Many thousands of spider species are found worldwide, and nearly all possess venom glands connected to fangs in the large, paired jaw-like structures known as chelicerae. Fortunately, only a very few spider species have fangs long and tough enough to pierce human skin. In the United States, these spiders include Latrodectus (widow spider) and Loxosceles (brown spider) species, tarantulas (a common name given to several large spider species), and a few others.
Patient complaints of "spider bites" occur much more commonly than do actual spider bites. Unexplained skin lesions, especially those with a necrotic component, are often ascribed to spiders, especially the brown recluse spider. Health care providers should consider alternative etiologies in the absence of a convincing clinical history and presentation. Many alleged "spider bites" are actually infections, with community-acquired methicillin-resistant Staphylococcus aureus (MRSA) being a common etiology.
Latrodectus species (black widow spiders) are ubiquitous in the continental United States, and the female can cause serious envenomations with rare fatalities. Black widows construct their chaotic webs in dark places, often near human habitation in garages, wood piles, outdoor toilets, and patio furniture. The spider has a body size of 1–2 cm and is characteristically shiny black with a red to orange-red hourglass shape on the ventral abdomen. The brown widow spider (L. geometricus) has recently been introduced into southern California and has spread along the Gulf of Mexico coast from Florida to Texas. This spider has variegated tan, brown, and black markings, also with a reddish hourglass on the abdomen, and envenomations result in the same clinical effects as from black widows.
Loxosceles reclusa (the brown recluse spider) is found only in the central and southeastern United States (e.g., Missouri, Kansas, Arkansas, and Tennessee). Rare individual specimens have been found in other areas, but they represent stowaways on shipments from endemic areas. Other Loxosceles species may be found in the desert southwest, although they tend to cause less serious envenomations. The spider's nocturnal hunting habits and reclusive temperament result in infrequent contact with humans, and bites are generally defensive in nature. The spider is 1–3 cm in length and light to dark brown in color, with a characteristic violin- or fiddle-shaped marking on the dorsum of the cephalothorax.
Tarantulas rarely cause significant envenomation but can produce a painful bite because of their large size. Tarantulas also bear urticating hairs that they can flick at predators and that cause intense mucosal irritation. People who keep pet tarantulas have developed ocular inflammation (ophthalmia nodosa) when these hairs embed in their corneas, usually while they are cleaning their spiders' cages.
Spiders use their hollow fangs (chelicerae) to inject their venoms, which contain various protein and polypeptide toxins that appear to be designed to induce rapid paralysis of their insect victims and aid in digestion.