Here’s another little mystery for y’all. Two months ago, Sheryl Smith-Rodgers posted to BugGuide’s ID Request the following three photos, which she had taken on March 10. These structures were found in a dry creek bed on a ranch in Mason County, smack in the middle of Texas. Each consisted of a pillar of sand and small pebbles, with a larger pebble forming a roof.
Unfortunately, that’s all we have to go on. We don’t know for sure that these are hollow turrets at the entrances to burrows, but it seems like they probably are. The first thing that came to my mind was that some kind of trapdoor spiders were using pebbles as lids to their burrows, and my fellow BugGuide editor Lynette Schimming had the same thought. But on further reflection, it seems to me that trapdoor spiders make their burrow entrances flush with the ground, and the related spiders that make turrets are the folding-door spiders (Antrodiaetus). Their turrets have flexible collars rather than lids, and apparently none of them occur in Texas, based on the map on page 397 of Coyle (1971)*.
These structures seem vaguely familiar, in a way that causes some inaccessible part of my brain to itch… possibly I came across them at one point and tried to forget about them because I couldn’t make sense of them. My favorite hypothesis at the moment is that these are pupal shelters of caddisfly larvae, which would have been active when there was water in the creek. On page 260 of Tracks & Sign of Insects there is a photo of a pupal shelter of a net-spinning caddisfly larva (Hydropsychidae), likewise consisting of tiny pebbles with a much larger pebble for a roof. As far as I know, however, hydropsychids don’t make anything approaching the height of these turrets. Larvae of Phylocentropus caddisflies (Dipseudopsidae) live in tubes that project vertically from sandy stream bottoms; might they ever put lids on their burrows when they’re ready to pupate? Any other ideas? Could they possibly just be the result of erosion–a miniature version of this? If only we could peek under one of those pebbles… [Edit: See John Pearson’s explanation supporting this last hypothesis in the comments below. I had noticed the sorting by particle size too, and wondered if that might be what was happening.]
* Coyle, Frederick A. 1971. Systematics and natural history of the mygalomorph spider genus Antrodiaetus and related genera (Araneae: Antrodiaetidae). Bulletin of the Museum of Comparative Zoology 141(6):269-402.
Awesome! Thank you for sharing this intriguing mystery that still amazes me.
These look to me like a geological feature–basically a tiny butte with the soft sediment underneath protected from erosion by the pebble. I’ve seen similar things trailside in many regions. I’ve never thought to dissect one, but it seems like that’s the next step!
Yes, I added a sentence about that possibility at the end this morning–you may not have seen that if you were reading the version that went to your email. I was going back and forth about that, but it does make a lot of sense!
I , too, lean toward the erosion pedestal explanation. Note the “fining upward” sequence of particle sizes from pebble-sized to sand-sized in the bottom-to-top vertical profile. When water flow in the streambed is fast, all particle sizes are transported downstream but as the flow begins to slow at the end of a runoff event, the biggest, heaviest particles (pebbles) drop out first and smaller, lighter particles later. Two runoff events would deposit a second fining-upward layer. Later erosion downcuts through these sediment layers except for scattered pedestals. In this case, all that is left of the second, upper layer is the basal cobbles, now capping the full sequence of the lower layer.
I agree that this is probably caused by erosion. It reminded me of something similar and more extensive we saw on a recent trip to Ecuador. I’m sending Charley a photo showing this, taken roadside near Wildsumaco Lodge. It’s a very interesting phenomenon. Thanks to others for the detailed explanations and for the posting.
http://www.alamy.com/stock-photo-one-pebble-remains-on-top-of-a-mound-of-sand-by-the-erosion-guilheta-60402916.html
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