Adult female northern widow spiders are shiny black or brown-black with two reddish triangles on the underside, resembling a split hourglass. Adult males are harmless, about half the female's size, with smaller bodies, longer legs and usually have yellow and red bands and spots over the back as do the immature stages. Newly hatched spiderlings are predominately white or yellowish-white, gradually acquiring more black and varying amounts of red and white with each molt. Juveniles of both sexes resemble the male and are harmless. These spiders spin tangled webs of coarse silk in dark places, usually outdoors.
Sacs are white at first, later turning pale brown.
About 300 to 400 eggs per sac is common with 4 to 9 egg sacs produced during a summer. Normally, only 1 to 12 young survive after the egg incubation period of 14 to 30 days due to cannibalism. Growth requires 2 to 4 months depending on availability of prey during which the females molt 6 to 8 times and the males 3 to 6 times. Females mature 92 days after egg sac emergence and live about 179 days longer, whereas males mature 71 days after emergence and live 30 days longer. Usually the female eats the male after mating.