Black widows are vanishing. Here's their new enemy.

In the spider world, even predators are hunted.

Biologists in regions of the Southern U.S. noticed that the iconic black widow spider — known for its telltale hourglass mark and venomous bite — often disappeared when a different species, brown widows, showed up. Now, new research published in the Annals of the Entomological Society of America provides compelling evidence that the brown widows are indeed driving Southern black widows out, in part by attacking and killing the well-known arachnids.

"They don't seem to be good neighbors with each other," Louis Coticchio, a biologist and graduate student at the University of South Florida who led the research, told Mashable.

When these two species occupied the same territory in the wild, Coticchio repeatedly observed that black widows slowly decline, until sometimes they were gone.

But to find out exactly what was happening in parts of the Southern U.S. wilderness, biologists needed to carefully observe the spiders interacting in a lab.

Black widows become the hunted

Black widows are a native spider in the U.S., and the Southern black widow, or Latrodectus mactans, is the species commonly found all over the Southeast. The brown widow, or Latrodectus geometricus, meanwhile, likely originated in Africa and has now established itself across the Southern U.S. after scientists first spotted the arachnid in 1935. It, too, has a distinct hourglass marking.

To best see what's transpiring between these two widow species, Coticchio tested close brown widow interactions with three related spider species: the Southern black widow, the red house spider (Nesticodes rufipes), and the triangulate cobweb spider (Steatoda triangulosa). The interactions happened in containers filled with twigs and branches.

The results were stark. Brown widows were 6.6 times more likely to aggressively attack the black widows compared with the other species.

Indeed, brown widows peacefully cohabitated with the red house and triangulate spiders at times, and sometimes themselves were eaten by their container mate.

But a key behavioral difference is that brown widows were often observed as aggressors, meaning invading their neighbor's space and potentially attacking. Meanwhile, black widows didn't invade. They weren't boldly encroaching on others' territory. Rather, they were often on the defensive. This allowed brown widows to easily attack.

"The brown widows are basically eating their competition. They're eating them preferentially," Mark Willis, an entomologist at Case Western Reserve University who was not involved in the research, told Mashable.

A brown widow (on right) killing a black widow in the researchers' lab experiment. Credit: Louis Coticchio / USF

Both Willis and Coticchio noted that this predation likely isn't the only factor driving brown widows to displace black widows in the wild. For example, brown widows can produce more offspring, because they can reproduce earlier in their lives and lay more eggs.

The reality that black widows, a well-known predator, have become popular prey for another spider isn't an unusual happening. Spiders are highly cannibalistic creatures.

"All spiders are predators. They have to kill something to eat. They don't care if it's another spider," Willis explained. "There are some spiders that specialize in hunting and eating other spiders. If there's anything that's 100 percent a predator, it's a spider."

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The impact of declining black widows

Biologists have much to learn about the decline of the Southern black widows and why it's happening. Is the brown widow in Africa an aggressor between the species, too? Or did the brown widow learn to attack the black widow in the U.S.?

"There are still a lot of questions that need to be answered," Coticchio said.

Perhaps the most important question of all is how the displacement and decline of Southern black widows will impact the existing native ecological relationships in the natural world. "If they're eliminating their competition by eating them and replacing them, that doesn't sound good to me," Willis noted.

After all, insects and arachnids are already burdened by the destruction of their habitat, climate change, and the widespread use of pesticides. Over 40 percent of insect species are in decline and a third are endangered, the U.N. Environment Programme found. "The rate of extinction is eight times faster than that of mammals, birds, and reptiles," the agency concluded.

And widow spiders, particularly black widows, certainly don't need to be unfairly villainized. Of course, you don't want to be bitten, because their venom can cause minor to severe symptoms. But a black widow's first line of defense, if you happen to disturb their web, is to hide, not bite. Bites are rarer than people think, Coticchio explained.

"They're not these out-to-get-you killer spiders," he said.