Adult brown recluse spiders are soft-bodied, yellowish-tan to dark brown, about 1/4 to 1/2 inch long and have long, delicate grayish to dark brown legs covered with short, dark hairs. The leg span is about the size of a half dollar. Distinguishing characteristics are the presence of three pairs of eyes arranged in a semicircle on the forepart of the head and a violin-shaped, dark marking immediately behind the semicircle of eyes with the neck of the violin pointing towards the bulbous abdomen.

The severity of a person's reaction to the bite depends on the amount of venom injected and individual sensitivity to it. Bite effects may be nothing at all, immediate or delayed. Some may not be aware of the bite for 2 to 8 hours, whereas others feel a stinging sensation usually followed by intense pain if there is a severe reaction. A small white blister usually rises at the bite site surrounded by a large congested and swollen area. Within 24 to 36 hours, a systemic reaction may occur with the victim characterized by restlessness, fever, chills, nausea, weakness and joint pain. The affected area enlarges, becomes inflamed and the tissue is hard to the touch. The spider's venom contains an enzyme that destroys cell membranes in the wound area with affected tissue gradually sloughing away, exposing underlying tissues. Within 24 hours, the bite site can erupt into a "volcano lesion" (a hole in the flesh due to damaged, gangrenous tissue).

The open wound may range from the size of an adult's thumbnail to the span of a hand. The sunken, ulcerating sore may heal slowly up to 6 to 8 weeks. Full recovery may take several months and scarring may remain. Plastic surgery and skin grafts are sometimes required. The brown recluse spider is not aggressive and normally bites only when crushed, handled or disturbed. It is aptly named since it is a reclusive creature seeking and preferring seclusion.