Spiders

Spiders were one of the first predators to walk on land, starting more than 350 million years ago, and they are still one of the most successful predators. Today, there are more than 38,000 types of spiders. Some spiders are as small as a grain of sand. The biggest, the Goliath birdeater tarantula, is quite large. Spiders are members of the group of animals called Arthropoda - animals with jointed legs and a hard outer skeleton. More specifically, they belong to the family Arachnida.

The spiders' ancestry is very different from that of the insects. Their origin lies in the group that gave rise to the horseshoe crabs, and the amblypygids (whip spiders and tailless whip scorpions) and the scorpions. In the consequence, their anatomy is fundamentally unlike that of insects. They have eight legs not six like an insect and their bodies are divided into two not three parts.

Their front half is aptly known as the cephalothorax, for it represents the head and thorax of their remote ancestors that have become fused into one. Like an insect's head, it has at the front a mouth which here is flanked on either side by a pair of poison fangs. It also carries the primary sense organs. And like the thorax of insects, it carries beneath it the legs - in this case, four pairs of them.

Vision

Spiders can have up to eight pairs of eyes, usually placed along the front edge of the cephalothorax but also, in some species, a little further back. some are tiny specks capable of registering little more than the level of light. Others are much larger. Nonetheless, even these big eyes are relatively simple in structure and cannot produce detailed images comparable to those provided by the compound eyes of insects. some spiders, in fact, are virtually blind. All of them rely on their sensitivity to vibrations as the primary source of information about what is happening around them.

In the place where one might expect antennae on an insect's head, a spider has palps. These are packed with sense organs which detect not only vibrations but scent. Those of a male are much bigger than those of a female.

Exoskeleton

Growth of a spider requires shedding its exoskelton, usually 4 to 12 times before maturity. Most spiders live one to two seasons. Orthognath spiders do not mature for several years; the males live less than a year thereafter, but the females may live up to 20 years. Some primitive spiders such as Sicarius, Loxosceles, and Filistata may live 5 to 10 years.

Are All Spiders Venomous?

Forest Wolf Spider Lycosa gulosa

It is true that every single species of spider is venomous, but the vast majority are unable to hurt anybody. Throughout the world there re quite a number that are large enough and venomous enough to bite a human being, but most of these cause no more discomfort than a sting from a wasp or a bee. However, some species of spider are very dangerous and have caused human deaths.

The most notorious spider is probably the Black Widow, Latrodectus mactans, which lives in the United States and other parts of North and South America, southern Europe, Africa, Asia, Australia and New Zealand. It is only the larger females that cause any problem, as the males are too small to be able to penetrate human skin with their fangs.

Funnel Web Spider Atrax robustus

Another notorious spider is the Funnel Web spider from Australia, Atrax robustus. There are three species of the Funnel Web, but it is this one, the species found in northern Sidney, which causes most concern. For a long time it was thought that the Funnel Web spider was probably the most venomous spider in the world. It seems nowadays that the thirteen spiders of the genus Hadronynche are more of a threat, and the Mouse spider, Missulena insignis, which had been thought only mildly venomous, is now acknowledged to be really dangerous. Affected tissue refuses to heal for along time where it has been dissolved by the venom, and when it finally does the victim is left with severe, disfiguring lesions.

Australia is not the only part of the world with dangerous spiders. South America is pretty well provided. Known locally as Aranha Armedeira, Phoneutria nigriventer, is the largest spider of South America which is also extraordinarily aggressive.

Aranha Armedeira Phoneutria nigriventer

Two other south American spiders, the Podadora or Bola Spider, Glyptocranium gasteracanthoides, of Argentia, and the Black Tarantula of Panama, Sericopelma communis, have been responsible for human deaths. Throughout the tropics one comes across a number of large, chunky spiders that are generally known in the United States as "Tarantulas." They are not really tarantulas, and although they look alarming they are for the most part of no danger to man. Many of them are reluctant to bite. They are often kept as pets. True Tarantulas, Lycosa narbonensis, are European spiders that also have a reputation of being lethal. This is not so.

Giant Wolf Spider
Hogna carolinensis

Spiders are an astonishingly varied group of animals. There are Bell Spiders, Argyronera aquatica, which live beneath the surface of a pond in a little bell of air contained inside a bell-shaped nest; the air is brought down, bubble by by laborious bubble from the surface. There are spiders that live in burrows with a trapdoor-like lid at the entrance, and there are crab spiders, so-called because of their shape which is somewhat crab-like.

Spider Silk

Spider silk is an albuminoid protein believed by most people to be quite similar to insect silk, It is secreted, however, in special glands within the abdomen. These glands differ in size, and number, their presence distinguishing the spiders from their close Arachnoid relatives.

The spider thread may be a very slender thread, sometimes only one millionth of a inch in diameter, or it may be ten or twenty times as thick; but it has great elasticity, being capable of stretching one-fifth of its length without breaking. Its tensile strength is thus greater than steel and is said to be second only to that of fused quartz fibers. The strength of a thread varies in different species.

Types of Spiders

The following list of types of spiders is far from complete and is provided for reference purpose only:

• Family Agelenidae (funnel weavers)
• Family Araneidae (orbweavers)
• Family Clubionidae (sac spiders)
• Family Linyphiidae (dwarf and sheetweb weavers)
• Family Loxoscelidae
• Family Lycosidae (wolf spiders)
• Family Philodromidae (running crab spiders)
• Family Salticidae (jumping spiders)
• Family Theridiidae (cobweb weavers)
• Family Thomisidae (crab spiders)
• Family Uloboridae (hackled orbweavers)
• Family Pholcidae (cellar spiders)
• Mygalomorphae (baboon spiders)

Additional resources:
The Scorpion Files
Life in the Undergrowth

References: