I hadn’t seen any for a long time, but last weekend I even accepted one of them in my car: hitchhikers. However, chances are you go for a hike in the Papuan bush (or tropical jungle for that matter) and you will come across some evil animal hitch-hikers, waving their sucker-armed front ends of their wormy bodies at you: leeches, terrestrial blood-thirsty ones. They sit on wet leaves or stones next to jungle paths, frequented by humans and animals, and wait to hitch a ride (and a blood meal) from a host and when satiated drop off again at a different location. But not all terrestrial leeches feed on blood (there are many species that prefer to gobble up earthworms) and there are other less blood-thirsty animals that get around by hitching a ride.
Examine some of the older, weaker blowflies buzzing against the window pane in a desperate attempt to get out, and you may occasionally find one that carries one, two, or even more 1-2 mm long pseudoscorpions on its body. The world record, incidentally, is 15 such tiny creatures on a single fly. Pseudoscorpions resemble scorpions, but lack the “stinger” (the long tail filament) and are also often referred to as book scorpions. They use flies as a means of transport without harming them. Pseudoscorpions are small, have tiny legs, cannot hop or fly and in order to disperse need to find transport – and what could be better than a big and tired blowfly, whose attempts to brush off the hitchhikers usually come to nothing?
The same can be said for many species of tiny mites, which climb “aboard” bigger insects like beetles and large spiders to get around. Dung beetles are a favourite vehicle for the tortoise mite, because it loves dung, but is too small and slow to crawl by itself from one dung pad to another. For it and other dung inhabitants like a roundworm by the name of Pelodera sp., to attach themselves to dung beetles is the perfect choice before a cow pad dries up. Similarly, pollen or nectar loving mites whose life span vastly extends that of the flower they are feeding on, need to reach another fresher flower before the old one wilts and such a journey is best achieved by hanging on to a flower-visiting transport insect as a stowaway.
In the sea there are many more examples of species of animals using others for transport. Remoras are fish whose dorsal fin has become a sucker organ and although the remoras can swim very well, to cover greater distances (and to be nearer some food that remains uneaten by their hosts) they attach themselves with their sucker to the underside of large turtles and fish like sharks and manta rays for example. Since the remora fish do not feed on their hosts and do not critically impair the latter’s swimming performance, they -just like the aforementioned pseudoscorpions, mites and round worms- cannot be termed parasites. Hitchhikers that do not negatively affect their hosts are termed “phoretics” and how they get around is known as “phoresy”. Having explained that, it becomes obvious that the blood-sucking terrestrial leeches, referred to at the start, or fleas are not phoretics, but parasites, ectoparasites to be precise. However, even if phoretics can’t be called parasites, they too, like human hitchhikers, can sometimes become a bit of a nuisance and difficult to shake off again. So, the step from being a phoretic to being a parasite isn’t a very big one.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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