Hitchhiking is a time-honoured tradition for many a backpacker, and is an inexpensive way of covering the distance between destinations. Nature, of course, has its own version called phoresis or phorecy. In this case a phoront (the hitchhiker) attaches itself to a host and travels along until said host reaches a suitable destination, where the phoront will then detach again.
Pseudoscorpions have mastered this art to the extent that some species even require phoresis to survive. Pseudoscorpions themselves are an ancient order of tiny predatory arachnids, usually no more than 3mm in size, that superficially resemble true scorpions but lack the elongated tail and sting. Many species possess venom which they can inject into prey from small venom teeth at the ends of their pincers. This makes the Pseudoscorpiones one of only three arachnid orders that possess venom, the other two being true spiders (Araneae) and true scorpions (Scorpiones). Pseudoscorpions occur in almost every part of the world, though most species are found within the tropics and subtropics. Their unique morphology makes these secretive generalists a very important predatory component of many terrestrial habitats, where they can readily be found among humid soil, leaf litter, compost piles, under stones, bark and logs, as well as harsh environments such as intertidal zones.
One of the better documented habits of pseudoscorpions is their phoretic association with many flying insects, whereby the pseudoscorpion attaches itself to a host insect by grasping it with its pincers, and travels along with its host to a new location where it would then detach. This is a pretty effective dispersal strategy as it allows these tiny arachnids to quickly travel great distances to find new hunting and breeding grounds.
For many pseudoscorpion species this is only an occasional behaviour and as such they do not rely on phorecy to survive. By clamping on for dear life, many pseudoscorpions also unwittingly damage the exoskeleton of their hosts’ appendages, or tire the host out during flight. On the other hand, some pseudoscorpion species form strong symbiotic relationships with a particular host and have specialized phoretic strategies.
If, for example, you rely on your host’s survival over long distance travel, would it not then be beneficial to reduce any potential damage to said host? Examples of this can be seen with the bee-associated pseudoscorpions of the Ellengsenius genus, where they survive by feeding on parasites found within beehives. If a bee colony thus moves the pseudoscorpions have to tag along with them without causing damage to their flying hosts, since an injured bee may not be able to keep up with the rest of the hive. To reduce potential damage to their hosts the associated pseudoscorpions have pincers modified with both a ventral-curved movable finger and a divot near the end of the fixed finger. Together these modifications allow the pseudoscorpion to effectively grasp around the bee’s leg and not directly onto it, reducing the risk of damage caused by crushing the exoskeleton of the appendage.
Members of the chernetid Cordylochernes scorpioides pseudoscorpion species have gone a step further, not only utilizing their host for transport but for a mating arena as well. Their host of choice, the Harlequin beetle (Acrocinus longimanus), is a giant Longhorn beetle found within indigenous forests of Central and South America. The beetle-riding pseudoscorpions prefer fallen trees, especially Ficus trees, where they scour the crevices of the decaying wood in search of small invertebrate meals such as wood-boring termites. The giant Harlequin beetle also prefers fallen Ficus trees to lay its eggs on. The emerging larvae then burrow into the dead tree and feed on the decaying wood for several months before pupating.
Upon emerging the adult beetles would quickly disperse in search of newly fallen trees to mate on and lay eggs. The pseudoscorpions, also wanting to disperse to greener pastures, thus only have a brief window in which to board their flight. Thus, as soon as the newly pupated beetles emerge they are often set upon by a large number of pseudoscorpions, each vying for a place on their host transport. The pseudoscorpions use both olfactory cues as well as beetle stridulation to locate their host, then quickly move to the beetle’s posterior. Their preferred space is the sub-elytral area on the beetle’s abdomen, just below the flight wings. To gain access to this space, the pseudoscorpions pinch the beetle’s rear abdominal plates. The beetle responds by flexing its abdomen as well as partially opening its wings, giving the phoronts access. Both male and female pseudoscorpions vie for space, though males often engage in competition with each other to block potential suitors from the females.
Once a beetle departs, large pseudoscorpion males would try to wrestle remaining males off the host, often leaving only a single dominant male with a harem of females. To stop themselves from falling off the beetle in flight, males construct silken safety harnesses from galea (spinneret-like structures) on their mouthparts to attach themselves to the beetle’s abdomen. Over time the males may expand on the harness to produce a complex nest-like structure. The male then engages in courtship where he would deposit a spermatophore on the abdomen of the beetle. The spermatophore is comprised of a stalk with a drop of liquid on top and contains the sperm. Approaching a female the male then beckons to her by moving his pedipalps up and down. If the female accepts, the male then grasps her with both his pincers and guides her over his spermatophore, where it attaches to the female’s genital plate. The sperm is then transferred. When the host beetle lands on a different Ficus tree the females disembark to develop their eggs in an external brood sac they carry around and protect until the young emerge. New females and males then embark the host beetle to start the process once more.
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