Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization. As of 2008, at least 43,678 spider species, and 109 families have been recorded by taxonomists; however, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900.
Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. Unlike insects, spiders do not have antennae. In all except the most primitive group, the Mesothelae, spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure.
Black Widow Spiders
Black widow spiders are most recognized for the red hourglass shape under their abdomen. This spider gets its name from the popular belief that the female black widow spider eats the male after mating, although this rarely happens. Black widows are poisonous when ingested during the first 17 days of their life.
Brown Recluse Spiders
Brown recluse spiders have a characteristic dark brown violin marking on their back. These spiders often infest cedar shake roofs and spin irregular webs, which are used as a retreat.
Common House Spiders
The common house spider is usually the spider most often encountered indoors. It is a nuisance pest, probably more because of its webs than the spider itself. The house spider is found worldwide and is common throughout the United States and Canada.
The common name for the hobo spider comes from its presumed method of expanding its distribution by hitching rides with humans along major highways in the Pacific Northwest. In fact, the hobo spider was introduced from Europe in the 1930s and has become established in at least 6 states since, displacing many native spider species as it spreads.
About 300 species of jumping spiders are found in the United States and Canada. Unlike most spiders, jumping spiders are active during the daytime and seem to like sunshine.
Long-bodied Cellar Spiders
Long-bodied cellar spiders are commonly referred to as “daddy-long-legs” because of their very long, thin legs, and as their name implies, are found in dark and damp places like cellars and basements. There are about 20 species of cellar spiders in the United States and Canada.
Unlike most spiders, wolf spiders don’t hunt with webs. Instead, they chase their prey using their fast running ability. These spiders are often big and hairy which alarms some people, but they are primarily nuisance pests. Over 100 species of wolf spiders are found in the United States and Canada.