Pseudoscorpions! Small, strange arachnids

Yes, this is a blog about spiders, and no, pseudoscorpions are not spiders. But they are members of the class Arachnida, like spiders, and fascinating, like spiders! I encountered them for the first time on a recent trip to the Okanagan with Sean, so here’s a post about some of their natural history.

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Two tiny pseudoscorpions Sean and I found while doing some rock flipping on a hillside near Vaseux Lake, BC. Photo: Sean McCann

Pseudoscorpions are really weird, and really awesome, creatures. Their name means “false scorpion” (which is also a common name for this order of arachnids), because superficially they look a lot like scorpions (members of another arachnid order), minus the “tail” with its stinger on the end.

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Drawings of a pseudoscorpion and a scorpion from JH Comstock’s book. The illustrations are most likely by Anna C. Stryke, but possibly by Mrs. Comstock. Note that the scale is different for each drawing.

Pseudoscorpions are tiny. The one below (photographed under a microscope) is only about 1 mm long! The largest pseudoscorpions can get as long as 10 mm. Their small size means they can live in tight spaces, like between floorboards, under tree bark, or even under the elytra (hardened forewings) of beetles – but more on this later.

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Pseudoscorpion found in Vero Beach, FL. Photo: Sean McCann

Pseudoscorpion morphology is strange. Like other arachnids, pseudoscorpions have two main body segments: a cephalothorax (the front part, a combined head-and-thorax bearing all of their appendages), and an abdomen. They may have one or two pairs of simple eyes on the sides of the cephalothorax (or none at all) and their vision is generally poor. As well as four pairs of legs, they have enormous chelate (pincer-like) pedipalps that they use for capturing prey and sensing their environment.

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A neobisiid pseudoscorpion nicely displaying its chelate jaws and pedipalps. Photo: Marshal Hedin, licensed under CC BY-NC 2.0.

Order Pseudoscorpionida

Pincer-like chelicera (singular of chelicerae) of a pseudoscorpion, bearing spinnerets on the movable finger. From Comstock’s book.

 

Their jaws (called chelicerae) are also like miniature pincers. Like spiders, pseudoscorpions can produce silk. Unlike spiders, who have abdominal silk glands and spinnerets, pseudoscorpions’ silk glands are in their cephalothoraxes, and their spinnerets are on the tips of their chelicerae! (The spitting spiders in the family Scytotidae are exceptional among spiders in also having silk glands in their cephalothoraxes, and “spitting” the silk out of their fangs along with venom.)

Pseudoscorpions use silk to build retreats* or cocoons for moulting, overwintering, and sometimes brooding their young.

Also like spiders, many pseudoscorpions use venom to subdue their prey, which includes mites and other tiny arthropods. The venomous pseudoscorpions are in suborder Iocheirata, which means “poison hands”. Their venom glands are in their modified pedipalps, with openings in the tips of one or both of the fingers of their claws.

 

To me, pseudoscorpion anatomy is all topsy-turvy (at least compared to spiders):        their silk comes out the wrong end and their venom comes out of clawed “hands” instead of fangs. 

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Pseudoscorpion – modified from the illustration in Comstock’s book.

Pseudoscorpions also have pretty strange reproduction. Males deposit a spermatophore (a package of sperm) on the ground, which a female must then pick up and insert into her reproductive opening. Males of different kinds of pseudoscorpions have various methods of ensuring that a female finds and uses their sperm. Some are carefree: the male deposits the spermatophore, walks away and (figuratively) crosses his fingers and hopes that a female will encounter it by chance. Other tactics are rather more direct and reliable: the male engages the female in an elaborate “mating dance” and eventually pulls her over his spermatophore to ensure that she picks it up. In one species, Serianus carolinensis, males only produce spermatophores when a female is nearby, then they spin special silk webs that direct her to the package.

Once a female’s eggs are fertilized, she keeps them in a brood pouch under her abdomen (or sometimes next to her in a silken brooding chamber). This brood-sac contains food for the developing pseudoscorpion embryos, which grow and moult into protonymphs (juveniles that look just like adults, but smaller) before emerging.

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A female Arctic pseudoscorpion, Wyochernes asiaticus, with her brood pouch. Photo: Crystal Ernst (used with permission)

Pseudoscorpions love books! Although I would like to think the little pseudoscorpion in the photograph below enjoys reading, what they really like about books is the booklice that sometimes live in them. Because pseudoscorpions can sometimes be found living between the pages of books and feeding on booklice, one common name for them is “book scorpions”.

