Cribellate Spiders, and Some More Thoughts on that Mysterious Thing from Peru

Photos of this intricate silken structure, to which I shared my immediate reactions here, have been spread around sufficiently that it’s pretty clear nobody (at least nobody with access to the internet) knows what made it.

fencemystery

I did have another thought or two about it that I wanted to share, though.  Troy Alexander, the photographer and discoverer of this thing, left a comment on my previous post with a link to some additional photos he’d posted on the Rainforest Expeditions Facebook page.  One of them was this closeup:

FenceMysteryDetail

As I said previously, I believe the object in the middle is a spider egg sac rather than an insect cocoon because all the delicate silk threads anchoring it to the substrate had to have been spun from the outside, which would be impossible for an insect larva that was already sealed inside it.  True, there are a few insects that cover their eggs with silk, but as far as I know, those that do (such as barklice and water scavenger beetles) all construct a simple mesh of webbing or a plain silken case, not anything approaching this ornate structure. Mimetus notius, a pirate spider (Mimetidae), makes a silken protection for its eggs that is more similar than anything I know of in the insect world.  The egg sac itself is a dense sphere of white silk with a loose covering of curly golden silk, but this in turn is surrounded by a large bubble of crisscrossing silk threads that present a barrier to any insect predators or parasitoids attempting to reach the egg sac.

DSC_0911

This outer bubble may not resemble the construction of the “fence” surrounding the Peruvian egg sac, but what about this?

DSCN2754

This, of course, is a detail of the snare web of an orbweaver (Araneidae).  The construction is the same as in the “fence”; a continuous strand of silk is spun perpendicularly to a series of evenly spaced supports.

As for the shape of the egg sac itself, Neospintharus trigonum (Theridiidae) demonstrates that a spider is capable of constructing an egg sac with a thick stalk projecting from a conical base.

DSC_4099

Anyway, Troy’s closeup reminded me of the frizzy silk produced by cribellate spiders. Upon reading this National Geographic article today, I saw that someone else had made the same connection.  Though the article states at the beginning that “scientists have no idea what made [the object] or what its purpose might be” and that “most agree it was likely built by an insect,” near the end it mentions that Troy’s leading hypothesis is that a spider is responsible, citing a suggestion by a reddit reader.  The article goes on, “Instead of spinnerets, or silk-spinning organs, some spiders have what’s called a cribellum, which, instead of spinning silk fibers, pushes the molecules through a fine mesh.”  I don’t see any mention of cribellate spiders (those that possess a cribellum) on the reddit post devoted to this mystery object, so I’m not sure who that suggestion came from*, but I thought I’d add a few visuals to explain and support this idea.

Cribellate spiders have two structures that distinguish them from other spiders: the cribellum, which pushes out many fine strands of silk, and the calamistrum, which is a row of leg bristles that the spider uses to comb the silk and produce a woolly texture.  Cribellate spiders do not produce silk with sticky droplets; instead, their insect prey simply becomes caught in the tangle of fine, woolly fibers*.  This difference is evident in a close-up photograph.  The sticky droplets can be clearly seen in my web photo above; in fact, the silk itself is nearly invisible.  In a similar detail of a cribellate orb web, you would see no droplets and the silk would appear more thick and messy.  Apparently I haven’t yet taken a picture with the specific purpose of illustrating this, but you can see a bit of what I’m talking about in this detail of the web of a feather-legged orbweaver (Uloboridae: Uloborus glomosus), which was originally a nice, neat orb web, but was damaged by the struggle to subdue a jumping spider:

IMG_9466

Compare the (somewhat out-of-focus) thread across the lower half of the photo with the ones parallel to the substrate in the detail of the mystery fence.  (The round, yellow objects in the right side of my photo are pollen grains.)

In addition to the cribellate orbweavers (Uloboridae), other cribellate spiders include lampshade weavers (Hypochilidae) and crevice weavers (Filistatidae).  The frizziness of the silk in some of these spiders’ webs is evident even at the macro level, as in the following examples.

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It is believed that the common ancestor of all true spiders (i.e. infraorder Araneomorphae as opposed to Mygalomorphae, the latter being tarantulas, trapdoor spiders, etc.) was a cribellate spider, and that the cribellum and calamistrum have been lost by many groups in the course of evolution.  I think it is also generally believed that the first silken structures produced by spiders were egg sacs, and that insect-catching snare webs followed from these.  At first a loose tangle of webbing around egg sacs might have incidentally trapped a few insects, and since many female spiders stay with their eggs to guard them, they would have been able to take advantage of this accident and eat what was caught.  Those that spun more elaborate networks of webbing around their egg sacs (like the pirate spider example above) would have a distinct evolutionary advantage.

