8 Facts About the Black Widow Spider

Black widow spiders are more than just their iconic appearance and venomous bite.

moving gif of facts about the black spider widow in her web

Treehugger / Julie Bang

Mention the black widow spider and you'll likely be greeted with a few frantic glances and exclamations of "What?! Where?!"

The black widow, however, isn't as dangerous or as one-dimensional a creature as popular culture suggests. It certainly has a venomous bite, but it also has an incredibly strong web and an odd courting ritual in which male spiders become homewreckers.

1. Widow Spiders Are More than Just Black

brown widow spider, Latrodectus geometricus, in web
A brown widow spider, Latrodectus geometricus, climbs in its web. Joao Paulo Burini / Getty Images

Belonging to the genus Latrodectus, widow spiders encompass 31 known species that exist on every continent in the world except Antarctica. While the three species common in North America — southern (L. mactans), western (L. hesperus), and northern (L. variolus) — are black, other species are a light to dark brown, like the appropriately named brown widow spider (L. geometricus). Some widow species — but not all of them — have a distinct red marking on their abdomen. In black widows, that often takes the form of a red or orange hourglass shape, which contrasts sharply with their otherwise black body. The shape can vary among individuals, though, and it doesn't always closely resemble an hourglass.

2. Female Black Widow Spiders' Venom Is Potent but Rarely Deadly

A female black widow spider sits near its egg sac while a male black widow spider approaches
Male black widow spiders, one of which is pictured above at right, have a variety of techniques for mating with females. Mark_Kostich / Shutterstock

The venom of the black widow is decidedly potent, rated about 15 times stronger than that of a rattlesnake, but a bite is not usually deadly. The spider's bite will bring on muscle pain along with a variety of other symptoms, including trouble breathing, nausea, and numbness around the bite site. These symptoms can become severe — especially for small children or people with a weakened immune system — but in most cases a black widow bite is not life-threatening. Only female black widows will envenomate a human since only their chelicerae — the hollow, needle-like mouth part — is long enough to inject the venom into humans. Additionally, black widow spiders are unlikely to bite you in low-threatening situations, and they may not even use their venom if they do bite you. Whenever possible, they'd almost always prefer to escape than confront a creature as big as us.

3. Black Widows Don't Often Eat Their Mates

Aside from their distinct appearance and venomous bite, the thing female black widows spiders are best known for is killing their mates and devouring them after sex. This trait is so commonly associated with the spiders that the phrase "black widow" is also sometimes used to refer to a human woman who has killed her partner or lover. This reputation for killing, however, is generally undeserved. Mate eating has never been recorded in the wild for most North American species, according to the Burke Museum of Natural History & Culture in Seattle; it has only been observed in lab settings where the male couldn't escape. This doesn't mean it doesn't happen with other members of the genus, but it isn't the norm.

4. Male Black Widows Do Their Best to Avoid Being Eaten

Despite the fact that sexual cannibalism is fairly rare among the black widows, males do try their best to not become a post-coital snack. A 2014 study published in Animal Behavior found that male black widows seek out well-fed virgins for mating. In both controlled field studies and in the wild, researchers observed that males prefer such females, telling them apart from other females due to pheromones they release. In addition to avoiding being devoured by hungry females, researchers believe the males seek a more robust female to increase the chances of fathering healthier and more numerous offspring.

Male black widows will also send out vibrations on a female's web to indicate they are there for mating and not eating. According to a 2014 study published in Frontiers in Zoology, the web plucks performed by males differ significantly from those generated by prey stuck in a web. When researchers played these vibrations back to female black widows, the spiders were less likely to provide a predatory response than when the researchers played back prey vibrations.

5. Male Black Widows Are Literal Homewreckers

As in much of the animal kingdom, competition for mating can be fierce, so males often resort to all kinds of tactics to ensure their genes are the ones carried on. In the case of the western black widow, this apparently involves destroying a female's web. Black widows' webs tend to be messy and tangled, unlike the orderly webs created by some other types of spiders, and when they're ready to mate, females deposit pheromones onto the webs. Males will destroy the web, reducing the females' pheromones and making the web less attractive to other males. For their part, females don't seem to mind the destruction of their property. Researchers believe this is because it cuts down on the potential harassment they experience during mating periods. Indeed, the web reduction even seems to make the females more receptive to mating.

6. The Webs of Black Widow Spiders Are Incredibly Strong

Spider silk has an array of amazing properties. On a per-weight basis, for example, it can be five times stronger than steel. Black widows' web silk particularly is known for its strength, so much so that researchers are striving to replicate its powers in synthetic materials. Attempts to do so have not yielded materials with the same strength or properties, although a 2018 study published in the Proceedings of the National Academy of Sciences may have remedied this issue. Using state-of-the-art imaging techniques, researchers took a closer look than ever before at the protein gland where the web silk is created. There, they discovered a more complex protein-assembly process. Being able to synthetically replicate this process could result in stronger materials for bridges, better materials for plastics, and more durable fabrics for military personnel and athletes.

7. Black Widows Are Not House Spiders

black widow spider female, with red hourglass marking on abdomen
Black widows may bite if they feel threatened, but they generally prefer to flee from humans. Shenrich91 / Wikimedia Commons / CC BY-SA 3.0

Although black widows belong to a group known as "cobweb spiders" (due to their habit of building irregular webs), they are unlikely to be responsible for the cobwebs you find in your house. Some spider species have adapted to share habitats with humans, but black widows are generally not among them. Their preferred habitats are outdoors, in places like vegetation, hollow tree stumps, abandoned rodent burrows, and piles of wood or rocks, although they do sometimes end up in outhouses, garages, or basements. Research has found that black widows can be beneficial for humans, by helping to control pest populations like red imported fire ants and harvester ants, but that still may not be quite enough to offset their frightening reputations for many people.

8. Black Widow Spiders Are Heading North

As climates increasingly shift and change across much of their range, the distribution of the northern black widow is expanding into what used to be prohibitively cold habitats. Outlined in a 2018 PLOS One article, Canadian researchers, relying on citizen science data, found that the northernmost range of the species has increased by some 31 miles (50 kilometers) between 1960 and 2016, creeping into eastern Ontario and Quebec.