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An Eight-Legged Mystery

A Reddit user found a pile of motionless spiders underneath their table. The reason why is a fascinating look at arachnids, their predators, and just how brutal nature can get.

Note: I originally posted this thread on Twitter. Since no one knows how long that will last, I'm sharing a cleaned-up version of it here.

There's a fascinating mystery unfolding on Reddit: a poster from Australia discovered a pile of dead spiders, all different kinds, had appeared overnight under their dining room table.

There are a couple interesting things about this photo. First of all, few of these are really indoor spiders: While I'm admittedly not an expert on Australian spiders, I see what looks like a crab spider and maybe some orb weavers here. Neither of those families really thrive in houses—crab spiders largely specialize in ambushing pollinators and other flying insects, and orb weavers make big, circular webs in bugs' flight paths.

Astute redditors noticed these spiders had splayed-out, instead of curled-up, legs, which suggested that they were paralyzed instead of dead. (Spiders' legs are hydraulic and depend on, basically, blood pressure to stay extended. No heartbeat, their legs curl up.)

Someone suggested that this looked like the stash of a mud dauber wasp—a kind of solitary wasp that captures spiders to feed their young. But if that was so, where was the nest? The mystery was solved when the original poster saw a wasp crawl into one of the empty screw holes on their table, and a limp spider fall out.

This is not unique to Australia: around the world, different wasps paralyze spiders with their sting and use them as living food sources for their young. Here's a photo from Colorado that a coworker sent me of a wasp dragging off a large but now-defenseless wolf spider, probably a member of genus Hogna.

Photo: Shannon Davis

One example that hikers in the US may/should be familiar with: the tarantula hawks, very large wasps from the genera Pepsis and Hemipepsis which have a legendarily painful, though not lethal, sting, and a pretty gruesome way of reproducing. When a tarantula hawk is ready to lay eggs, it finds a tarantula's burrow, enters, dresses it up in motley and offers it a glass of amontillado (just kidding, the wasp stings it). The wasp then lays its eggs on the spider's abdomen and bricks it up inside its own burrow. I've watched one bury a tarantula, and if you've never seen a 2-inch wasp carrying a rock in its jaws, it will make a real impression on you.

Photo: Astrobradley

The tarantula will remain there, alive but unable to move, until the egg hatches, whereupon the larva will eat its way into the spider's abdomen and consume it from the inside-out, saving its vital organs for last, before chewing its way out to freedom.

If you find this whole concept thoroughly disturbing, you're in good company: Parasitoid wasps were one of the things Darwin cited to justify his disbelief that God had designed all of the world's animals. As he wrote in one letter:

"I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice."

Put simply, he didn't think a loving God would create something that would do that. (For my money, though, the best treatment of the theological implications of parasitoid wasps was in Zach Weinersmith's daily webcomic Saturday Morning Breakfast Cereal.)

In conclusion? It sucks to be a spider sometimes, man.