What's a Loxosceles?
Loxosceles is the genus of spiders that includes the infamous brown recluse spider Loxosceles reclusa. These spiders, also called the violin spiders or fiddleback spiders because of violinlike marks on the back of their cephalothorax, are brownish-yellow in color, have six eyes, and spin small, irregular webs under rocks or in nooks and crannies of your house. Loxosceles species are "medically significant" - in other words, do not touch. Interesting sidenote: I took the photos below of a spider that was in the bathroom of my parents' house near Pittsburgh; I didn't realize until researching this writeup that the spider looks just like the Brown Recluse Loxosceles reclusa. (please note: an entomologist has told me that I am probably wrong about that. In any case, the photos below are of an arachnid that looks confusingly similar to a Loxosceles, so, close enough :)
Range and appearance
Thirteen species of violin spiders are found in the southwest and midwest USA, with L. reclusa having the largest range. Other Loxosceles can be found in South America (including L. laeta), Europe (including L. rufescens) and Africa. (I am still looking for a comprehensive list of species, preferably with range maps ... write me if you have information).
The characteristic violin marking takes the form of a large dark spot around the spider's "head", with the skinny neck of the violin pointing back toward the abdomen. It doesn't look much like a violin to me, and it isn't always present in young spiders, or in some of the more western species. The most characteristic feature of Loxosceles species is actually the eyes: most spiders have eight eyes, but Loxosceles spiders have six eyes, arranged in three pairs, or dyads, that sit side-by-side.
Loxosceles spiders are nocturnal. They spin their "small, irregular webs" as a lining in their hiding places, which tend to be under rocks or bark. This hiding behavior has earned these spiders the name of "recluse" (as in, the Brown Recluse L. reclusa, the Desert Recluse L. deserta, etc). They do leave their retreats, however, which distinguishes them from the Pholcidae, also called the Daddy Long Legs Spiders.
The family Sicariidae includes, in addition to Loxosceles, another genus called Sicarius. Sicarius hahnii, a South African spider, is considered by some to be the world's most dangerous spider; its bite can result in death. Loxosceles, however, usually only causes soft tissue damage.
Many spider bites are attributed to Loxosceles spiders, even in states where they are never found. If you think you have been bitten by a violin spider, make sure to bring the spider with you to the emergency room so that it can be turned over to an entomologist for proper identification. Some house spiders, for instance, have bites that look like Loxosceles bites at first.
The Loxosceles spiders are nocturnal, and spend the daytime hidden under rocks or in folds of clothing. (watch out!) The bite of a Loxosceles spider is not deadly, but it is very unpleasant - the venom is a necrotoxin, meaning it causes tissue to die and fall off. Pain usually won't begin until 6-12 hours after the bite occurs. Not all bites result in the painful ulcers and scarring that is associated with the worst bites. In these cases, the skin sloughs off, leaving a raw area anywhere from half an inch to around 8 inches in diameter. This wound may take anywhere from several weeks to a year to heal properly, and can leave a nasty scar. There may also be a systemic reaction within 24-36 hours characterized by restlessness, fever, chills, nausea, weakness, and joint pain.
Loxosceles's necrotoxic venom is cytotoxic and hemolytic. It contains at least 8 enzymes; the enzyme thought to be responsible for most of the destructive effects is called Sphingomyelinase D.
Mortality and morbidity figures are unknown, because the spider is not usually positively identified. However, death is rare, if it occurs at all. Most envenomations only result in soft tissue damage.
References and further reading:Family: Sicariidae
arachnology.org links for Loxosceles
Entomology and means of controlling L. reclusa
Biological and Biochemical Properties of Loxosceles Venom
Brown recluse spider biology and elimination
Brown recluse spider envenomations