A weird tarantula with a honking big horn on its back

So they’ve discovered a tarantula with a very strange “horn” on its back. Reading the paper below, which recounts the discovery (access free; pdf here), one discovers that these “foveal horns” are uniquely large and weird; as the authors say, “no other spider in the world possesses a similar foveal protuberance.” Now other species in the genus Ceratogyrus do have smaller horns (see below), but not like this one.

Here it is, with captions taken from the paper. Look at that big thing sticking up off the cephalothorax!

Figure 3. Ceratogyrus attonitifer sp. n. paratype, cephalothorax. A retrolateral view B dorsal view C ventral view. Scale bar: 10mm (A).

But wait! There’s more, including a defensive posture:

Figure 2. Habitat, burrow and live habitus of Ceratogyrus attonitifer sp. n. in south-eastern Angola. A Aerial view of habitat at the type locality showing a dambo (wetland) amongst miombo (Brachystegia) woodland. The expedition campsite is to the right of the dambo. Specimens were collected primarily along the margins of the wetland area B live habitus, dorsal, showing full size of the foveal protuberance in life C specimen in defensive posture typical for baboon spiders; background is white sand at the type locality D burrow entrance amongst grass tussocks; entrance approximately 2cm wide.

 

The species, C. attonitifer, got its name this way:

Etymology

The specific epithet is derived from the Latin root attonit–, meaning astonishment or fascination, and the suffix –fer, bearer of or carrier, and refers to the astonishment felt by the authors at the discovery of this remarkable species.

And the formal description of the horn:

Fovea strongly procurved with prominent, elongate protuberance extending over dorsal aspect of abdomen, as long as or longer than carapace length, anterior part extending from carapace sclerotized, remainder soft and membranous, bag-like in living specimens, becoming shrivelled when preserved, dark in colour.

As I said, other species in the genus have foveal “horns”, like C. darlingi below, but they are much smaller:

Photo from Wikipedia

However, as the authors say (my emphasis):

Members of other theraphosid genera from the Neotropics, namely Cyrtopholis Simon, 1892, Sphaerobothria Karsch, 1879 and Umbyquyra Gargiulo, Brescovit & Lucas, 2018, also possess similar foveal structures, as do some species of the ctenizid genus Stasimopus in South Africa, and several aganippine idiopid genera from Australia (M. Rix pers. comm.). The protuberance of C. attonitifer is unique in its length, as well as being soft, whereas this structure is fully sclerotized in all other genera where it is known to occur.

But what are these horns used for? Wikipedia (which we must now take with a grain of salt), says this:

C. marshalli features the biggest horn, where it stands straight up about 1 cm. There are several probable functions for this horn: according to a study by Rick C. West in 1986, it provides an increased surface for the attachment of the dorsal dilator muscle, which aids in drawing in liquefied food into the sucking stomach at a faster rate; this way, the spider can retreat to a safe place faster. It also increases the area for the midgut diverticula to expand during times of nutrient and water availability, analogous to a camel’s hump, helping it to survive in its arid habitat during droughts.

Okay, so that’s speculation: “probable functions.” But this big floppy horn? Who knows? The authors don’t even try to speculate. Note that every specimen of C. attonitifer they found, however, was a female, so it’s imperative to see what the males look like. That would tell if it has some kind of sexual function.

In the meantime, speculate away!

_________

Midgley, J. M. and I. Engelbrecht. 2019. New collection records for Theraphosidae (Araneae, Mygalomorphae) in Angola, with the description of a remarkable new species of Ceratogyrus. African Invertebrates 60:1-13.

31 Comments

  1. Diana MacPherson
    Posted February 17, 2019 at 2:16 pm | Permalink

    It’s a stand to prop it up as it terrifies you with its fangs an also to allow it to read books.

    • merilee
      Posted February 17, 2019 at 2:17 pm | Permalink

      +1
      And we thought it couldn’t get any scarier looking.

