While most spiders are probably harmless, several southwestern spiders have venoms that are toxic, in varying degrees, to humans. These include the Western Black Widow, Latrodectus hesperus Chamberlin and Ivie and other species of Latrodectus which may on occasion be imported to the area. The Brown Widow, Latrodectus geometricus Koch, has recently been reported in Arizona and is expected to invade New Mexico eventually, if it has not already. It is more retiring than the Black Widow species and less likely to bite. While some true Western Black Widows my be chocolate brown, the general pattern and the egg sacs can easily distinguish the two species. The Brown Widow is usually lighter brown with dark curved markings and has egg sacs with tufts scattered over the surface, while all Black Widow species have smooth egg sacs. Several species of violin spiders, Loxosceles spp., are also quite venomous. These include L. arizonica, L. deserta, L. apachea, L. blanda and (in eastern New Mexico and by introduction in various parts of the Southwest) L. reclusa. This latter species is now established in the Roswell, New Mexico, area. Other spiders found in the Southwest that have been reported to have venoms that cause local reactions include Dysdera crocata, Plectrurys tristis, Peucetia viridans, Chiracanthium inclusum, and Phidippus audax. The widows and the violin spiders are probably the most dangerous. Jumping spiders in the genus Phidippus produce very painful bites, but these are usually without sequelae.
The venom of the widow spiders is neurotoxic and is known to attack the vesicles containing the neurotransmitter acetylcholine at synapses, including those at neuro-muscular junctions. The effects of the bite are very similar to flu, with muscle aches and a general run-down feeling. Agitation and restlessness symptoms may also occur. In more serious cases there may be respiratory paralysis and even death. This is very rare and only about 5% of the cases are fatal, usually because of other factors, including heart disorders. The venom is most dangerous to older people and children. Fortunately, black widows are very shy and very few bites are recorded for the number of spiders occurring in the region. Bites were somewhat more numerous during the age of outside toilets. Females are the dangerous sex and these usually only bite to defend egg sacs or if pressed between clothing and skin (see Gertsch 1979, Smith 1982, Goddard 2003).
The venom of the violin spiders (including the brown recluse) is quite different. It is primarily composed of enzymes that have a tissue-destroying effect. Whereas there is little to note at the site of a black widow bite, the area of a bite of a violin spider swells and hardens. The swelling eventually drops off, leaving an angry ulcerating wound in its place. All of the species seem to have this effect, although a similar symptomology may be found in some other spiders as well. Occasionally (very rare) systemic effects may occur, as when venom is carried to the liver or other internal organ. This is more serious and can result in death. The wound on the skin may not heal for some time, even months, and is a source for possible infection. Occasionally this may lead to a massive bacterial infection and even death. However, these effects are rare. Usually the sore slowly heals, often leaving a nasty scar (see Gertsch 1979, Smith 1982, Goddard 2003). In addition violin spiders are not especially aggressive and may live in numbers in a house without a bite occurring.
Remember that if a spider bites you SAVE THE REMAINS OR CATCH THE SPIDER IN A VIAL OR JAR. Having the specimen may help to identify it and thus help with deciding the treatment to give for the bite. To avoid bites, never place your hand in an area you cannot see into and never place your hand into a tangled web. Spider bites may, paradoxically, be both underreported and over reported. True bites may be ignored by the person involved or are misinterpreted. However, as often as not "spider bite" is written down as a diagnosis even when no spider is associated with the bite. Up to 80% of 600 "spider" bites studied in California were actually caused by ticks or reduviid bugs (Vetter and Visher 1998). Some "bites" may actually be bacterial or fungus infections or possibly the onset of Lyme Disease, so it is wise to be suspicious of "easy" diagnosis without a specimen.
In recent years there have been several Internet hoaxes involving the mythical deadly "toilet spider," supposedly imported from South America or the Orient and found in a toilet in a restaurant near a major airport. This story has caused quite a bit of worry, but there is no such animal. The creature in the earlier version of the hoax was called the South American blush spider, "Arachnis gluteus," and this totally fictitious name was later replaced with Telamonia dimidiata, a valid name for a harmless (as far as anybody knows) Asiatic jumping spider.