"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."
- Jonathan Swift

April 11, 2016

Pseudolynchia canariensis (revisited)

Ever since birds and mammals have evolved to have feathers and fur respectively, many different orders of insects have also evolved to take advantage of the opportunities that they provide. Fleas, lice and some families of flies have become ectoparasites that dwell in the cosy environments offered by animals covered in feathers or fur.

Top: P. canarienesis with hitch-hiking lice
Bottom: Pigeon lice; (A) Columbicola columbae,
(B) Campanulotes compar, (C) Hohorstiella lata,
and (D) Menacanthus stramineus. Image from the paper
While there are many biting flies that feed on the blood of feather- and fur-covered animals, few are as specialised as the hippoboscid flies - also known as Louse Flies. Louse flies are flies that have evolved to be obligate ectoparasite - some of them can fly, but they prefer spending most of their time crawling around the feathers of birds or the fur of mammals. The species featured in the study we are covering today is Pseudolynchia canariensis, which parasitises the common rock pigeon (Columba livia). The flattened body and long legs of the louse fly allows it to go scrambling amidst the feathers of its host, as it finds a sweet spot to chow down on some pigeon blood. The prime spot to do so is on the pigeon's belly amongst all the soft downy feathers

But P. canariensis is not the only ectoparasite on pigeons - as would be expected, pigeons are also home to many other parasites include a variety of actual lice. There are four species of lice that regularly hang out around the pigeon's belly, Columbicola columbae (which has been featured on this blog before), Campanulotes compar, Hohorstiella lata, and Menacanthus stramineus. So it can get pretty crowded on a pigeon's belly and there are plenty of opportunities for these parasites to mingle

Unlike the louse flies, lice don't have wings - so to travel from one host to another, they have to either crawl the whole way themselves, or borrow someone else's wings. By that I mean some lice hitch a ride on their fly-based namesake. In this study, scientists measured the mobility of the above mentioned four lice species commonly found on the rock pigeon, and their ability to hitch a ride on those P. canariensis.

They first tested how well each of those lice move about on their own by placing them on a piece of filter paper, and watch how far they managed to move in two minutes. Because lice tend to dislike light, the scientist shone a small light on them to coax them to move. Next, they tested the lice's ability to attach to louse flies by placing louse individually in a clear tube with a louse fly, and see how quickly they climb onboard - if at all. Finally, they test how well these lice managed to stay on the louse fly by repeating the attachment experiment, but this time they let P. canariensis does its thing and fly to the other side of a small room with a closed window, then recapture it to see whether the hitch-hiking louse had manage to hang on.

They found that not all lice are equally adept when it comes either moving on their own, or the finer art of leaving on a louse fly - and it seems aptitude in those two skills are inversely related. The most athletic lice like M. stramineus are also the worst at attaching themselves to P. canariensis, whereas those that can't move all that well off-host, such as C. columbae, are louse fly riders par excellence. Rock pigeons are pretty gregarious, so for the more mobile lice, they can easily cover the distance under their own steam. At the speed which the scientists recorded, M. stramineus is capable of covering one metre in the period of six minutes, which makes it quite the marathon runner in the louse world. In contrast, Co. columbae and Ca. compar are downright helpless anywhere away from a bed of pigeon feathers, but they are very skillful when it comes to piggybacking on a louse fly.

For some lice, leaving on a louse fly is not such a lousy way to travel.

Bartlow, A. W., Villa, S. M., Thompson, M. W., & Bush, S. E. (2016). Walk or ride? Phoretic behaviour of amblyceran and ischnoceran lice. International Journal for Parasitology 46: 221-227.

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