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Native Californian, biologist, wildlife conservation consultant, retired Smithsonian scientist, father of two daughters, grandfather of 4 small primates. INTJ. Believes nature is infinitely more interesting than shopping malls. Born 100 years too late.

Thursday, June 28, 2007

Got you under my skin

Notice anything unusual about this squirrel?

Yeah, I've noticed that it's a male too, but that's not unusual. (Arrow "x" is there only for reference.)

What I'm talking about is the dark thing hanging down from the squirrel's belly? Can you see it? Look again below the scrotum. Arrow "y" points to it.

The luckless rodent seems to be carrying an alien from the underworld of the Arthropoda. What you probably see is a repugnant swelling with an oozing sore, a warble. It's the domicile of a bot, the parasitic larva of the bot fly.

As flies go, bot flies are rather pleasing. They are hairy, somewhat blocky in build, and resemble bees. They have small or vestigial mouthparts. The raison d'etre of their brief adult existence is reproduction, and during their quiet interlude under the skin of squirrels they accumulate the energy stores needed to go forth and procreate.

There are 34 species of the bot fly genus Cuterebra in the US, and their larvae, the bots, must feed on small mammals, mainly rodents and rabbits in order to reach maturity. Each species of Cuterebra has its preferred host. In other words, gophers, wood rats, white-footed mice, and squirrels have their own dedicated species of bot fly, but they are not totally loyal to one host. Occasionally they meet up with the wrong host. Forty species of insects, including bot flies have been known to "accidentally" parasitize people in the US.

Finding a bot on your own body or in something you intend to eat is not a pleasant experience. In the southern US, only 2% of squirrel hunters were reported to eat bot-infected squirrels. The rest of them got really turned off. So a lot of hunted squirrels cycle back into the food chain without benefit of human digestion.

The fly bot is the closest thing to the slimy xenomorph (read Alien) from Planet LV-426 that exploded from the chests of its human hosts. Surely you remember Sigourney Weaver and aliens that got scarier with each new movie.

The difference of course is that aliens are fictional. True, they are all pretty much based on biological themes, though embellished and recombined to be extremely scary. Bot flies and their parasitic larvae however are REAL. So let's concentrate on the kinds of zoological reality that gave the producers of Alien its wildest ideas. There's a good reason for this, because our youth often confuse reality and fantasy, and in the age of intelligent design people seem to be losing their curiosity about biology and the natural world.

You will recall that the infective phase of the Alien life cycle is the Facehugger, which leaps onto the head of the human host and shoves an egg down its throat. This is extremely disconcerting for the human host. Cuterebra have far more finesse.
The female uses olfactory cues to home in on the nests of rodents, and simply lays her eggs on the vegetation nearby. When the eggs hatch the tiny bullet-shaped larvae (the first instar) zero in on body heat and attach themselves to the passing rodent.

There is no need for forced entry. The tiny larva crawls undetected into the body through the eye, nose, or mouth -- for that matter any opening they find, natural or otherwise.

The Alien's embryo (the "crawler") settles in the thoracic cavity and matures in a remarkable 24 hrs. Our baby bot (the first instar) on the other hand, takes about a week to migrate from the point of entry to a comfortable space beneath the skin, often in the squirrel's hindquarters. There it prepares its lodging for the next several months. The first order of business is to rasp a small breathing hole to the outside word. It also uses this porthole as a latrine, periodically expelling its liquid brownish excreta on the surface, which the squirrel periodically grooms away.

For the next month and a half the bot is the squirrel's constant companion. The squirrel's body reacts to it by encapsulating it in a fibrous sac, not unlike a mummy bag which seals it off from the rest of the body. The host's white blood cells and antibodies also increase. but this has little effect on the bot. It is basically a couch potato. Its daily routine is to lounge in its mummy bag, feeding continually on tissue fluids and cellular debris.

When it reaches the stage known as the third instar, it's almost as big as the end of your thumb. But unlike the Alien, it doesn't explode through the body wall, killing its host and scaring the hell out of everyone. On the contrary, it goes gently into that good night of pupation by squeezing through the breathing pore and dropping to the ground. There it burrows and undergoes its final transformation. It may emerge as an adult in another month or pass into a diapause and emerge next year.

It is hard to say how squirrels and other bot-infested rodents feel about all of this. They seem to carry on normally and ignore the bot, but in the laboratory at least, rodents become inactive about the time the bots emerge, and we may infer from this that harboring a bot has its discomforts.

