Solifugids – arachnid teddybears (with big teeth)

The body of a solifugid is covered with long hairs. These are sense organs, capable of detecting the tiniest changes in the temperature, humidity, or air movement (South Africa) [Canon 5D, Canon 100mm, speedlight Canon 580EX + Canon MT-24EX twin light]
Like everybody else, I have a soft spot for things that are fuzzy and look at me with expressive, big eyes. But of course being furry and having eyes isn’t necessarily equivalent to being cuddly, as I learned during a painful lesson delivered to me once by a silky anteater, whom I had foolishly picked up from a branch in Costa Rica – turns out that its claws were not only good for ripping termite mounds open, they also worked great on human skin. And so when I saw my first solifugid, arguably the hairiest of all arachnids, it wasn’t too difficult to resist the temptation to pet it and snuggle against my cheek. In retrospect, probably a good decision.

The “head”, or propeltidium, of a solifugid is a giant ball of muscles that power their huge chelicarea, and the first pair of “legs” (pedipals) are usually held in the air, sensing for prey and danger. (Mozambique) [Canon 7D, Canon 100mm macro, 3 speedlights Canon 580EXII]
Solifugids (Solifugae) are the arachnid equivalent of a honey badger, the tireless, famously cantankerous African mammal. Like the badger, they are lonesome, opportunistic predators, constantly on the run, looking for something, anything, to sink their jaws in. They are big and they are fast. Really, really fast. Most are nocturnal and hate being exposed to the light (hence their name, Solifugae, Latin for “fleeing from the sun”.) But some are active in the middle of the day, although still preferring to stay in the shade as much as possible. This, of course, sometimes leads to comical misunderstandings – when you see a giant, hairy arachnid chasing you in the desert (and I can attest to the fact that it does happen!), be assured that it is after your shadow, not you per se. Just let it curl up against your leg, and all will be fine.

Solifugids are efficient hunters, capable of overpowering very large prey. Before swallowing their food they must macerate it very well, using a “cheliceral mill.” [Canon 5D, Canon 100mm, Canon MT-24EX twin light]
Solifugids are found across the globe, mostly in hot, dry environments (curiously, they are absent from both Australia and Madagascar.) A handful of species live on the verges of wet, tropical forests, but these are rare (in all my years of working Costa Rica I have only seen one solifugid.) Little is known about the physiology of these animals, but they appear to be extremely tolerant of wide temperature changes, from near freezing to over 50°C (122°F), which is an adaptation to the daily temperature changes in the desert environment. At the same time, a solifugid may drop dead the moment it is exposed to the sun after being dug out from its burrow, likely indicating that their adjustment to extreme temperatures must be conducted in a gradual fashion.
Unlike spiders, with which they are often confused, solifugids are not venomous. Probably not venomous. Those species that have been studied in detail lack venom glands, but there is some anecdotal evidence of possible envenomation of large prey items by these arachnids. Bites from solifuigids may also cause localized pain and swelling in humans, further indicating injection of toxins or histamines. Regardless, they are efficient killers of insects and other small animals, including frogs and lizards. They pursue their prey relentlessly. I once watched a solifugid going after an insect that hid under a rock, and never before had I been as grateful for being large as at that moment: these animals do not know the meaning of the words “tired” and “give up.”

The evolutionary history of soifugids is murky. Few fossils unequivocally recognized as solifugids exist, but it appears that they may be at least 300 million years old. Their closest relatives are also uncertain, but most arachnologists agree that they are closely related to pseudoscorpions. Among other arachnids they are unmistakable: their chelicerae (“jaws”) are so huge and muscular that it is easy to confuse them with the entire head of the animal. Solifugids are also equipped with strange, mushroom-like organs on the underside of their fourth pair of legs, known as the “racuquet organs” or malleoli. Their function is not entirely clear, but they seem to be extremely sensitive organs of smell, and play a role in both detection of prey and finding a mating partner. Solifugids look as if they had five pairs of legs, but the first pair, the pedipalps, is in fact an element of the mouthparts. At the tip of the pedipals solifugids have unique suctorial organs that, like the suction cups on the arms of an octopus, help them catch and hold their prey.

I like solifugids, I like them very much. But I can understand why some people may be afraid of them. To be completely honest, solifugids are the only animals that I have never caught with my bare hands (and I have caught a lot of animals with my bare hands, vipers and scorpions included.) I know that they cannot really hurt me, even if they are mildly venomous (of which there is no firm evidence), but their gremlinish body and behavior tap right into my primordial fear of the agile and the unknown. Maybe next time I will try to pet one. Maybe.

