It was suggested that this might go better in the article section.
One of the most common questions I see asked on Bugguide is "does it bite/ is it dangerous", most of these comments are in the context of spiders or other Arachnids. So I thought I'd provide some information on this interesting Class.
The Class Arachnida is comprised of 10 extant orders; Scorpiones, Opiliones, Pseudoscorpiones, Solfugae, Palpigradi, Acari, Ricinulei, Araneae, Amblypigida, and Urypygida. There are also several extinct orders, including what were the largest arthropods, the Eurypterida (Sea-scorpions).
There's one other group, called Schizomida, but they're often lumped in with other groups. I have no idea how, but Jeff managed to find one
Palpigradi, Ricinulei are both very small groups and seldom encountered. Acari are megadiverse and would take way more time to go into than necessary, so I won't go over these three orders.
Out of the seven that are last, only 3 of them are poisonous; Aranaea, Scoriopnes and Pseudoscorpiones. Since they're the most pertinent I'll treat them last. What follows is a VERY brief description of the remaining 4 groups, two of which have some nasty rumors attached to them.
are familiar to everyone, they're the ubiquitous Harvestmen/Daddy Long-legs. There is a persistent rumor that I always get asked about that says they're the most poisonous Arachnid in the world, but they can't bite you. Half of this is true; they have extremely small chelicerae and couldn't bite you if they wanted to. The other part, about them being venemous, is completely false. While there are some species that are predatory on very small invertebrates, most are detrivores and eat decaying animal matter. There was never a need for venom to be developed for this group.
are known variously as Sun Spiders or Camel Spiders. Back when we first went to the Middle East, a notorious photo of a Solfugid the size of a dinner plate was circulated as having been found in Iraq. Along with this photo came various rumors that they could kill Camels, hence their name, and had put soldiers in hospitals from their bites. So first, the image was based on the camera angle, most Solfugids don't get very large at all. The second part is simply not true; they lack venom, though they can bite, and have much larger chelicerae than most Arachnids. They are very fast and highly predatory, lacking any silk glands. They're a fascinating group, and if you'd like more info, I can send you a term paper on them that a friend of mine wrote.
are commonly known as Vinegaroons or Whip-tailed Scorpions, and are exceedingly odd to look at. These have large palps but very small chelicerae, and you'd have to bother one a lot to get it to bite you. It's most distinctive feature is a long, whip-like projection from its abdomen. For defense, they can excrete a rather foul-smelling compound.
are a seriously bizarre-looking group. They're known as Wind-scorpions or tail-less whipscorpions. Like Vinegaroons and Opiliones, these have small chelicerae, but very large palps that look like claws almost. The long 'whips' are really a highly modified first pair of legs. These are widespread in the tropics and can also be found in the southwest.
The remaining three groups are the only orders that are venomous, and one of which might as well not be.
are minute arachnids that look like tailess, chubby scorpions. They can be found all over the place, I've found them in dirt, under logs, under seaweed at the beach, they're very common. They do in fact have venom, but it's used for catching things smaller than itself, and the venom has virtually no effect on people, if it could actually bite you. Two interesting sidenotes; they're not closely related to Scorpions, and they're the only order besides Araneae with silk glands.
are obviously enough, the group we know as scorpions. These arachnids are found throughout the world, and along with spiders, the only Arachnids that pose a threat to people, and only a minor one, except in rare cases. Since everyone is pretty familiar with what a Scorpion looks like, I'll stick to the essentials. The stinger on the tail delivers venom into the scorpions prey, or whoever might pick it up carelessly. For the most part, a sting from a scorpion is similar to a bee sting in terms of pain. Save for a few species in the Southwest, none of the scorpions in the US are dangerous to people. The few that are still won't kill you, but might lay you up for a day, or necessitate a visit to the doctor. As long as you don't go bother one, the odds of being stung by one of these species is relatively low.
are what most people who write in to Bugguide are concerned with. It is true that spiders are venomous, it is not true that they are aggressive, go out seeking to bit people, or that there is any species in the US that will likely kill you if you're bitten. In North America, there are three major genera of spiders that will cause any more of a problem than a simple bug bite. These are Tegenaria, Latrodectus sp, and Loxosceles
are known as Hobo siders. They don't spin webs and are hunters. If you're bitten by one, it might make you a bit sick, but otherwise you'll be fine.
These are the infamous Widow spiders. In the United States there are 5 species of Widow; Latrodectus mactans (Souther Black Widow), L. variolus (Northern Black Widow), L. hesperus (Western Black Widow), L. geometricus (Brown Widow), and L. bishopi (Red Widow). There combined ranges cover most of North America, though L. bishopi is confined to palm scrub habitat in central Florida. There are about 27 other species throughout the world. Widow spiders are members of the family Therediidae, the Cobweb Spiders. They build, dense, irregular webs in corners of sheds, under rocks, woodpiles and similar habitat (Red Widows are the exception, and build their webs only between palmetto fronds). Because of the places they like to build their webs, they're not something that randomly shows up in your house, and so bites are relatively uncommon. Black Widows use a mild neurotoxin as their venom. If you're bitten, it will hurt, you'll likely get dizzy and nauseous and you'll ache. If the symptoms last longer than a day, you might want to go see a doctor and they can give you antivenom if they deem it necessary. There are less than 1 person a year who dies from a bite from one of these spiders, and the most susceptible are infants and the elderly. The former shouldn't be in any place to encounter them in general. As with all things, allergic reactions can happen, so if the effects seem out of proportion to these symptoms, a doctors visit would be a good idea.
are the equally infamous Recluse spiders or Violin Spiders. These are members of the family Sicariidae, the Six-eyed Spiders. There are several species in the southwest, but the most commonly encountered, and occaisionally problematic species is Loxosceles reclusa, the Brown Recluse. These are hunting spiders and don't build webs of any significant kind. They enjoy hiding in dark places, which is unfortunate because this includes clothing/bedsheets lying on the ground and other sundry items. This is what puts them in contact with people much more than Widows. They won't bit unless they are harassed however, and would rather avoid you if they could. The best thing to prevent these unwelcomed reunions is to keep things off the floor, something I fail miserably at doing. These spiders reach their North and western range here in Kansas. They are mostly confined to the southeast, so reports of them from California or other westerly locales are to be treated dubiously. As I've said, there are a few other species but aren't usually a problem. Brown Recluse spiders use a much different cocktail of venom than Widows. Recluse venom is a necrotoxin, which means it kills the cells around the bite, which can lead to the ugly photos you find on the internet of gaping wounds. That kind of reaction is pretty rare though, You won't notice you've been bitten, but the bite will get red and inflamed, then if you have a bad reaction it will turn dark and a divot around the bite will occur and... Unfortunately, antivenom won't help with this and there's not a whole lot a doctor can do, but I can't stress this enough that this is a really rare thing to have happen, and encounters with these spiders can be kept to a minimum with some simple housekeeping.
To sum it all up. The vast, vast, vast majority of Arachids are harmless. The few that remain aren't inclined to bite you, and you can avoid them pretty easily. If you do get bitten, don't be worried, a visit to the doctor will likely sort any problems out for most spider bites. Like bee stings, the biggest danger is that of an allergic reaction, which is extremely rare. There are 300 million people in the US, and a year can come and go without a single spider-related fatality. Respect spiders, and they won't bother you. Demonizing these cool creatures is doing them a great injustice.
If anyone would like to add or change parts of this, let me know.