There are over 40,000 different spider species that have been identified by taxonomists and spider specialists, called arachnologists. These fall into two main categories: mesothelae and opisthothelae. These two sub-orders are differentiated mainly by their abdomen.
Mesothelae spiders have segmented plates and ganglia in the abdomen and four pairs of spinnerets. Opisthothelae spiders do not have segmented plates or ganglia in the abdomen, and they have one, two or three pairs of spinnerets. The vast majority of spiders fall into the second category, with only about 87 species that fall into the first category.
All spiders are arthropods, which means that they are invertebrates with exoskeletons, segmented bodies and jointed appendages. Spider species are found on every continent except Antarctica, and in every natural habitat except for sea or air. They comprise the largest order of arachnids and they are the seventh most diverse among organisms.
Almost all known spider species are predators, although some can derive as much as 90% or more of their food from plants, making them vegetarian species. Most feed on insects, but some go after larger prey like birds and lizards. They all have fangs that can inject venom, but only a few spider species have venom that is potentially harmful to humans. While the average life cycle of a spider is only about two years, some spider species like tarantulas have been known to live in captivity for up to 25 years.
Most spider species live and work independently, but a few have been known to demonstrate social characteristics similar to, but not nearly as complex as, insects. One species in the Family Theridiidae forms colonies of up to 50,000 spiders. Some cooperate just to protect eggs and young spiders, but others join forces to hunt and capture prey as well.
The web of a spider is its best known method of capturing prey, and the characteristics of webs vary greatly. Unfortunately, there is no relation to the type of web spun by a spider species and its classification in the animal kingdom. Despite this, spiders can be grouped into orb web spinners and non-orb web spinners. Orb webs have a distinct pattern with straight lines extending from the center like spokes on a wheel, and radial web lines connecting the spokes. Non-orb webs follow no distinct pattern and are often called cobwebs.
The more than 40,000 spider species have been broken into over a hundred different families according to modern taxonomists, but that has not simplified the matter enough for some scientists. Since 1900 there have been 20 or more proposals to categorize spiders in different ways. Plus, there is ongoing research on the subject as well as scientific confusion and controversy.