Life Science

Hitchhiking Pseudoscorpions (Phoresy)

Here’s a word to add to your naturalist vocabulary: phoresy. Phoretic associations between organisms involve one party (the symbiont) using a typically larger other party (the phoront), as a means of transportation. In other words, one hitchhikes on the other. You can observe this strategy right in your own backyard. For example, some tiny nectar-eating mites use the bodies of larger pollinators to move between flowers.

But we’re not here to talk about hitchhiking mites. We’re here to talk about hitchhiking pseudoscorpions. If you already know a bit about pseudoscorpions, feel free to skip ahead. Otherwise, buckle up for a crash course below.

Pseudoscorpion Origin and Evolution

Pseudoscorpions are brought to you by the same evolutionary lineage (arachnids) behind spiders and scorpions. It’s important to note, however, that these are distinctly not spiders, nor are they scorpions. They are something else entirely.

A Neobisiid pseudoscorpion on a woody substrate.
A Neobisiid pseudoscorpion.
Image by Marshal Hedin | CC BY-SA 2.0 | via Wikimedia Commons

They are also a very old and very successful branch of the tree of life. The earliest fossil record of pseudoscorpions dates to around 390 million years ago, during the Devonian period. That makes pseudoscorpions one of Earth’s earliest terrestrial life forms. Incidentally, the only arachnid lineage with an older fossil record is the one that gave rise to scorpions, harvestmen, and acariform mites around 420 million years ago.

Remarkably, the morphology of pseudoscorpions has barely changed in all that time. In that regard, these animals are a great example of morphological stasis – a phenomenon in which a species doesn’t change much over time despite changes in the environment. Evolution was clearly pleased with itself when – to paraphrase science writer Jake Buehler – it strapped the forelimbs of a crab to the body of a termite.

Photographs of prehistoric pseudoscorpion specimens trapped in amber.
Prehistoric pseudoscorpion amber inclusions. Many are nearly identical to pseudoscorpions you see today.
Image by Harms, D. and Dunlop, J. A. | CC BY-SA 3.0 |Wikimedia Commons

Pseudoscorpion Morphology

Pseudoscorpions are small. The largest ones, members of the species Garypus titanius, only reach around 12mm in length. Most, however, are just 2-8mm long.

As with other arachnids, their bodies are organized into two distinct regions. The posterior region is called the abdomen (opisthosoma). It’s rather malleable and consists of 12 conspicuously delineated segments (much like scorpions) partially covered in hardened plates called tergites on the dorsal (top) aspect and sternites on the ventral (underside) aspect. Typically, only the first ten segments are visible dorsally. You’d have to flip the pseudoscorpion over to see the last two.

Internally, the abdomen contains organs and structures involved in excretion, respiration, circulation, reproduction, nervous system functions, etc.

House pseudoscorpion (Chelifer cancroides) anatomy diagram showing key parts of the pseudoscorpion body.
House pseudoscorpion (Chelifer cancroides) general anatomy
The Common Naturalist | CC-BY-SA

The anterior body region is called the cephalothorax (prosoma) – a fusion of the pseudoscorpion’s “head” and “thorax” – containing important structures like simple eyes, mouthparts, digestive tract, and brain.

At the front of the cephalothorax are two scissor-like chelicerae (i.e. chelate chelicerae). They look like a pair of tiny crab claws and are used to rip prey into manageable pieces for digestion. They also contain a grooming organ and spinnerets that produce silk. You read that right. They produce silk from their mouthparts. Whereas arachnids like spiders often use their silk – produced from spinnerets on their abdomen – to create webs, pseudoscorpions spin their silk into cocoons for protection during a molt, to help develop their brood-sac, or to shelter from unfavorable environmental conditions.

As you look back along the sides of the cephalothorax, you’ll see two oversized pedipalps and four pairs of walking legs. The walking legs are fairly straightforward and are the primary means of locomotion. Depending on the species, they consist of five to seven segments and terminate with a pair of tiny tarsal claws.

Macro photograph of the tarsal claws on leg IV of a Chelifer cancroides pseudoscorpion.
Tarsal claws on leg IV of a Chelifer cancroides specimen
Image by Kyron Basu via Bugguide.net | Creative Commons License

The pedipalps, however, are much more interesting. They are almost comically long relative to the pseudoscorpion’s overall body size. Extending outward from either side of the body, they each terminate in a large claw. The pseudoscorpion uses these “chelal hands” for defense, hunting, and interacting with mates.