What else? Well, if you are a nestling squirrel and unfortunate enough to be selected by several bots, you may not live long. If you are an adult female and the bots settle in your back side (as they often do) you may develop a pseudo-scrotum, and fool all the other squirrels. Male's with a bot-infested scrotum on the other hand may sport an appendage that resembles a duffel bag, but this renders them reproductively useless for the time being.

The affinity of bots for the scrotum accounted for the name Cuterebra emasculator, but Drs Bob Timm and Robert E. Lee Jr showed that scrotal bots don't affect subsequent reproduction. Unlike a lot of other parasites, bot flies don't practice "parasitic castration" upon their hosts.

If that wasn't so, a lot of southern squirrel hunters might be wearing camou codpieces made of fly screen.


Baudoin, M. 1975. Host castration as a parasitic strategy. Evolution 29:335-52.

Bennett, G.F. 1973. Some effects of Cuterebra emasculator Fitch (Cuterebridae, Diptera) on the blood and activity of its host, the Eastern chipmunk. Journal of Wildlife Diseases 9:85-93.

Jacobson, H.A., D.C. Guynn, and A. Hackett. 1979. Impact of the bot fly on squirrel hunting in Mississippi. Wildlife Society Bulletin 7(1):46-48.

Scott, H.D. 1964. Human myiasis in North America (1952-1962 inclusive). The Florida Entomologist 47(4):255-261.

Slansky, F. 2006. Cuterebra bot flies (Diptera: Oestridae) and the indigenous hosts and potential hosts in Florida. Florida Entomologist 89(2):152-160.

Timm, R.M. and R.E. Lee Jr. 1982. Is host castration an evolutionary strategy of bot flies? Evolution 36(2):416-417.


Anonymous said...

Good stuff, Codger. By limiting this to a discussion of rodent bots, you have protected your readers from the fact that bot flies also tackle humans from time to time. Folks working in the neotropics are exposed to bot flies, particularly if they are working at night, or around dusk or dawn. Monkeys are frequently infected with bots, sometimes in the scrotum like your unfortunate squirrel, but also around the head and neck, probably in areas that are not covered with a dense coat of hair. I assume that it is one of these Primate parasites that occasionally infects humans.

Interestingly enough, there seems to be strong individual variation in whether or not you are bothered by bots. Some folks get hit by several on their first trip working in the field in the tropics, and others can work there their whole careers and never get one.

I thought I was in the former category, as I worked in both Central and South America in areas where students were always getting them, and never had one for many years. Then, suddenly, I had a couple of years when I got several. Some were in the back of my neck, and some in my arms, and at least one in my ankle.

I had colleagues who managed to rear them out until they pupated, but I never had the patience. The breathing hole oozes the goo you mentioned, which is obnoxious to anyone around you, and in fact, they do cause a certain amount of pain from time to time. The larva attaches with little hooks on its hiney, and occasionally they reposition themselves in such a way as to give you a cheap thrill.

The locals deal with them by blocking the breathing hole with a small piece of raw meat. Then, the larva begins to chew its way up through the meat to reopen the breathing hole, and when it is more out into the meat then into the hole in you, you can give it a squeeze and pop it out.

Because I was always back in this country by the time I wanted to eject the little rascals, the raw meat treatment was vetoed by my wife, who instead covered the hole with a little nail polish, and then when I could feel the critter wiggling around to try to open it up, she would squeeze the bejesus out of the wound, and pop it out. And yes, I am married to a saint willing to put up with a maggot-ridden field biologist of a husband.

The best part of the story is the version I have heard of how the human variety (or Monkey, if that is the normal host) deposits the egg. Supposedly the adult fly catches a mosquito in flight, and lays the egg on it. Then, when the mosquito bites you, the heat from your body causes the egg to hatch, and the larva crawls into the mosquito bite. I haven't looked into the literature to see if this is fact or fiction, but I like the sound of it.

Anyway, enjoy your next trip to the tropics, and try to spend a little time in the forest at night...


Camera Trap Codger said...

Thanks for filling us in, Batfinger. I don't recall hearing your adventures with fly bots, but no doubt our readers are now champing at the bit to book their vacations in Latin America. I DO remember when you got falciparum malaria in Africa and didn't know what was wrong until you went to Walter Reed. You looked like death warmed over when you came back to work, but breathing the fumes from the specimen cases outside your office restored you to the pink of health. [Readers, Batfinger works in a very large museum back east.]

Jackie said...

I just got up, expected some light hearted animal tales, and instead got grossed out tales more suitable for later in the day! Fortunately, my cup of java is a strong one and its bitter taste makes the reading almost palatable.

