Gravel-Covered “Cocoon”
The road leading up to our house goes through a section where it was cut down through congomerate rock. This produced a rather pronounced cliff face running alongside the road for several hundred feet. So, on September 13, 2014, while pushing my bike up the hill, I looked over at the cliff and noticed what looked like a cocoon made out of gravel hanging suspended underneath a projecting rock.
It was a couple of inches long, and consisted of gravel/sand particles about 1/8 inch in diameter, loosely bound together by silk. There appeared to be something inside of it, although I couldn’t tell for sure without ripping it open.
I had hopes of hatching out whatever was inside, so I suspended it from the lid of one of my little bug jars, and put it out on the side of the front porch to overwinter.
But then, when spring rolled around, it became obvious that nothing was likely to happen. So, I finally took it down and opened it up.
And it turned out it wasn’t actually a cocoon at all. Inside, I found several papery-silk pouches that I’m pretty sure were spider egg cases. No eggs were inside, though. Evidently they had already hatched out by the time I found it, so nothing was ever going to come out. There was also at least one dead beetle in there that looked like it had been wrapped in silk and sucked dry by a spider.
Anyway, I understand that this kind of papery, brown, roughly spherical silk egg cases are mostly made by cobweb weaver spiders in the family Theridiidae. The bit with having them wrapped in a gravel/silk mixture is not something I’m finding anyone else describing, however. Which made me wonder if it was really something the spider did intentionally, or if the web simply picked up gravel from the disintegrating cliff face as the wind blew it back and forth to make the gravel-covered pseudo-cocoon that I found.
So, when it finally got warm enough that the spiders would become obvious again, I thought I’d start looking to see if there was any evidence of spiders doing this sort of thing on purpose. And on June 1, 2015, I was looking at the same cliff face –
and saw a number of cobweb-type webs, each with a little ball of coarse sand suspended in the middle!
And if we zoom in on this one, we can see the spider hiding in there, camouflaged as if she’s just another grain of sand!
So, it looks like these spiders do, in fact, intentionally build little shelters in their webs by assembling sand and fine gravel, along with the occasional corpses of their prey. This would probably be really useful in evading predators and parasitic wasps, since the spider is really hard to spot in the gravel lump.
I brought two of the spiders home to look at more closely, and here is one of them:
They look a lot like immature Common House Spiders. But, following up on BugGuide, it looks more likely that they are one of a couple of very similar species, like Cryptachaea rupicola, or maybe Parasteatoda tabulata. Both of which have the trait of “camouflaging its retreat using available debris”.
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Fascinating!
Are you sure this isn’t simply a bag worm cacoon?
Thanks, Carole
Arblebarg: Yes, pretty sure. I’ve opened up bagworm cocoons before, and this doesn’t look anything like them. Plus our local bagworms are about 10 times smaller than this.