Dear Straight Dope:
Why would spiders carry large rocks around? Several students of mine have seen this before. I told them it was possibly spider eggs they saw but several assured me it was not eggs but rocks. I thought they may be mistaking spiders for ants. But, once again they were certain it was spiders. Please e-mail me if you know anything about this. It was a big question on our Rock Study Unit.
SDStaff Doug replies:
Spiders can’t carry rocks. They have no way to do so, because spiders have fangs, not mandibles. If they can’t sink their fangs into something (I suspect a rock would be classified as such a thing), they can’t carry it in their jaws. They also can’t carry things in their legs. The only way they carry their egg sacs is by holding them with their spinnerets, which latch onto the silk. Rocks, not being covered with silk, cannot be held in spinnerets.
In other words, if you saw something carrying rocks, it wasn’t a spider. Moreover, if there was more than one critter doing it in the same place at the same time, then the ONLY thing possible is ants, since there’s no way you could mistake termites for spiders, and no other flightless insects live in groups and have debris-carrying behavior.
You may ask: isn’t there some type of desert spider that builds rock burrows to hide in? No doubt you’re thinking of Solifugids, also known as sun scorpions, sun spiders, camel spiders, and about a bajillion other names. They live in the desert and pounce on prey, and live in little burrows. They’re only remotely related to spiders. I have photos and info on the UCR Insect FAQ page linked to at the top of my home page (http://cache.ucr.edu/~heraty/yanega.html#ENTOMOLOGY LINKS).
Could someone could have mistaken a spider carrying her egg sac as a spider carrying a rock? Certainly, given the size, shape, and color of many spider egg sacs. Since the sac is made of silk, the spider can grip it with the spinnerets, a feat that would be impossible with even a tiny pebble. No true spiders are capable of carrying rocks, even if they can shovel them out of their way when digging a burrow.
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