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“Book scorpion” enjoying some literature while waiting for booklice. Photo: Sean McCann

Finally, pseudoscorpions are hitchhikers! Because they are so small and don’t fly, pseudoscorpions can’t get very far on their own. To overcome this obstacle, they hitch lifts on other organisms – usually larger arthropods like beetles and flies. This particular kind of symbiosis – in which the individual doing the carrying is apparently unharmed – is called phoresis.

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A longhorn beetle, Xylotrechus sagittatus, with phoretic pseudoscorpions hitching a ride on its leg. Photo: Sean McCann

Phoresis is a fantastic word that comes from Greek: phor means “carry, bear; movement” but it can also mean “thief”. Phoretic pseudoscorpions latch onto the bodies of their transportation with their pincers to steal a free ride. Some pseudoscorpions are stowaways under the elytra of comparatively gigantic harlequin beetles, and feed on phoretic mites and find mates while they travel. Piotr Naskreki has a wonderful blog post with pictures of these tiny ecosystems on a beetle’s back.

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A closer view of the cluster of tiny travelers. Photo: Sean McCann

Want to find out more about pseudoscorpions? Of course you do! Here are some references and further reading from around the web:

10 Facts about Pseudoscorpions. Fantastic blog post by arachnologist Chris Buddle, who also created this great photographic key to the pseudoscorpions of Canada.

Comstock JH (1912). THE SPIDER BOOK: a manual for the study of the spiders and their near relatives, the scorpions, pseudoscorpions, whip-scorpions, harvestmen, and other members of the class Arachnida, found in America north of Mexico, with analytical keys for their classification and popular accounts of their habits. Doubleday, Page & Company. (Available from the Biodiversity Heritage Library, with wonderful illustrations)

Harvey MS (2013). Pseudoscorpions of the World, version 3.0. Western Australian Museum, Perth.

Pseudoscorpions page on the Massey University Guide to New Zealand Soil Invertebrates website. *Includes photos of pseudoscorpions’ silk retreats!

Pseudoscorpions page on the Encyclopedia of Life.

The order of the Pseudoscorpiones. Nice summary of pseudoscorpion biology by F. Schramm, with lots of photos and scholarly references.

Zeh DW & Zeh JA (1992). On the function of harlequin beetle-riding in the pseudoscorpion, Cordylochernes scorpioides (Pseudoscorpionida: Chernetidae). Journal of Arachnology, 47-51.

What happens when you poke, prod and pinch black widow spiders? You might be surprised…

This post originally appeared on Chris Buddle’s blog Expiscor at Scilogs.com.

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A stunning female western black widow (Photo: S. McCann)

People seem to have a particular fear mixed with fascination when it comes to venomous animals, and whenever I talk about my work with black widows I am invariably asked questions like, “have you been bitten yet?” The answer is, of course, no. Spiders almost never bite people. I’m always quick to relate that in my experience black widows are not aggressive, even when I go around poking and prodding them with my bare hands.

Replicated experimental results always carry more weight than anecdotes, however, so I am delighted to share this recent paper: Poke but dont pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperus.

Hey look!! An actual peer-reviewed research paper about the poking, prodding, and pinching of black widows, confirming that they are reluctant to bite, even when threatened. Not only that, but the study provides some cool data suggesting that these spiders are capable of assessing risks to make decisions about how to defend themselves.

Here are the details:

David Nelson and his coauthors wanted to know if black widows change their defensive behaviour depending on the level of threat they are faced with. To find out, they used gelatin ‘fingers’ to place spiders in three different threatening situations: a ‘low threat’ attack was a single poke with one finger, a ‘medium threat’ was a series of prods simulating a more persistent attacker, and the ‘high threat’ was three a series of long pinches of the spider’s entire body between two fingers, as might be experienced if being grasped by a predator.

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Figure 1 from Nelson et al. 2014

They found that the spiders engaged in several distinct defensive behaviours during these experimental attacks: retracting the legs toward the body, moving (often retreating), ‘silk-flicking’ (drawing sticky silk out of the spinnerets with last pair of legs and flinging it toward the attacking finger), ‘playing dead’ (curling up into a ball), and biting.

