Tony Carpenter, age 13, of Jackson, Miss., for his question:
HOW MANY KINDS OF SPIDERS DO WE HAVE?
Spider is the common name for about aU,00U species of anthropod animals. The spider, or arachnid, has eight walking legs, anterior appendages bearing fangs and poison glands and specialized reproductive organs on the second appendages of the male.
Spiders can be found in all parts of the world. Most live on the land but a few have adapted to freshwater life by trapping air bubbles underwater and carrying the bubbles with them. In the spider family are the scorpions, mites and ticks. Although most spiders are less than a half inch long, some grow to be much larger. The largest is a species that can be found in Guyana. It has a body length of about three and a half Inches.
A spider's body is divided into an anterior cephalothorax, or prosoma, and a posterior abdomen, or opisthosoma. The parts are separated by a narrow stalk, or pedicel, which gives the animal a flexibility that facilitates its use of silk. Spiders make silk like webs.
The cephalothorax ordinarily bears several pairs of simple eyes that tend to be larger in hunting spiders and smaller in spinners of elaborate webs. Each of the first pair of appendages bears a foray with an opening from a poison gland at the tip. The next two appendages are rather leglike but generally modifiea into feelers.
On the abdomen are modified appendages, the spinnerets, used in secreting silk. Respiratory openings on the abdomen lead to the so called book lungs (named for their layered structure) or a system of tubes (tracheae) for carrying air, or both.
The digestive system of spiders is adapted exclusively to taking up liquid food, because the animals generally digest their prey outside the body and then suck the fluid. The fairly complex brain is larger or smaller in certain parts, depending on whether the animal locates prey mainly by touch or vision.
Spider silk is a fibrous protein that is secreted as a fluid and forms a polymer, on being stretched, that is mucn stronger than steel and further resists breakage by its elasticity. A single spider can spin several Kinds of silk. Although some other invertebrates also spin silk, spiders put this ability to the most spectacular variety of uses. For example, they form draglines that help them to find their way about and to catch themselves if they tall. Small and, especially, young spiders spin a "parachute" thread that enables them to be carried by the wind, some for hundreds of yards. The males use silk in transferring sperm to the palpal organ, and the females make cocoons with it. Silk is also used to make nests and other chambers and to line burrows. The most familiar and amazing use of silk by many species, however, is in making insect traps called spider webs. Once prey is caught in such a web, the spider may wrap it in more silk. A spider does not have to learn how to make a web. It is instinctive behavior that enables the animal to spin a web.