Fiddleback Spider

Since the violin pattern is not diagnostic, and other spiders may have similar markings (such as cellar spiders and pirate spiders), for more assurance in identification it is imperative to examine the eyes. Most spiders have eight eyes; recluse spiders have six eyes arranged in pairs (dyads) with one median pair and two lateral pairs. Only a few other spiders have three pairs of eyes arranged in this way (e.g., scytodids). Recluses have no obvious coloration patterns on the abdomen or legs, and the legs lack spines. The abdomen is covered with fine short hairs that (viewed without magnification) give it the appearance of soft fur. The leg joints may appear to be a slightly lighter color.

As suggested by its specific epithet reclusa ("recluse"), brown recluses are rarely aggressive, and actual bites from the species are rare. (In 2001, more than 2,000 brown recluse spiders were removed from a heavily infested home in Kansas, yet the four residents who had lived there for years were never harmed by the spiders, despite many encounters with them. The spider usually bites only when pressed against the skin, such as when tangled up within clothes, towels, bedding, inside work gloves, etc. Many human victims of brown recluse bites report having been bitten after putting on clothes that had not recently been worn or lying undisturbed on the floor. The initial brown recluse bite frequently is not felt and may not be immediately painful, yet such a bite can be serious. However, the fangs of the brown recluse are so tiny they are unable to penetrate most fabric, including socks.

The brown recluse bears a potentially deadly hemotoxic venom. Most bites are minor with no necrosis. However, a small number of clinically-diagnosed brown recluse bites do produce severe dermonecrotic lesions (i.e., necrosis); an even smaller number of clinically-diagnosed brown recluse bites produce severe cutaneous (skin) or viscerocutaneous (systemic) symptoms. In one study of clinically-diagnosed brown recluse bites, the incidence of skin necrosis was 37% and the incidence of systemic illness was 14%. In these instances the bites produced a range of symptoms common to many members of the Loxosceles genus known as loxoscelism, which may be cutaneous (skin) and viscerocutaneous (systemic).

A brown recluse's stance on a flat surface is usually with all legs radially extended. When alarmed it may lower its body, withdraw the forward two legs straight rearward into a defensive position, withdraw the rearmost pair of legs into a position for lunging forward, and stand motionless with pedipalps raised. The pedipalps in mature specimens are dark, quite prominent, and are normally held horizontally forward. When threatened it usually flees, seemingly to avoid a conflict, and if detained may further avoid contact with quick horizontal rotating movements. The spider does not usually jump unless touched brusquely, and even then its avoidance movement is more of a horizontal lunge rather than a vaulting of itself entirely off the surface. When running the brown recluse does not appear to leave a silk line behind, which at any rate might make it more easily tracked when it is being pursued. Movement at virtually any speed is an evenly paced gait with legs extended. When missing a leg or two it appears to favor this same gait, although (presumably when a leg has been injured) it may move and stand at rest with one leg slightly withdrawn. During travel it stops naturally and periodically when renewing its internal hydraulic blood pressure that, like most spiders, it requires to renew strength in the legs.