard exoskeleton that is a dark red substance called sclerotin.
Spiders have jointed appendages in front of the mouth opening called the shelicerae. Each of these has a stout base with a groove on one side and small fangs, which folds back into the groove. Near the tip of the fang there is an opening from the duct from a poison gland. There a re pediapalps behind the mouth, which have six segments. The basic segments act as jaws for chewing. Along the cephalothorax there are four pairs of walking legs that are covered with hairs or bristles.
Each spinneret has many fine hairlike tubes and large tubes called spigots. These lead to an opening from several silk glands. The silk forms as a liquid in the abdominal glands. As the silk is drawn through the tubes, it hardens and forms a filament that is strong and elastic.
The blood of a spider is a clear liquid that is pumped through a tubular heart. The heart is located in the middle of the abdomen and the blood passes through arteries in the spaces in the body cavity. From these spaces the blood travels through the respiratory structures, where it releases carbon dioxide and takes in oxygen.
A spider breathes through tracheal tubes or book lungs or both. These tubes are simple passages that open to the outside through special openings. The book lungs are air-filled sacs that contain many leaflike folds. These open into a furrow and the exchange of gases takes place within this folds.
A tube that extends from the mouth to the anus is the method of digestion in spiders. When a spider captures another insect, it pierces it with its fangs and pumps enzymes into its victim’s body. These enzymes digest and liquefy the tissues of the victim. Then the spider sucks the liquefied tissues into its pharynx, where it is strained.
The life cycle of a spider consists of the egg, the larvae, the spiderling and the adult. They grow by molting, which means they periodically shed their hard outer shell. There are about 60 different kinds of spiders, divided into two major groups – tarantulas and true spiders. Tarantulas are found mainly in tropical areas and are large. True spiders are much smaller and are more dependent on silk. In both groups, there are some varieties that are poisonous to humans. The most dangerous are the Lycosa, the black widow and the brown recluse.