The recluse spiders or brown spiders (genus Loxosceles), also known as fiddle-back, violin spiders or reapers, are a genus of venomous spiders known for their bite, which sometimes produces a characteristic set of symptoms known as loxoscelism. Recluse spiders are now identified as members of the family Sicariidae, having formerly been placed in their own family, the Loxoscelidae.
Loxosceles is distributed nearly worldwide in warmer areas. All have six eyes arranged in three groups of two (dyads) and some are brownish with a darker brown characteristic violin marking on the cephalothorax. However, the "violin marking" cannot be used as a reliable way to identify the spider as literally thousands of species of spider have the same markings. Spiders come with many markings varying greatly within the same species. Most Loxosceles can live for one and a half to two years. Members of both genera can live for very long times without food or water. They are about 7–12 mm long.
Familiar species in the United States include the brown recluse spider (Loxosceles reclusa). It is found in a large area of the Midwest, west to Colorado and the New Mexico state line and east to northern Georgia. Sporadic records from other locations only represent incidental introductions, not established populations. Other notable members of this genus include the Chilean recluse spider (L. laeta) and the Mediterranean recluse spider (L. rufescens).
Recently, concerns have been raised regarding recluses spreading faster due to warmer air carrying them farther as a result of changing climate. On the contrary, newly hatched recluses do not travel via ballooning and thus the populations are confined to very tight spaces with dense populations.
Venom components and effects
Loxosceles spiders, like Sicarius species, have potent tissue-destroyingvenoms containing the dermonecrotic agent, sphingomyelinase D, which is otherwise found only in a few pathogenic bacteria. Recent research has indicated the venom is composed largely of sulfatednucleosides, though these compounds are relatively new discoveries, so little is known about them. The venom produces necroticlesions that are slow to heal and may require skin grafts. The wounds are also prone to infection. Rarely, the venom is carried by the bloodstream to internal organs, causing systemic effects.
The venom is identical in male and female spiders, but females can have almost twice the concentration of toxins. For unknown reasons, the toxicity of the venom to mammalian species varies; recluse bites will cause necrosis in humans, rabbits, and guinea pigs, but not in mice or rats.
The Chilean recluse (L. laeta) supposedly has a more potent venom, which results in systemic involvement more often. This spider was accidentally introduced to the Los Angeles area (Alhambra, Sierra Madre, and Monterey Park). This spider, however, seems to be confined to a very limited area, even though it has been known there for over 30 years. All Loxosceles species that have been tested have venoms similar to that of the brown recluse and all should be avoided. In general, though, they are not aggressive and commonly occupy human dwellings without causing problems.
Many types of skin wounds are mistaken for or assumed to be the result of a recluse spider bite. Several diseases can mimic the lesions of the bite, including Lyme disease, various fungal and bacterial infections, and the first sore of syphilis. It is important to associate the spider directly with the bite to initiate proper treatment, and to consider alternative diagnoses if no spider was seen.
A recluse spider is usually found in the center of its web, which often contains the remnants of prey items. The most common food items for the Arizona recluse (L. arizonica) are night-active ants such as carpenter ants. The brown recluse feeds on whatever small prey is available, and has been observed to prefer scavenging over actively hunting.
Bites most often occur as a defense when the spider is trapped against the skin, in clothing, for example. Insecticides often fail to kill the spider, instead intoxicating its nervous system and inducing aggressive behavior.
The bite of a recluse spider can generally be categorized into one of the following groups:
Unremarkable - self-healing minute damage
Mild reaction - self-healing damage with itchiness, redness, and a mild lesion.
Dermonecrotic - the uncommon, "classic" recluse bite, producing a necrotic skin lesion. About 66% of necrotic bite lesions heal with no complications. In extreme cases, the lesion may be up to 40 centimeters wide, last for several months, and heal with a permanent scar.
Systemic or viscerocutaneous - an extremely rare, sometimes fatal systemic reaction to envenomation of the bloodstream. This reaction is more common in obese victims, because the venom destroys adipose tissue. It is more often fatal in children.
^Vetter, R. and D. Barger. (2002). "An infestation of 2,055 brown recluse spiders (Araneae: Sicariidae) and no envenomations in a Kansas home: implications for bite diagnoses in nonendemic areas". J Med Entomol39 (6): 948–51. doi:10.1603/0022-2585-39.6.948. PMID12495200.
^Vetter, R. and S. Bush. (2002). "The diagnosis of brown recluse spider bite is overused for dermonecrotic wounds of uncertain etiology". Ann Emerg Med39 (5): 544–6. doi:10.1067/mem.2002.123594. PMID11973562.