Sends a message by color

Female black widows are particularly notable for having jet black skin with red patterns on the underside. A female black widow's crimson hourglass on the abdomen clearly signals danger. However, animals other than humans are also on the watch for a black widow's signals. Black widows chase insects, and those insects try to avoid getting caught in their teeth. Spiders are preyed upon by birds and wasps, who often stay away from red creatures since they are often venomous. (The black widow's poison is weak as the victim is being devoured.) Black widows therefore had to find a balance as they developed between concealing from prey and scaring predators away.

According to trials by Colorado College spider expert Nicholas Brandley using 3D-printed widows, the synthetic spiders were shielded from bird assaults by conspicuous red markings, he told Smithsonian magazine in 2016. Attacks on plain plastic spiders occurred three times as frequently as those on red-spotted ones. A live black widow with numerous red dots preferred to build its web higher up in terrariums than its less colorful cousin, according to a different experiment. It may be more protected from predators below and in the sky thanks to the numerous spots.