City Cast

Black Widows in a Time of Heat Islands

Scott Dickensheets
Scott Dickensheets
Posted on July 25
Photo of black widow spider

Wait until things heat up, lady. (Lubomir Volnar/Getty)

🕷️ Life in the big city is a mixed blessing if you’re a black widow.

On the upside, the lifestyle can be great. Humans create waste, which ensures a smorgasbord of the spider’s favorite bugs. Lotta water, too, at least compared to the desert. The crime rate is favorable: “Natural predators like parasitic wasps, which eat spider eggs, don’t tolerate cities well,” the Phoenix New Times notes. Perhaps their only natural enemy is the Orkin man. The human-built environment is better, too; more out-of-the way places to hide, spin webs, and lay eggs. “A cactus can do that,” says Arizona State University biologist Chad Johnson, who’s studied this, “but not as reliably as just a city full of concrete walls.”

The downside? The heat — specifically the scorching heat-island effect so prevalent in urban settings. Studies by scientists at Arizona State indicate that extreme urban heat hinders black widows in key ways. High temps often render the widows more lethargic. Baby spiders raised in het “were underweight … and more prone to cannibalizing their fellow spiderlings,” for one. It’s thought that heat-related hormone spikes alter their metabolism and lead to premature molting. “If it’s hot enough,” says Dr. Allen Gibbs, life sciences professor at UNLV, many spiderlings “don’t make it to adulthood.”

And yet, paradoxically, they thrive in cities. “Populations of urban spiders,” says the New Times, “are 10 to 15 times denser than in the desert.”

Oh, lovely.

(More shudders: My take on scorpions, and why they're so hard to get rid of.)

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