Now I wonder how this Peruvian structure fits into this story.  It’s interesting to imagine that orb webs arose from similar “fences” surrounding egg sacs, though if this were the case I would think these things would be found in more places than one spot in the Amazon.  Could the fence, conversely, be derived from an orb web, or is it a totally unrelated structure?  The idea that it is made of cribellate silk suggests that it may not only prevent ants and tiny parasitic flies and wasps from getting access to the egg sac, but may also capture them as they attempt to approach it.  If so, the females that construct the fences apparently don’t hang around to take advantage of this; do the emerging spiderlings feast on their trapped would-be assailants?

* See Leslie Brunetta’s comments below.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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17 Responses to Cribellate Spiders, and Some More Thoughts on that Mysterious Thing from Peru

  1. ophis says:

    fascinating discussion

  2. Charley, I think I may be the person who suggested cribellate spiders, and I think you’re on the right track here. The majority of all spider species are cribellate spiders (there are tens of thousands of species): the original glue-affixing orb weavers and their descendants are a very successful offshoot of one of the cribellate lineages. The first use of spider silk probably was egg casing, but this may have been simultaneous with burrow lining, because it’s clear, especially when you look at the mesothele spiders, that spiders started out hundreds of millions of years ago as burrow dwellers. In one way, nearly all later silk constructions, including this corral, can be seen as modifications of burrow linings. I’m pretty sure that this will turn out to be a cribellate construction, although I may have to eat my hat. There’s nothing in it that can’t be seen in other spider constructions, reaching all the way back to the ropy trip lines of the mesotheles. (Self-promotion alert:) If you’re interested in how spider silk and spider silk constructions have evolved, please see our book, “Spider Silk: Evolution and 400 Million Years of Spinning, Waiting, Snagging, and Mating” (Yale Univ. Press). More info at our website if you click on my name.

  3. Also, another cool thing about cribellate silk is that it doesn’t entangle arthropod prey. It holds them by van der Waals force. Brent Opell has done lots of work on this.

    • Aha! I always found it hard to picture how the microscopically woolly texture was enough to entangle anything but a very tiny insect, but that was the explanation I’d read in various sources. I’ll be sure to fix that in my own book if I get a chance to revise it.

    • William Holz says:

      See, this is why I read comments.

      In retrospect this makes perfect sense. . but. . wow. It’s right up there with ‘no, echinoderms don’t have suction cup feet’ and ‘the Komodo Monitor has a perfectly normal bite and you kind of have to blame the Water Buffalo for hiding in knee deep murky water’

      I love having my mind blown :)

      • That’s just one fascinating thing about just one kind of spider silk. The whole “stronger than steel” thing, while true about one particular kind of spider silk, is the least of it. If this construction was made by a cribellate spider, I would guess that the spider used perhaps 5 different silk proteins, produced in 5 different types of glands, to make it.

  4. The cribellate attribution is tempting, but I have seen plenty of spider egg sacs that are frizzy like this and not made by cribellate spiders. There are also many spiders that make an egg sac within a protective tent, including Cheiracanthium inclusum (yellow sac spider), Pisauridae (nursery web spiders), and Loxosceles reclusa (brown recluse). None of these are cribellate spiders. Oddly, last night I dreamt that an artist admitted to creating these constructs. However, now that I’m awake again, I’m not inclined to think they are man-made.

  5. Thinking about this further, I would guess that this is an orbweaver, or at least a spider in the group Orbiculariae. This whole construct looks like an orb web. The fence posts are radii. The wires on the fence could be the spiral line. There is a hub at the center. In this case, the spiral line *does* look most like a uloborid spiral, which would make it cribellate. Uloboridae may be a good guess. Deinopidae is closely related. What does a deinopid egg sac look like?

    • Joe, I wouldn’t be surprised if this spider (if it is a spider) turns out to be closely related to the uloborids or deinopids for the reasons you point out here. But I don’t know what their egg sacs look like. Also, I wonder about the construction being anchored to a substrate rather than spanning a space. In that way it reminds me of lampshade webs, except it is much more orderly.

  6. It’s also possible that this could be the spider’s catching web. That is likely an egg sac, but it could also be the spider’s refugium.

  7. Lola says:

    The first pic made me think “ET phone home”.

  8. amszafraniec says:

    Fascinating! I found your hypothesis about the spiderlings feeding on prey trapped in the fence especially intriguing. I want to know the answer to that question!

  9. I’m still blown away that this structure is unknown.
    Troy is part of our extended family and we have been following his Peru research.
    We all saw this well before it went viral.
    No one up here had a clue that he would stump the internet!

  10. Pingback: Morsels for the mind – 13/9/2013 › Six Incredible Things Before Breakfast

  11. David McIntyre says:

    Mystery solved (tentatively) and sensationalized (improbably):

    http://www.weather.com/video/amazon-mysterysolved-43069

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