Black Widow

This picture helps illustrate the difference between the female and male black widows the female has the red "hour glass" on her bottom the male which is the smaller of the two is not at all poisonous to humans or animals.

Black Widow spiders inhabit most of the warmer regions of the world to latitude of about 45 degrees N. and S. They occur throughout all four deserts of the American Southwest. The female Black Widow is shiny black, usually with a reddish hourglass shape on the underside of her spherical abdomen. Her body is about .5 inches long, 1.5 inches when the legs are spread. Adult males are harmless to humans, about half the female's size, with smaller bodies, and longer legs and usually have yellow and red bands and spots over the back as do the immature stages.
Newly hatched spiderlings are predominately white or yellowish-white, gradually acquiring more black and varying amounts of red and white with each molt. Juveniles of both sexes resemble the male and are harmless to humans. The venom of the female black widow spider is 15 times as toxic as the venom of the Prairie Rattlesnake. Only a minute amount of the toxin is injected in a single bite by the spider however, so they are rarely fatal. By comparison, the relatively large amount of injected rattlesnake venom results in about 15 to 25 percent mortality among those bitten.