Both claws are covered in sensitive bristles called setae that provide the pseudoscorpion with a wealth of useful sensory information about their surroundings. The arrangement of these setae is sometimes so unique that it can aid in species identification.

Along the surface of each claw’s “fingers” are 1-2 rows of “teeth” to enhance grip and damage the tissues of prey.

The fingers of some species even have venom glands that produce a mixture of neurotoxic peptides, enzymes, protease inhibitors, and defensins to further help with prey control. The glands run the length of at least one of each claw’s fingers and feed into a venom tooth at the tip of the finger. When this tooth pierces the pseudoscorpion prey, the venom gland is compressed; injecting the toxic cocktail.

The pedipalps serve another, more unusual function, but we’ll get to that later.

Chelifer cancroides pseudoscorpion female scientific specimen photograph.
Chelifer cancroides (Linnaeus, 1761), female.
Image by Artiom M. Ostrovsky

Pseudoscorpion Distribution and Habitat

Pseudoscorpions have a cosmopolitan distribution. You can find representatives of their more than 3300 species on every continent except Antarctica. Most, however, are concentrated in parts of the world with tropical and subtropical climates. That’s not surprising since pseudoscorpions, and their prey, are typically associated with vegetation and rich, productive soils. You’ll commonly find them in moss, humus, compost, rotting logs, under tree bark, around rocks, and in leaf litter.

Psuedoscorpions and other arthropods observed in wet leaves.

Other species prefer caves. Chthonius ressli and Neobisium carcinoides have been documented in caves in Hungary. Lagynochthonius fragilis can be found in limestone caverns in Vietnam and at least seven Microcreagris species are found in caves of the United States. That list is not exhaustive and there are many, many more cavernous pseudoscorpion species out there.

Some pseudoscorpions have even carved out a niche living among birds. A 2010 study by Turienzo, et. al. outlined 85 species of pseudoscorpion that are associated with the nests of 98 species of bird. Of those, the Chernetidae family of pseudoscorpions was the most represented.

Human homes have also become a habitat for pseudoscorpions. The synanthropic species Chelifer cancroides, for example, is often called the “house pseudoscorpion” due to its apparent preference for human dwellings. You may be able to find them scuttling around shelved books that have been bound with starchy adhesives. Such books often contain booklice and dust mites that feed on that glue and Chelifer cancroides is more than happy to eat them.

Getting Around by Hitchhiking (Phoresy)

Pseudoscorpions are so small that even traveling the length of a backyard is a considerable risk and energy investment. Moving from place to place, however, is usually crucial for their survival. Many pseudoscorpions live in unstable, ephemeral environments like rotten logs or mulch. They need to be able to relocate fairly quickly in response to prey abundance, environmental conditions, and habitat availability. That’s not easy when you’re super small. To put their size in perspective, take a look at the image of a pseudoscorpion on a US Penny below.

Photograph of a pseudoscorpion on a US penny coin
A pseudoscorpion on a US penny coin
Image by MSU

Several pseudoscorpion species, however, have come up with an unorthodox way to compensate. They will attach themselves to larger animals and hitch a ride!

This behavior (phoresy) is extensively documented in pseudoscorpions. It’s not a new behavior either. There are examples of pseudoscorpion phoresy going back at least 44 million years based on Baltic and Dominican amber inclusions. Long ago, phoretic pseudoscorpions and their “rides” fell into amber with the pseudoscorpions still attached. That amber solidified and was preserved for paleoecologists to find and analyze today. Some of the amber inclusions they’ve found include associations with prehistoric crane flies, parasitoid wasps, caddisflies, and etc. At least one recorded instance in amber involves a moth, but there is some debate about that one.

