Although many spiders are both frightening and venomous, among the spiders of the United States and Canada only the Black Widow (Lactrodectus mactans) and the Brown Recluse (Laxosceles reclusa) are considered dangerous.

Female Black Widows (the only ones that cause a problem) are shiny black, medium-sized, and rather plump. They are characterized by a more or less hourglass shaped red mark on their underside (they generally hang upside down). They spin tangled, coarse webs in darkened areas indoors or out. If not well-fed, the females notoriously eat their mates after mating. Eggs are laid in white to brown sacs, with the young emerging in a few weeks. Fortunately for us, most of the young are consumed by their fellows long before maturity. Maturity takes about three months from birth, and the longest life spans are well under a year.

The Brown Recluse is a much less common spider, with its major concentration in an area southwest, south, and southeast from southern Iowa and northern Illinois. Brown in color, it has a violin-shaped mark of a slightly different hue extending along the back from the eyes to the abdomen. As the name implies, it prefers dark areas such as the underside of a rock. Unfortunately, it also favors basements and fabrics (stored, for example, in boxes in the basement).

The females of both spiders bite humans in self-defense or defense of their eggs. Fatalities from the bites of either are rare. A Black Widow bite can produce severe symptoms, including sweating, nausea, cramping, and difficulty in breathing. The bite of the Brown Recluse can produce an alarming, festering, oozing sore that refuses to heal. Scarring frequently occurs.