strange behaviors

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  • Richard Conniff writes about behavior, in humans and other animals, on two, four, six, and eight legs, plus the occasional slither.

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OK, This is Gross: Botflies

Posted by Richard Conniff on September 9, 2009

This is, oh joy, a video of a guy graciously removing a botfly larva from his partner’s scalp.  And to accompany it, here is an excerpt about botflies from my book, Swimming With Piranhas at Feeding Time:  My Life Doing Dumb Stuff with Animals:

So it takes an almost unnatural objectivity to suggest that our ectoparasites can also be fascinating. Like any species colonizing difficult terrain, they have adapted ingeniously to our flesh. They use sophisticated chemosensors to find us; saws and scalpels to penetrate our skin; siphons and a small pharmaceutical warehouse, including anesthetics and anticoagulants, to steal a blood meal and get away undetected. If we can suspend for a moment the uneasy awareness that all this evolution is geared to extracting our blood, and if we can forget that our parasites mostly use this blood to produce the eggs for their future pestiferous generations, then it is possible to regard them with awe.

They are capable of extraordinary subterfuge. For example, the adult botfly of Middle and South America manages to parasitize us quite gruesomely without ever actually making physical contact. To avoid being swatted by some balky human or other host, she captures an insect, a mosquito for example, glues her eggs to her prisoner’s abdomen, then sets it free.

The mosquito ignores the eggs (as will we for a moment) and goes off to employ subterfuges of her own. Many mosquitoes feed at night, for obvious reasons (“Consider the outcome if you were to approach an elephant with a syringe,” one entomologist says). But this mosquito is a day feeder, finding a victim with her eyes and with sensors attuned to carbon dioxide, warmth, lactic acid, and other bodily emanations.

Having deftly touched down, the mosquito stabs and saws her way into the fine web of blood vessels in the skin. The damaged vessels instantly attempt to plug their leaks with aggregating platelets in the blood. But host and parasite have evolved together, with all the one-upmanship of any arms race. So the mosquito is equipped with a powerful enzyme in her saliva to disable the platelets. The more saliva she pours down one tube in her proboscis, the faster she can suck up blood through another. Humans in turn have an immune response to the saliva, which alerts us with itching and swelling, but only after about a minute. We swat ploddingly—and are likely to kill only the slowest feeders. Thus we do our bit for natural selection, helping ensure that future generations come only from mosquitoes that are quick enough to get away with our blood in a minute or less.

But the co-evolutionary arms race on the human ecosystem is even more disheartening than all this might suggest. The mosquito may leave behind other gifts, along with her saliva. After having been driven out in mid-century, malaria and dengue fever have lately begun to reappear in the United States and other developed nations. Insect-borne diseases are on the increase worldwide, largely because so many species have developed resistance to insecticides and their pathogens have developed resistance to our best medical therapies. In the New World tropics the insects may arrive bearing not just agents of disease but at least one other gift: Let’s say we get bitten by the mosquito that was briefly held prisoner a few days earlier by a botfly. As the mosquito feeds, our own body heat triggers the botfly eggs glued to her abdomen to hatch. A botfly larva promptly crawls into the fresh bite wound, where it matures with time into the ripest sort of traveler’s horror story.

The larva has a segmented, yellow-brown Michelin-man body, belted with rows of raked-back spines for lodging itself mouth-first in the skin. It also anchors itself with two tusklike hooks sticking out from the mouth. Its tail is a breathing tube, which can lift up, periscope-like, just above the surface at the point of entry. As it develops, the larva wriggles visibly and painfully under the skin. Removing the botfly is relatively simple (one remedy involves applying bacon to the breathing hole, so the botfly has to burrow up through it for air). But a Harvard biology student, curious about his own potential as an ecosystem, once nurtured a botfly in his flesh for six weeks. Finally a one-inch-long [2.5-centimeter-long] botfly larva, ready to move on to its pupal stage, started to emerge from his scalp as he sat in the bleachers during a Red Sox-Yankees game at Fenway Park. The Sox lost, and despite the biologist’s heroic efforts to protect it, the botfly died.

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