Earl Rolfsrud, age 9, of Nashville, Tenn., for his question:
DO SPIDERS HAVE SENSE ORGANS?
Most spiders are helpful to man because they eat harmful insects. There are 29,000 known kinds of arachnids, but scientists believe there may be as many as 50,000 kinds. Many people think they are insects, but they are called arachnids.
A spider gains knowledge of its surroundings through its sense organs. Included is a brain which controls the activities of all parts of the body.
Most kinds of hunting spiders can see better than web spinning spiders. But all spiders can see only short distances.
The sense of touch is the most highly developed of the spider's senses. Special hairs on its body serve as organs of touch and perhaps as organs of hearing and smell.
Each hair on the spider's body contains a nerve. These nerves send messages to the brain that tell the spider how to respond to changes in its surroundings.
Spiders can easily sense vibrations and the presence of certain chemicals.
A spider's eyes are on top and near the front of its head. The size, number and position of the eyes vary among different species. Most species have eight eyes, arranged in two rows of four each. Other kinds have six, four or two eyes. Some species of spiders that live in caves or other dark places have no eyes at all.
Spiders as a group have two kinds of breathing organs: tracheae and book lungs.
Tracheae, found in almost all kinds of true spiders, are small tubes which carry air throughout the body. Air enters the tubes through the spiracle, an opening in front of the spinneretts in most kinds of true spiders.
Book lungs are in cavities in the spider's abdomen. Air enters the cavities through a tiny slit on each side and near the front of the abdomen. Each lung consists of 15 or more thin, flat folds of tissue arranged like the pages of a book. The sheets contain blood vessels.
As the air circulates between the thin sheets of the lung, oxygen passes into the blood. Tarantulas have two pairs of book lungs while most true spiders have one pair.
The blood of spiders contains many blood cells and is transparent. The heart, a long, slender tube in the abdomen, pumps the blood to all parts of the body.
As the spider's blood circulates, it flows through open passages instead of closed tubes, much as those of the human body. If the spider's skin is broken, the blood quickly drains from its body. The insect's tough skin often prevents this from happening.
A digestive tube extends the length of the spider's body. Near the mouth the tube is wider and larger and forms a sucking stomach. When the stomach''s powerful muscles contract, the size of the stomach increases. This causes a strong sucking action that pulls the food through the stomach into the intestine.
Juices in the digestive tube break the liquid food into particles small enough to pass through the walls of the intestine into the blood. The food is then distributed to all parts of the body.