The venomous spiders are nimble, secretive and dangerous
Sexual cannibalism is surprisingly rare
Black widows earned their name because scientists witnessed the females eat their mates after copulation. But research has shown that in a related species, redback spiders, females only cannibalize their mates about two percent of the time, so experts suspect that American black widows have similar rates of cannibalism in the wild.
The widows’ cannibalistic behavior was first observed in the lab, where males had nowhere to run away from their larger, hungrier counterparts. But in the spiders’ natural habitats, males have the opportunity to make an escape.
Male black widows also have strategies to avoid riskier sexual encounters in the first place; for instance, research suggests they can tell whether or not a female is hungry by her pheromones, so they can avoid potential mates who seem a bit peckish.
And some related species take an aggressive approach. Brown widow and redback spiders sometimes use a process called “traumatic insemination.” If a male happens upon a juvenile female that has developed only its internal plumbing, the male can pierce a hole in the female’s shell with its fangs and mate. The practice didn’t seem to cause permanent harm to the female spiders, and it gave the males a chance to pass on their genes without getting eaten, and search for another mate down the line.
Tiny slits are used for “Spidey” senses
All of the spiders in the Latrodectus genus have a few things in common: curved feet covered in bristles, earning them the name comb-footed spiders, and messy, irregular nests of silk called tangle webs. Western black widows take two different strategies to build their webs depending on how well-fed they are: starving spiders build more sticky threads, which snag prey, and healthy spiders invest more time in supporting threads, which may stop them from overeating.
The spiders rely on strands of silk in their tangle webs as extensions of their own senses. Thousands of organs called slit sensilla, which look like cracks in the exoskeleton and are especially common on their leg joints, feel vibrations in the silk. By changing its posture, a spider changes the shape of the slit sensilla, so a black widow can tune its senses to certain frequencies of vibrations coming down its web.
Coloring sends a message
The red hourglass on a female black widow’s abdomen sends a clear message: Danger. But humans aren’t the only ones on the lookout for a black widow’s signals. The insects hunted by black widows want to avoid falling into their jaws. Birds and wasps, which generally avoid red critters since it’s a common sign of venom, prey on spiders. (The black widow’s venom doesn’t pack a punch when it’s the one getting eaten.) So as the black widows evolved, they needed to strike a balance between hiding from prey and warning predators off.
Colorado College spider researcher Nicholas Brandley conducted experiments with 3D-printed widows showed that bright red spots protected the fake spiders from bird attacks, he told Smithsonian magazine in 2016. Unadorned plastic spiders were attacked three times more often than the red-spotted ones. In another experiment, a live black widow with many red spots tended to build its web higher up in terrariums than its less-colorful counterpart. The extra spots may give it more protection from predators up high and lurking below.
Climate change is expanding their range
Black widows are most common in the warm environments of the southern and southwestern United States. While they tend to disappear when winter weather arrives, they don’t actually get killed when the temperature starts to drop. Instead, black widows find a protected area and go into a dormant state called overwintering. In spring, they emerge, and the tricky business of mating begins.
Black widows are rare at the northernmost stretches of their range, but climate change may soon change that. Northern black widows today live about 31 miles further into Canada than they did in the 1960s.