“Brown Widow” is taking out “Black Widow”

Scientists have found that black widow spiders in the southern United States are declining, and the culprit may be brown widow spiders. The study was published March 13 in the Annals of the Entomological Society of America.

A brown widow spider. Photo by Shutterstock/Vinicius R. Souza

Black widow spiders, known for their red hourglass shape on their belly, are gradually disappearing in Florida in recent decades, while brown widow spider populations have been on the rise. The latter is a spider native to southern Africa and Madagascar that came to the United States in the 90s of the 20th century. However, the inhabitants of the area can get some comfort because brown widow spiders are less toxic than black widow spiders. Symptoms after being bitten are usually limited to mild irritation, as if bitten by a common spider.

Initially, scientists wondered if the decline in black widow spider numbers was related to brown widow spiders competing for their food and space. But since Florida seems to have enough food and space for these two spiders, entomologists think there may be other reasons, for example, perhaps Black Widow is being hunted by Brown Widow.

To test this idea in the lab, researchers like Richard Vetter of the University of California, Riverside, provided a “spider buffet” for brown widow spiders, which included red-footed island spiders, triangular fat-bellied spiders and southern black widow spiders. They found that brown widow spiders were 6.6 times more likely to kill southern black widow spiders than any other arachnid. It is rare for spiders to prey on organisms of the same genus.

When the brown widow spider and the southern black widow spider met face to face, the researchers found that both species were rapidly “slapping” each other’s legs. They also noticed that female brown widow spiders actively track, capture, and devour southern black widow spiders at all stages of development.

Compared to the black widow spider, the brown widow spider is more aggressive to other spiders provided by the researchers and does not spare the young. “Brown widow spiders wipe them out before they can take action, and this spider is very good at timing,” Vetter said. “In the face of brown widow spiders, southern black widow spiders are never intruders, but they also protect themselves from attack. Researchers have found that defensive black widow spiders can occasionally trap aggressive brown widow spiders with cobwebs and inject venom; The latter are more daring, often venturing into the web of Southern Black Widow spiders.

When the researchers compared the two species, they found that female brown widow spiders were superior to southern black widow spiders in terms of size and reproductive ability, which may be one of the reasons why black widow spiders are gradually disappearing in Florida. Young female brown widow spiders are 9.5% larger than southern black widow spiders, and mature female brown widow spiders can produce multiple oocysts at once, while southern black widow spiders produce only 1. (Source: China Science News, Wang Jianzhuo)

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