    • Michael Fisher
      Posted February 17, 2019 at 3:50 pm | Permalink

      Ms. Itchy Crawly Spider’s favourite author is E. B. White.

      • Diana MacPherson
        Posted February 17, 2019 at 3:53 pm | Permalink

        Ha ha – nice one!

  2. rickflick
    Posted February 17, 2019 at 2:22 pm | Permalink

    Very puzzling. I can only guess it might have some utility when shedding the exoskeleton. Like a pull handle? It might make a useful handle for researchers to pick up the spider for analysis, although I can’t imaging what selective force would be operative. French Berets sometimes have a little thingy on top, but I have no Idea what that’s used for either.

  3. ThyroidPlanet
    Posted February 17, 2019 at 2:27 pm | Permalink

    Sub

  4. Posted February 17, 2019 at 3:10 pm | Permalink

    No idea. The putative function for drawing in liquified food is as good as any.

  5. Achrachno
    Posted February 17, 2019 at 3:14 pm | Permalink

    Maybe used to scare off/discourage predatory wasps that try to land on the tarantula’s backs?

    • merilee
      Posted February 17, 2019 at 3:56 pm | Permalink

      Reminds me of the story my college roommate tells of stepping on a tarantula barefoot!! one evening on her driveway in La Cañada (near Pasadena). Eeeek!

    • Diana MacPherson
      Posted February 17, 2019 at 4:04 pm | Permalink

      I don’t know if we can trust you with a name like “Arachno”. 🙂

      • Achrachno
        Posted February 17, 2019 at 11:15 pm | Permalink

        You can trust me, I’m an Achrachno not an Arachno. Much more trustworthy and with far fewer legs.

        • Diana MacPherson
          Posted February 18, 2019 at 7:23 am | Permalink

          🤣

        • Posted February 18, 2019 at 12:05 pm | Permalink

          I am reminded of a typo in a paper that one of my proofreaders caught: I accidentally wrote “arachnocapitalists” to describe US Liberatarians, meaning “anarchocapitalists”. I got back “8 legged financiers?”

          • merilee
            Posted February 18, 2019 at 1:18 pm | Permalink

            Love it!

  6. Posted February 17, 2019 at 3:23 pm | Permalink

    The ‘horn’ inflates and when needs be, blows it, namely traditional music of Angola, but every now and then a standard jazz tune.
    In other words i’m dumbfounded as to what it would use it for, one stab at it, something to do with mating. On the asumption, no males have been seen and a mating ritual is yet to be observed.

  7. Randall Schenck
    Posted February 17, 2019 at 3:43 pm | Permalink

    It’s a camel-back. That is a newer version of the canteen used for water by soldiers in the field. The camel-back holds more water and has a small hose going round to the mouth so you get your water on the move. This one on the tarantula’s back, of course does not need a hose. Probably not purchased at the PX.

    • merilee
      Posted February 17, 2019 at 3:59 pm | Permalink

      Well it certainly has the tarantula’s back😼

  8. Posted February 17, 2019 at 3:57 pm | Permalink

    What if the horn is a tiny parasitic male permanently attached to the female’s body?
    Like in anglerfish.

  9. Christopher
    Posted February 17, 2019 at 4:09 pm | Permalink

    Does this “horn” get larger with each successive molt? Is it present in males, is it present in the freshly hatched young? Are there any sensory hairs on the outside, what about nerve ganglia on the inside? If it’s movable, perhaps it is used by the species as identification or as an indicator for sexual interest so as to prevent pre-coital cannibalism? What a fun thing to find, not just a new species, but a whole new series of questions.

  10. Thanny
    Posted February 17, 2019 at 4:13 pm | Permalink

    Does this species carry young on its back for a period of time? If so, could it serve as increased surface area for the young to hold on to?

    Pretty thin idea, but the only one that comes to mind at the moment.

  11. Neil Taylor
    Posted February 17, 2019 at 4:21 pm | Permalink

    It also has huge pedipalps.

