Don’t expect deep thoughts today, for I have writing to do!
For some reason I’ve been posting a lot of arachnids lately. But this one is special: it weaves a web that contains a “fake spider” used as a decoy, so it’s a case of one of my favorite evolutionary phenomena—mimicry. Have a look:
That big “spidery” thing in the middle isn’t a real spider, even though it has the requisite eight legs. It’s a dummy, woven (and with stuff added to it) by the much smaller real spider, which you can spot right above the dummy.
A piece by Nadia Drake at Wired Science explains (although leaves out some of the science):
A spider that builds elaborate, fake spiders and hangs them in its web has been discovered in the Peruvian Amazon.
Believed to be a new species in the genus Cyclosa, the arachnid crafts the larger spider from leaves, debris and dead insects. Though Cyclosa includes other sculpting arachnids, this is the first one observed to build a replica with multiple, spidery legs.
Scientists suspect the fake spiders serve as decoys, part of a defense mechanism meant to confuse or distract predators. “It seems like a really well evolved and very specialized behavior,” said Phil Torres, who described the find in a blog entry written for Rainforest Expeditions. Torres, a biologist and science educator, divides his time between Southern California and Peru, where he’s involved in research and education projects.
“Considering that spiders can already make really impressive geometric designs with their webs, it’s no surprise that they can take that leap to make an impressive design with debris and other things,” he said.
I teach about Cyclosa in my favorite lecture (on mimicry) for my introductory evolutionary biology class, but the species I show makes only a crude, spider-shaped figure in its web. I’ve never seen anything like this, and will be including it in my future lectures. Drake notes:
Though Cyclosa are known for building decoys, most of the described spiders’ constructions are clumpy, made out of multiple little balls built from egg sacs, debris or prey, rather than something resembling an actual spider. “Known Cyclosa don’t have that spider-with-leg looking thing, which is why we think it’s a new species,” Torres said.
As Drake does note, this dummy (an “extended phenotype” using Dawkins’s argot) gives the real spider protection from predators. Birds often eat spiders, and do so by striking the web. If there’s a big, juicy-looking dummy there, it’s much more likely to be struck by predators than the real spider, which can then escape being nommed. Ergo, the behavior is adaptive.
Now that’s a hypothesis, of course, and I’m not sure it’s been tested. One way to do that would be simply to monitor webs, seeing how often birds strike the dummy versus the real spider, and compare that to successful strikes of spiders that lack dummies. That would be hard work given the infrequency of predation events, but it’s the only way to buttress the story. But I suspect the story is real, simply because I can’t think of an alternative (a “Jerry-of-the-gaps” argument!).
Here’s another photo. It’s a remarkable likeness, attesting to the efficacy of natural selection, and I’m sure it would fool most human observers at first glance (and a glance is all that birds get). The fact that the real spider jiggles the dummy adds to the illusion that it’s real.
A bit more on this amazing case of mimicry:
In September, Torres was leading visitors into a floodplain surrounding Peru’s Tambopata Research Center, located near the western edge of the Amazon. From a distance, they saw what resembled a smallish, dead spider in a web. It looked kind of flaky, like the fungus-covered corpse of an arthropod.
But then the flaky spider started moving.
A closer looked revealed the illusion. Above the 1-inch-long decoy sat a much smaller spider. Striped, and less than a quarter-inch long, the spider was shaking the web. It was unlike anything Torres had ever seen. “It blew my mind,” he said.
So Torres got in touch with arachnologist Linda Rayor of Cornell University who confirmed the find was unusual. “The odds are that this [species] is unidentified,” she said, “and even if it has been named, that this behavior hasn’t previously been reported.” Rayor notes that while more observations are necessary to confirm a new species, decoys with legs — and the web-shaking behavior — aren’t common in known Cyclosa. “That’s really kind of cool,” she said.