Chances are that when most people hear the word “bot”, they think of software that makes a computer seem like a human or undertakes some routine online tasks. But long before those kinds of bots, a bot was a fly that parasitized humans or other mammals. Today’s Found Object is the larva of the human botfly, Dermatobia hominis, that I extracted from my arm after a visit to my brother in Costa Rica during which we made some trips into the wilder areas of the country.
After my return, I attended a particularly boring meeting that gave me plenty of time to focus on what appeared to be a persistent, non-healing insect bite on my arm. Staring at it from close up, I gradually realized that something in that bite was moving, and when I poked the bite, the moving object quickly withdrew to deeper levels, then reappeared after a few minutes. Aha! Being an entomologist, I concluded that I had acquired a bot fly larva in Costa Rica!
What to do? I had acquaintances (entomologists, of course) who had reared out the larvae under their skin until they had emerged of their own free will to pupate in the soil, but these larvae get quite large, and when they are large, having them as guests can really hurt. Their mandibles are modified into a pair of hooks (see image below) with which the larva rips tissue for ingestion. While it does so, its other end is sticking out through a hole in your skin, taking the air through its posterior, spiracle-containing end to breathe. I tried nabbing the larvae as it poked its hind end to the surface, but it was fast, and there wasn’t much to grab. Still, it had to breathe, right, so it had to keep poking out.
It was man against larva! Who was smarter? I hoped I was, so I put a big blob of KY Jelly over the wound, and waited until the larva was desperate for a breath of fresh air. Out came the breathing tube end, but oh oh, … no air! Farther and farther out as it searched for air until finally, a long stretch of its body was in the transparent blob of KY Jelly (who knew it had other uses?). I nabbed it with my No. 5 Inox Swiss Dumont watchmaker’s forceps and held fast. It was like roping a dogie, with the larva trying to retract and Walter holding fast--- what a contest! I knew that the larva couldn’t simply be pulled out slick as a whistle because its body is covered with recurved spines that anchor it in the skin. I also knew that if I pulled hard enough to break the larva in two, I would court an infection. Millimeter by millimeter I felt my skin give way as the larva was pulled toward its inevitable (I thought) demise. Finally, with a last uncomfortable tug, there it was, row and row of spines that had failed to save it from its fate--- a Found Object in alcohol. You could quibble about who found who, but let’s just agree that I am the final, more crucial finder.
I got another bot on the back of my calf on a trip to Honduras, but by then I was a bot expert. I showed my animal behavior class, but they were so grossed out that none of them got closer than 10 feet. Maybe they thought the bot would jump out and burrow into their hides. I guess growing up in the USA did not prepare them for having personal parasites.
The adult bot fly is a very large insect, about the size of a bumblebee. Like many parasitic flies, it lays ready-to-hatch eggs or freshly hatched larvae. The larvae quickly burrow into the host’s skin at the base of a hair follicle. However, in neither of my bot adventures could I remember having been approached by a large, buzzing fly about as subtle as an Apache attack helicopter. As an entomologist, I am sure would have noticed.
But that’s not how it works. The fly doesn’t search out humans on whom to lay its eggs. No, it searches out mosquitoes because mosquitoes can easily approach humans in a stealthy manner (as we all know). The female bot fly dive bombs a female mosquito (there is no point in dive bombing a male, as they don’t feed on blood) and sticks a fully developed egg to its belly. The mosquito proceeds to do what mosquitoes do --- it follows a trail of carbon dioxide and body odor to find its human host, and as she settles on the host, the host’s body heat signals the bot egg that it’s time to hatch, and it does, burrowing into the host’s skin to safety. Is this intricate enough, and high-risk enough (for the fly) that you might not even mind having this amazing creature consuming the tissue under your skin? It is, right?! It certainly gives you the experience of truly being part of nature, of “giving back” as the popular term goes. How can that be bad?
Bot flies and their relatives belong to the family Oestridae, and include species that parasitize several species of mammals, sometimes in rather gruesome ways. Some oestrids live in the stomachs of horses, under the skin of cattle, and in the nasal cavities of deer, caribou and sheep, giving them the blind staggers, or even killing them. The human bot fly isn’t limited to people, but also parasitizes several other large mammals including livestock and deer. Many other mammals, including rodents, are unwilling hosts to bot fly larvae, but in most of them, the fly lays the eggs directly on the host without the middleman (middleskeeter?) that the human botfly uses. During our morning tea, we sometimes see a squirrel with a huge lump under the skin of its back or side. Almost for certain, this means a botfly larva is dining on the squirrel’s subcutaneous tissue while the squirrel dines on the seeds intended for our wild birds.
Before there was Chatbot, GoogleBot and Botnet, there was Ratbot, Sheepbot and Horsebot. My little adventures with NatBots reminded me of how rarely we in the developed world serve as hosts for a long roster of parasites from all across the animal kingdom--- protists, sporozoans, nematodes, platyhelminths, acanthocephalans and insects. They enter us through food, soil, human and sexual contact, insect bites, drinking water or even just contact with water. They then settle in our skin, lungs, blood, liver, muscles, kidneys, and brains. The tropics are a special land of endless opportunity for these creatures, and eliminating these hordes requires major public health programs, vector control, and a range of cultural and behavioral changes.
Finally, some reflections on vectors—- should the mosquito that carried the botfly egg to my skin be considered a vector? Carrying somebody else’s egg doesn’t seem like a big deal, just sort of a delivery service. But being a vector isn’t always a walk in the park. Many insect vectors are themselves sickened by the parasite. Does anyone feel sorry for a female mosquito with malaria or fecund with filariasis? A tsetse fly teeming with trypanosomes? A tick buffeted by babesia? A blackfly oppressed by Onchocerca? I don’t expect many takers for my proposition of a touch of empathy, but sometimes it’s important to see the biological world from a perspective other than our own. As in everything else in the living world, we are just opportunities for other creatures. So, after reading this essay, if you must go to the tropics, don’t eat, don’t drink, don’t sleep, and take Tom Lehrer’s advice, “don't drink the water and don't breathe the air!”
Great essay! Not what I was expecting to read while eating breakfast, but fascinating nonetheless! I have a friend who picked up one of these in Central America when it burrowed into her neck. She had great bragging rights afterwards. Thank you for the science behind the freakiness!
Thank you! Only one egg per mosquito? Is the mosquito injured when the larva exits?