There are three spiders in the United States that are venomous. The Black Widow, the Brown Recluse and the Hobo Spider. The Black Widow is probably the easiest to identify and also the most famous of the three. The Brown Recluse is the most common species and the most likely to get bit by. We do not have Hobo Spiders in my area so I am not real familar with them.
The Black Widow is a somewhat small spider. The female is usually solid black with red markings or an hour glass on its belly. The hour glass can also be a light orange color. The male Black Widow is brown and about 1/10th the size of the female. The male Black Widow is not poisonous. The name Black Widow comes from the fact that the Female will usually eat the male after mating occurs. Black Widows are not agressive but will bite in self defense. Black Widow Spiders are usually found under rocks, in old barns and under debris. Black Widow Spiders eat insects which is catches in its web. Black Widow Spiers do not make an open web like most spiders but instead make more like a cobb web under debris.
The Brown Recluse is also a small spider. Reaching about the size of a quarter when full grown. The Brown Recluse is most famous for the violin shaped spot on its head. The Brown Recuse is nicknamed the Fiddleback because of this marking. Brown Recluse spiders are tan to light brown in color with the tail section usually being dark. They also have a fiddle or violin shaped dark brown spot on there head. The male and female Brown Recluse Spiders look identical and can not be sexed by a layman.
The Hobo Spider is the third species of spider in the United States considered dangerous. Many studies have been done on the Hobo Spider and no one can seem to agree on whether this species is dangerous. Some scientist believe the bite of a Hobo Spider can cause necrosis but other studis have found no sign of necrosis in Hobo Spider Bites. The Hobo Spider is found in the Northern States and rarley bites unless forced to.