As a field biologist by profession I spent much of my life studying predatory arthropods. My experience was not usually with really venomous animals, but I was often asked to determine the identity of suspected dangerous spiders. One of these critters is the much feared Black Widow (Latrodectus spp., Family Theridiidae.) Actually there are also White Widows (Middle East - Israel), Red Widows (Florida), Red Back Spiders (Australia) and Brown Widows (Cosmotropical.) However in North America it is the black widows (three species - Latrodectus mactans, L. variolus and L. hesperus) that get the most attention. These are (in same order) called the Southern Black Widow, the Northern Black Widow and the Western Black Widow. All of these have neurotoxic venom (which attacks the neurotransmitter vesicles at synapses), but the Southern Black Widow is thought to be the more dangerous species. Even so the mortality rate for all the widows has been essentially zero for ten to twenty years. This is not to say that such a bite is inconsequential and indeed the symptoms of the neurotoxic venom can be quite alarming. These often include muscle aches, agitation and sometimes a light fever. In more serious cases respiratory paralysis might occur, but this is fairly rare. My experience has been mostly with the Western Black Widow, which is common through much of the western states and into Mexico.
Unlike the other black widows, the Western Black Widow is highly variable and I have seen everything from nearly all black with no hourglass mark, to being dorsally covered with canary yellow spots. In between some were plum-colored or chocolate brown! The typical color (see below) is shiny black with a bright red hourglass marking on the underside. They are primarily nocturnal, but on occasion I have seen them on their haphazard webs in broad daylight. The diminutive males often cohabit with the females and although females will kill males and eat them, it is by no means the rule as it appears to be with the Australian Red Back Spider.
Western Black Widow (Latrodectus hesperus) typical female, Mesilla Park, New Mexico. (Photo by me).
Black Widows were more often encountered in biting situations during the era of outdoor toilets, leading to some embarrassing bite locations. I have seen extremely dense populations of these spiders in Arizona and New Mexico. For example I noticed strong haphazard webbing in the large arborvitae hedge around the public library in Yuma, Arizona. I was able to examine the hedge at night with a flashlight and discovered at least one female Western Black Widow every cubic foot! The population must have reached the hundreds at least!
Unlike the Southern Black Widow, which usually builds its web close to the ground (often in an animal burrow), or the Northern Black Widow, which usually builds its webs in trees or shrubs, the Western Black Widow builds webs just about anywhere. I have seen small to huge webs, some high in trees or (as in the library population) up to over ten feet in a hedge, as well as much lower in animal burrows, rock crevasses, junk yards, and inside houses and sheds.
Widows are often confused with the False Black Widow (Steatoda grossa - also in the family Theridiidae, or comb-footed spiders) and the female of the Southern House Spider (Kukulcania spp. - in the family Filistatidae) The False Black Widow is an import from Europe, but the Southern House Spider is native. There are at least two very similar species of Southern House Spider in the Southwest. Neither of these has red markings and the Southern House spider is fuzzy, unlike true black widows.
In general, widow spiders do not bite easily, unless protecting their egg sacs, or trapped between clothing and skin. Reasonable precautions will minimize the possibility of being bitten, even if you are in a densely populated area.