Batfinger, your botfly encounters sound awful. Interesting, make for good story telling, but awful too.

Camera Trap Codger said...

Surprised you, huh? Well, your reaction gave me a good chuckle, Zhakee. Biologists seem to have a weakness for scatology and gross stories. (The excuse is that it's science.) Batfinger in particular is well-known for his colorful tales of field work.

Anonymous said...

Chris: Science is 'eye-opening'!! My only earlier bot fly experience was limited to a squeeze or 'Coke-bottle compress' on a warble found ready to emerge from a cow's back!!


Anonymous said...


Good one on the bots. Nicely mounted too. That's the one we used to get at Corpse Ridge with Cactus Mac.


Camera Trap Codger said...

You are right, country boys and girls know all about bots; it's the city kids who are out of touch with nature.

The two specimens I have drowned in the pool. I've never gotten one sweeping vegetation or aerial netting. But tabanids (horseflies) are an entirely different matter, right?

Jace Stansbury said...

Chris- Thanks for a very
disgusting, though informative entry. One thing's for
sure....I'll never eat squirrel :-)

Batfinger- nice tale and equally gut wrenching!

By the way Chris- what brand of game camera do you use? Excellent photos and entries my friend!


Camera Trap Codger said...

I am only using "homebrewed" Sony Cybershot s600s with Pixcontroller universal controller boards and Pelican 1060 or 1040 cases. Now that you are getting addicted, its time to subscribe to the various camera trap forums -- I am on the Pixcontroller Forum and Real Deal Hunting Chat. They can prepare you to hack your own 6 MP cam for about $200 or less.

Glad the bot post had the desired effect (LOL).

Owlman said...

My first real job as a biologist was working with Frank Radovsky at the George Williams Hooper Foundation at the UC Medical Center in S.F. while I was a graduate student at S.F.State. Besides Ticks, Fleas and mites We were propagating Cuterebra latifrons in the lab.
As I recall they mate in flight at the top of hills and one of the sexes (I forget which) flys up from a perch to copulate with the opposite sex in mid air.
We attempted to do that by attaching light thread with wax to the thorax of each sex. We took them out in the large hallway of the 15th floor of the then new building. Dressed in our white lab coats Radovsky with one sex and I with other were flying our fly's like model airplanes. I can still see the look on the faces of passers by who knew nothing about what we were up to. I'm not completely certain that we even knew.

Camera Trap Codger said...

Great story, which proves my basic premise in this blog--biologists know how to have fun in weird, sometimes socially-unacceptable, but always educational ways. In my experience, the experiments that didn't work were almost always the most fun. I can see the passersby looking up in the sky wondering what the hell was going on. LOL

Anonymous said...

Wow. I remember squirrel warbles from growing up in eastern Kentucky ("Something for the foxes to eat" my dad always said when we shot one), and had always been curious about the life cycle. Thanks for the lesson. You mention that it isn't unheard of for the bots to jump host. How common is it for humans to be infected by native North American bots? After reading batfinger's comments, I wonder if I have one more thing to watch for next time I'm out swamp stomping in west Lousianna/east Texas (as though mosquitoes, ticks, poison ivy, and giardiasis weren't enough.)

Camera Trap Codger said...

Sorry I missed your comment Frog_man, but here's the answer. A lot of different species of flies have jumped host now and then. I came across a paper on the subject of US cases, and it was far more than one would guess. But the chances of it happening are still rather slim. You are far more likely to get ticks and chiggers. LOL

Anonymous said...

Here ya go--just 1 of several instances I've found


Anonymous said...

Chris-- I just stumbled across you pix of the squirrel with the bot fly larva. I'm a bot fly researcher, and I wonder where that pix was taken? The only known species of bot fly (Cuterebra) that infests tree squirrels occurs east of the Mississippi River-- it is Cuterebra emasculator. If that is a western gray squirrel in that pic from California or somewhere out west, then this is of scientific interest-- it could be that C. emasculator has made it to the west coast, along with eastern gray sqs, or that sq was infested by some other species, such as one that usually infests mice and characteristically infests the groin area. So you may have a new scientific discovery on your hands-- cool, huh?!?! I'd appreciate it if you would contact me directly at fslansky@ufl.edu. I've got an incomplete bot fly website at botfly.ifas.ufl.edu with some closeup pix of infested sqs, if anyone is interested.

Camera Trap Codger said...

Frank, I'm sending you an email.