Dendrochernes cyrneus pseudoscorpion hitchhiking on the antenna of a Rhyssa persuasoria wasp
Dendrochernes cyrneus attached to the antenna of Rhyssa persuasoria from Austria
Photo by G. Kunz via Bioone | CC-BY

It’s unclear how this hitchhiking behavior emerged in the first place. It could be that it started as failed predation attempts or perhaps some pseudoscorpions millions of years ago were reacting defensively and got transiently stuck when they latched onto other animals. Over time, the clingier pseudoscorpions could have gained an evolutionary edge over the less-clingy pseudoscorpions and passed that heritable behavior to their offspring. Whatever the origin, this behavior has persisted over generations, become widespread, and become more complex. Fortunately for the hosts, it also appears that this behavior has evolved in ways that cause only minor damage (if any) to the host’s body. How thoughtful.

As I’ve hinted earlier, phoresy is most common in pseudoscorpions that live in ephemeral habitats like rotting wood. Eventually, their rotten log breaks down and the smorgasbord of tasty microfauna dries up. The pseudoscorpion needs to relocate if it’s going to survive. When a fly or beetle (or bird) gets close enough, the tiny pseudoscorpions take advantage of the lucky opportunity, latch on, and are carried to greener pastures.

Several pseudoscorpions hitching a ride on a fly's leg
Several pseudoscorpions hitching a ride on a fly’s leg
Photo by Tom Murray | BugGuide.net
Several pseudoscorpions hitching a ride on a fly's leg (alternate angle)
Several pseudoscorpions hitching a ride on a fly’s leg (alternate angle)
Photo by Tom Murray | BugGuide.net

Speaking of beetles, at least one species of pseudoscorpion utilizes beetle-based transport for more than just moving from place to place. Harlequin beetles (Acrocinus longimanus) are large, even by human standards. An adult harlequin beetle, with legs outstretched, is about the size of an adult human hand. For something as small as a pseudoscorpion, a harlequin beetle might as well be an A320.

These goliath forest-dwelling beetles happen to love using rotting trees as nurseries. The crevices of fungus-covered rotting trees provide excellent cover and nourishment for their eggs. When the eggs hatch, the larva burrow into the wood and remain there for several months, munching away until they pupate and undergo a complete metamorphosis into their adult form.

Harlequin beetle specimen on a hand, Acrocinus longimanus
Harlequin beetle specimen on a hand (Acrocinus longimanus)
Image by Ben Sale | CC-BY | Wikimedia Commons

These beetles aren’t alone in their love of these rotting trees. Pseudoscorpions like Cordylochernes scorpioides also happen to love rotting trees and cluster there in large numbers to feast on termites and other tiny arthropods drawn to the decay. The pseudoscorpions, however, don’t like to stick around forever. Eventually, sometimes after several generations on the same log, they are compelled to search out a new log in response to changing environmental conditions like a decrease in prey supply or a lack of mating opportunities.

When a newly-minted harlequin beetle adult emerges and prepares to take flight, the pseudoscorpions see an opportunity to tag along. They scuttle aboard their Coleopteran airship, latch on with their clawed pedipalps and small amounts of silk, and prepare for takeoff. Compared to the beetles, they’re so small and delicate that the beetle probably doesn’t even realize they’re there.

Pseudoscorpion female (Cordylochernes scorpioides) hitchhiking on the abdomen of a harlequin beetle.
Pseudoscorpion female (Cordylochernes scorpioides) hitching a ride on the abdomen of a harlequin beetle in Trinidad.
Licensed image | All rights reserved | Alamy

Once airborne, the pseudoscorpions sometimes pass the time by…procreating. Typically, a large male pseudoscorpion will clear away smaller males and establish a kind of airborne mating arena. He then deposits a small patch of silk on the beetle’s abdomen and loads it with a ball of fluid containing his spermatophore (a small package of sperm.) He then coaxes nearby females over his sperm sculpture and pushes the spermatophore into her sexual aperture. He repeats that process as many times as he can with as many females as he can over the course of the flight.

Eventually, the beetle arrives at a new rotting tree. At this point, the female pseudoscorpions disembark to feast and lay their eggs. Large males, however, often remain behind to try their luck joining the “exhibitionist pseudoscorpion mile high club” with the next batch of ladies now clamoring aboard the harlequin.

As for the smaller males, they often do best by disembarking and trying their luck with the ladies in the rotting logs. Their small size and mobility seem to give them a bit of an edge over the larger lumbering males when it comes to copulating in tight spaces.

Wild times.

Further Reading and References:

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