The underside of solifugids’ last pair of legs carries strange organs known as malleoli. Their exact function is not known, but they appear to be very sensitive scent organs. [Canon 7D, Canon 100mm macro, 3 speedlights Canon 580EXII]

18 Comments Add yours

  1. More than 20 years ago I travelled from Vancouver (where I live) to the southern BC town of Osoyoos which is situated at the northern tip of the Sonoran Life Zone. It is the only place in BC where you can find scorpions, which is what I was after. Unfortunately I didn’t find any, but I was surprised and absolutely delighted to find solfugids! Great critters! I should try and dig out the slides from that trip…

    Excellent text and photos…as always!

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  3. Isaac Mallol says:

    Craig, I was thinking something similar to you, but a little bit different, I think those organ must come from mechanoreceptors as the scorpion’s Pecten (I don’t know the English name, comb). There is a paper about how these two groups are closely related on the arachnids’ tree: “molecular data has complicated this picture by suggesting that scorpions move higher on the tree, closer to sun spiders” .
    Probably because all the ectodermis of the solifugae is covered with sensilla able to act as a mechanoreceptor, detecting vibrations and air movement, this mushroom-like organs, became relaxed in their duty. Then evolution used this structures to detect chemical volatile compounds in the air, as Stephen Jay Gould would say “the opportunistic character of evolution” using an organ or structure that already exist for other porpoise.
    Well this is just my thought, actually I don’t know too much about arachnids as I would like.
    Isaac M.

    1. Craig says:

      Isaac, I wish I knew more about them as well! :)
      I think your example of the scorpions comb organ is probably better than mine. And thank you for sharing the interesting link.

  4. Sean McCann says:

    Really interesting animals! I wonder if anyone has managed to rear them successfully…An arachnid with a good sense of smell that is also a nocturnal, wandering, generalist predator….I suspect that they might have some interesting brain structure. It would be interesting to examine in detail their behavior.

    1. Solifugids are notoriously difficult to keep in captivity. To begin, they are apparently really short-lived, the longest recorded lifespan is, I think, only 2 years, with most not exceeding one year. They may also require huge enclosures because they constantly run. I tried keeping them several times, and never managed to have them survive for more than 2-3 months.

      1. Gil Wizen says:

        I feel that I can contribute something here – Solifugids can actually live for years, at least from my experience (longest lived individual I had was collected as a large specimen and survived 5 years in captivity).
        A few years ago, I and another colleague were keeping several Solifugid species from Israel successfully. These species were from two families, Galeodidae and Karschiidae. We had no success in keeping Rhagodidae species alive and well in captivity for more than 2-3 months, like Piotr said.
        The species we kept showed a seasonal life cycle – they became active for only 1-2 months per year, running relentlessly in search for food during time. Once they have consumed enough, they would stop running, relax, find a secure spot and hold their legs above their head. In this immobile stage, they stayed hidden until the next year, when they became active again. Galeodidae were mainly active in the early summer and the activity season for Karschiidae was winter. We never succeeded in breeding the arachnids, they were far too aggressive in the activity season. Maybe they mate very close to the end of their activity, but we never managed to figure this out.
        Of course, we never got to publish our observations – we need more observations to back this up, and we hope to do this one day. At some point I had to release all the Solifugids I had back to the wild because I moved in with my girlfriend and then later moved to Canada.

        My profound apologies for the long post!

      2. Thanks Gil, very interesting! You should definitely try to publish your observations, even if they are at this point only anecdotal. The fact that solifugids can live for several years is probably unknown to most arachnologists.

  5. Craig says:

    I suspect that the hairs on the body are not capable of detecting temperature or humidity. They likely have mechanoreceptors at the base, for detecting close environs or air movements. But thermo/hygroreceptors are more likely restricted to other parts, and are not likely associated with those body hairs.
    As for the organs on the legs, those are fascinating! I would suggest that they are analogous to the slit organs on the legs of spiders, and are modified mechanoreceptors used for detecting movement of prey. It would certainly fit with their method of hunting, and could be used for triangulation. I’d love to see some TEM’s of those, to get some idea of the sensory function. And I’d love to stick some electrodes in them, if I could. I suspect that they are highly specialised mechanoreceptors, rather than olfactory receptors. But if you have any papers that you could share on those, I would love to see them. If not, that is something that should be done. Fascinating!

    1. The truth is, nobody knows for sure what the solifugid hairs are for. Camel crickets use similar hairs to detect temperature changes. They are too diverse in their length and thickness to be all simple mechanoreceptors. But it is very likely that many of them are for proprioception. Regarding the malleoli, there is an old paper (Brownell & Farley, 1974, Tissue & Cell 6: 471-485) that examines the structure and possible function of these organs; the authors think that they are for hemoreception/olfaction. I also recall reading somewhere that solifugids can detect prey items with the malleoli in those cases when the pedipals missed them.

      1. Craig says:

        The sensory capabilities are definitely worthy of further study. It’s a pity we don’t have any solifugids in Australia!
        And thank you for reference, I’ll try to track it down and have a read.

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