Hitching a ride

posted by Christopher Laumer / on March 27th, 2010 / in Arthropods, Parasites, Symbiosis

When collecting bugs out in the field, it can be easy to get more than one bargained for. Many flies, beetles, and other mobile beasties such as the harvestman shown above (Megalopsalis sp. from New Zealand) find themselves regular host to hitchhikers of arachnid origin: the five orangish globules nestled among the bases of this unfortunate individual’s limbs are parasitic mites hunkered down for the long haul.

Indeed, many tens of thousands of mite species spend their early lives attached to a host, slowly drawing nutrition from its internal fluids until they become large enough to drop off wherever their hosts have carried them, where they begin life as free-living adults. However, these parasitic freeloaders aren’t the only kind of tenants one will find on harvestmen; there are also diverse sorts of more benign bedfellows who climb aboard with no appreciable harm to their unwitting ride.

One group of these tag-alongs is the pseudoscorpions – distant relatives of true scorpions, living secretive lives in forest soil and tree bark – which many would doubtless find abjectly terrifying… if they ever got larger than a few millimeters. Below, you can see a pseudoscorpion hanging for dear life onto the leg of another Megalopsalis. There are reports of pseudoscorpions waiting eagerly around a flower for a bee pollinator to grab onto, or clustering around the pupal bores of flies just before the airborne adult emerges. The traditional interpretation of this behavior suggests that climbing aboard larger, more mobile animals is an adaptation meant to transport these tinier critters to a wider range of habitats. Others, however, have suggested that perhaps pseudoscorpions simply grab onto whatever passes by them in the hopes that they might be able to eat it.

Strangely enough, even pseudoscorpions can bear hitchhikers, as the parasitic mites on this neobisiid I collected during field work in Alabama attest (gray bugs next to the greenish dots). It’s hard not to be reminded of the poet Jonathan Swift’s famous verses:

“The vermin only teaze and pinch
Their foes superior by an inch.
So, naturalists observe, a flea
Has smaller fleas that on him prey,
And these have smaller still to bite ’em,
And so proceed ad infinitum.”

First three photographs by Gonzalo Giribet. Last photograph by Christopher Laumer.