Spiders

Spiders are eight legged invertebrate animals classified as arachnids. There are over 100 families of spiders, comprised of over 40,000 species.World Spider Catalog: Intro: Counts

Interestingly, other arachnids include scorpions and tarantulas. Scorpions are in a sub-species, but tarantulas are technically spiders. The primary difference is in the way that their mouths are constructed. A tarantula has mouth parts known as pedipalps that strongly resemble short, extra legs. But just like the other spiders, tarantulas have eight legs. This is the defining similarity between all arachnids.

Spiders have special characteristics that differentiate them from insects. These features include only two main body parts: the cephalothorax and the abdomen. Each main section has four legs. Spiders also have no antennae, as most insects do. Spiders are distinct from other arachnids by the fact that they have a small waist and tough exterior over the cephalothorax.Explorit Science Center: Spider Facts

The more dangerous the spider, the more likely it will be reluctant to bite you. Classic examples of this would be the brown recluse or the black widow spider. The black widow builds her nest in very remote places and, unless you put your hand directly on the creature, she will attempt to flee rather than go on the attack. The black widow is referred to as such because of her interesting habit of immediately eating her mate after the mating process is complete. This is not uncommon among spiders. Most full-grown spiders you see are typically female, for this reason.

Another characteristic with the more deadly spiders is striking marking. The black widow is pitch black with a bright red "hourglass" on her abdomen. This means "steer clear!" It's nature's way of letting enemies know that she is very visible and very dangerous.

Contrary to what many people believe, spiders are not insects. In addition, all arachnids are not spiders. The "grandaddy longlegs," properly known as the harvester, is not actually a spider. All spiders are not male, and they do require a male and female in order to mate.Burke Museum of Natural History and Culture: Spider Myths