Not all adult female black widows exhibit the red hourglass on their abdomen, some may have a pair of red spots or have no marking at all. Any markings that are present are bright red. Adult male black widows are a quarter the size of the female and are usually gray or brown rather than black and red. While they may sometimes have an hourglass marking on their abdomen, it is most commonly yellow or white and not red. The bite of a male black widow is not considered dangerous to humans. It is the bite of the adult female black widow that has given this spider its dangerous reputation, due to her much larger venom sacs.
The widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very frequently hangs upside down near the center of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and wrap it in silk. As other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. While there are some more aggressive spiders, most widows are not aggressive. Many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched or in cases where a female is protecting an egg sac.