The term “poisonous” is often used to describe spiders, but scientifically, the correct term is “venomous,” as the toxin is injected through a bite. In Illinois, most of the hundreds of spider species are harmless to humans. However, two native species possess venom potent enough to be considered medically significant: the Brown Recluse and the Black Widow. Knowing the difference between these spiders and their potential effects is key to minimizing risk.
Medically Significant Species in Illinois
The two medically significant species in Illinois belong to different families, reflected in the type of venom they produce. The Brown Recluse (Loxosceles reclusa), most common in the southern two-thirds of the state, carries cytotoxic venom. This venom can cause localized tissue damage known as necrosis. While severe necrotic wounds are possible, most bites are minor, often resulting in only a small swelling that heals without serious complication.
The Black Widow spider (Latrodectus spp.) produces neurotoxic venom that affects the nervous system. Although this venom is highly potent, the amount injected during a defensive bite is usually small. Symptoms of a serious bite involve systemic effects, such as widespread muscle pain and cramping. Severe envenomations are rare in Illinois, and bites are seldom fatal, especially for healthy adults.
Identification and Common Habitats
Correctly identifying these spiders is crucial. The Brown Recluse is a small, light to dark brown spider. It is best recognized by the distinct violin-shaped marking on its cephalothorax, with the “neck” pointing toward the abdomen. The most reliable identifier is its eye arrangement: six eyes arranged in three pairs, unlike most spiders which have eight eyes.
The Black Widow is glossy black and features a distinctive red or reddish-orange hourglass shape on the underside of its abdomen. In some Illinois species, this hourglass may appear broken, forming two separate spots. The female, which is larger and carries the potent venom, displays these iconic markings.
Both species are reclusive, preferring dark, quiet, and undisturbed locations. Brown Recluse spiders frequently inhabit human structures, seeking refuge in cluttered areas like basements, closets, attics, and garages. They are often found in stored items, shoes, clothing, or bedding left on the floor. Black Widows typically build irregular webs closer to the ground, often in woodpiles, sheds, under patio furniture, or near outdoor openings. Reducing clutter helps eliminate their hiding spots.
Bite Symptoms and Emergency Action
If a spider bite is suspected, initial first aid focuses on wound care. The bite area should be thoroughly cleaned with mild soap and water to prevent secondary infection. Applying a cold compress or ice pack wrapped in a clean cloth can help reduce pain and swelling. It is also helpful to elevate the affected limb, and any jewelry, such as rings, should be removed promptly before swelling begins.
Symptoms from a Brown Recluse bite often progress slowly, sometimes beginning hours after the initial bite. The wound may develop a blister and a characteristic “bull’s-eye” or target lesion, with a purplish center indicating tissue damage. In contrast, a Black Widow bite typically causes immediate sharp pain, followed by severe muscle cramping, abdominal rigidity, nausea, and sweating, as the neurotoxin affects the body.
Any suspected bite from a medically significant spider requires immediate medical attention, even if symptoms initially appear mild. It is particularly important to seek care if the victim is a child or elderly person. Professional medical evaluation is necessary if systemic symptoms develop, such as difficulty breathing, intense muscle pain, fever, or vomiting, or if the wound shows signs of infection or begins to spread.