    It looks like it has 10 legs!

    Do mygalomorphs exhibit sexual dimorphism in their pedipalps?

    I’m used to looking for the boxing-gloves small male araneomorphae spiders have, but don’t have any experience with mygalomorphs.

    All I can say is it is one creepy spider. 10 legs and a horn. Shivers.

  12. Mark R.
    Posted February 17, 2019 at 5:23 pm | Permalink

    Those are some fierce looking fangs. No mention of how venomous this is.

    • merilee
      Posted February 17, 2019 at 6:23 pm | Permalink

      I’m under the impression that tarantulas aren’t terribly venous, just hairy (and not in a cute, furry sense) and scary.

      • Mark R.
        Posted February 18, 2019 at 11:39 am | Permalink

        I’ve always been under that impression too. Their looks are scarier than their bite.

        • merilee
          Posted February 18, 2019 at 12:01 pm | Permalink

          Possibly kill by inducing cardiac arrest on their “viewers”.

  13. ThyroidPlanet
    Posted February 17, 2019 at 6:05 pm | Permalink

    Oh I see what everyone is doing now

    In figure 3A it almost looks torn at the base – is that simply a fold, as I assume?

    My offerings:

    Inflatable bladder : float in water, or modulate buoyancy using … do they have hematolymph? The fluid that … well, that’s insects… I’ll have to read..

    Fills with pheromone – attracts mates or scares predators

    Makes sounds – attracts mates or scared predators

    Fills with fluid which makes the sac wet and sticky …

    Fills with fluid which it sprays ( is there a meatus?) at … enemies. Prey. Like the bombardier beetle does.

    Egg case – somehow. This is my personal favorite.

    A silly idea : appendage for the refueling plane

    This is fun – a whole set of questions could be written with this format, where you have to intuit the function of a body part.

    • ThyroidPlanet
      Posted February 17, 2019 at 6:13 pm | Permalink

      Oh one more – I promise ( I don’t really)

      Decoy – puffs up when under severe duress (it appears not to do this in the defense posture) and the predator bites it, the spider hits the eject button and gets away.

      Perhaps in the defense picture the protuberance is simply not being used, but it could.

      Or or or – uses the protuberance to root in the burrow of its favorite prey, to release the eggs or larvae.

      • James
        Posted February 17, 2019 at 8:42 pm | Permalink

        Oh, I like decoy! A predator grabs this, it breaks off, and the spider gets away–like a lizard tail or an octopus arm. It wouldn’t even have to puff up, it’d just have to be in a position to get attacked before the main body does.

  14. Posted February 17, 2019 at 7:07 pm | Permalink

    It’s a balloon!

    -Ryan

  15. James
    Posted February 17, 2019 at 8:40 pm | Permalink

    I don’t think there’s sufficient information in a static image to speculate. I mean, from my brief search online it sounds like it could be anything from a storage sack (these live in harsh environments) to extra muscle attachment surfaces, all the way to vestigial features. I think we need to see it do something before we can really do any informed speculation.

    Never let that stop me before, though, so here goes!

    They seem to be ambush predators, either above ground or from tunnels, so if I were to speculate wildly I’d say the organ likely serves to enhance their sensitivity to vibration.

    On the specimens I’ve seen so far there are often lines radiating from the outer edge of the head toward the horn. So another question I’d like to see answered is: What does this spider look like when viewed in the wavelengths visible to other spiders of the same species, and to its common predators? We know that flowers, for example, have colors that humans can’t see but which bees, certain birds, and other pollinators can. Could something similar be happening here?

    Finally, what other genes are associated with this one? Could it be a case where some other trait was selected for, and this sort of tagged along? Probably couldn’t explain its origin, but it could explain the later development. Assuming this IS a later development, and not an ancestral trait…I don’t know much about spiders in deep time, and have been burned on that concept in the past!

  16. Posted March 4, 2019 at 10:43 am | Permalink

    Shit .I have arachnophobia 😦


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