During low-threat, single pokes, no bites occurred. Most spiders were completely non-confrontational, simply moving away, and only rarely flicking silk.  When the threat level escalated to persistent prodding, the spiders changed their defensive behaviour: roughly half of them flicked silk, some played dead, and only one spider (out of 43) attempted to bite the offending finger. Silk-flicking is much safer than biting for a black widow – she can maintain her distance while flinging sticky silk to subdue or slow down her attacker. Biting, on the other hand, requires getting up close and personal with the assailant in order to pierce it with her tiny fangs, making her much more vulnerable to injury.

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See the red tips of this black widow’s puny fangs? It’s a lot safer for her to keep her distance in threatening situations than get close enough to use them (Photo: S. McCann)

Only when the spiders were being pinched between two fingers (with the mouthparts already positioned right up against their ‘attackers’) did biting start to become a more common, last-resort tactic: 60% of the spiders bit the fingers as a result of being squeezed for an extended period of time, delivering on average 2.7 bites each. Pinching also resulted in silk-flicking by about half of the spiders, and a few played dead.

This is all great information, but when the spiders did bite the gelatin fingers, there was no way of knowing how much venom they injected, if any at all (sometimes venomous animals deliver ‘dry’ bites). The next question the researchers wanted to answer was, do the spiders control whether and how much venom they inject when biting? In particular, they wanted to know if the amount of venom injected would vary depending on the type of threat (in this case either pinching a leg with forceps, or grasping the abdomen with gloved fingers).

For this experiment they came up with a clever method to collect the venom: a small vial with a thin membrane over the opening was presented as a target for the spiders to bite. If a spider did bite, her fangs would pierce the membrane (the number of holes would indicate how many times) and any venom she expelled would be collected in the vial so the volume could subsequently be measured.

It turned out that more than half of all bites were dry (no venom was detected in the vials). The black widows delivered more bites per target when they were pinched on a leg than on the abdomen, but more venom was released with each bite when the abdomen was pinched. Being grasped by the body is a high-risk situation for a black widow because her abdomen is unarmored and vulnerable; a strong squeeze or puncture can be deadly. Pinching a single leg, on the other hand, represents a non-life threatening attack. Spiders can autotomize (drop) their limbs and survive without significant ill effects.

The team also found evidence that the spiders delivered more venom per bite when repeated threats were spaced 5 minutes apart than 5 seconds apart. Attacks after the longer intervals might have been interpreted as coming from new assailants, each requiring a larger dose of venom than a second or third bite to the same persistent attacker.

The results all indicate that black widows have fine control over how much venom they inject when biting. First, they can decide whether or not to use venom at all. Some spiders gave dry bites, then wet bites, as well as vice versa, demonstrating that dry bites were not simply a result of running out of venom. Furthermore, they can vary the amount of venom they inject during individual bites and in response to different kinds of threats.

Both silk and venom are metabolically expensive to manufacture, so it makes sense that spiders would be selective about when and how much of these resources to deploy in defense. This study suggests that they are able to assess risks and adjust their responses accordingly, only dipping into their reserves of silk and venom as the threat level escalates towards a life-or-death situation.

What does this all mean for humans? Grabbing and pinching spiders is generally not a good idea – they might get injured and could bite defensively. This is just good sense and didn’t require a scientific study to confirm, but the new data suggest that even if a black widow does bite, she’s not necessarily going to inject any venom. It’s also important to note that in the experiments where bites did occur, the spiders always had a ‘finger’ or target placed in direct contact with their mouthparts.

An unaggressive female black widow takes a stroll across my hand. Although I never grab spiders to pick them up, coaxing them onto my hands and letting them wander around on their own steam has never been a problem. (Photo: S. McCann)

The most exciting thing this study tells us is that spiders can make decisions about how to respond to threats (which sometimes include humans) – further evidence of their incredible sophistication. Perhaps more importantly for the arachnophobic, it suggests that black widows would much rather conserve their valuable venom for use in dispatching their next meal than waste it on a human who is of no interest as prey!

Spiders in general are amazing creatures worthy of our admiration and respect. I hope that this new information about black widows might convince some that there is more about them to be fascinated by than to fear!

References and related reading:

Nelsen, D. R., Kelln, W., & Hayes, W. K. (2014). Poke but don’t pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperusAnimal Behaviour89, 107-114. http://dx.doi.org/10.1016/j.anbehav.2013.12.019

Vetter, R. S. (1980). Defensive behavior of the black widow spider Latrodectus hesperus (Araneae: Theridiidae). Behavioral Ecology and Sociobiology7(3), 187-193. doi:10.1007/BF00299363

W. Cranshaw (2014). Western widow fact sheet: http://www.ext.colostate.edu/pubs/insect/05605.html