Something to howl about: to earn her spurs as a tropical biologist, the author decided to study a parasite that even her colleagues wanted to avoid

Natural History, Oct, 2003 by Katharine Milton

In 1974, as a greenhorn to the tropics, I traveled to Panama to begin a study of the dietary behavior of wild howler monkeys on Barro Colorado Island. The island was separated from the mainland in 1912, during the construction of the Panama Canal: its six square miles of forest now serve as a field station managed by the Smithsonian Tropical Research Institute. In my first exhausting but exciting weeks settling into new quarters and venturing on my own into the forest, I noticed that many howler monkeys had peculiar lumps under their fur, usually around the neck and throat but sometimes on the chest or stomach, on the back, even on a cheek or above an eye. The lumps were large, and they often made the monkeys appear grotesque. Infants looked as if they had two heads or a massive goiter; many adults resembled something out of B-movie sci-fi.

Curious, I asked other biologists on the island about the lumps. They, too, were fairly new to the site, but their answer was immediate: "Bot fly larvae." Bot fly larvae? Eek! I'd never heard of them, but they sounded pretty alarming. I learned that Dermatobia hominis, the "human" bot fly, is well known to science because of the diabolically clever way it finds hosts for its offspring. A female ready to deposit her eggs seeks out a bloodsucking insect, generally a fly or mosquito. She grasps the insect--known in the trade as an egg porter--and holds it firmly in flight while she attaches rows of her eggs to its abdomen with a water-insoluble glue. She then releases the insect unharmed. Now, though, it is neatly decorated with twenty-odd bot fly eggs. There the bot fly embryos grow quietly until they're ready to hatch.

The trigger for hatching comes from a third animal species. When the egg porter makes a meal from the blood of a mammal--a meal required for the insect's own reproduction--the bot fly embryos, by now developed into tiny threadlike larvae, sense the heat from the mammal's body and burst from their eggs. The larvae burrow directly into the mammal's skin, where they make themselves at home.

Each larva lives in what is known as a warble, a pocket or chamber that forms in the host's skin. In its warble, which has a small breathing hole open to the air, the larva feeds on a rich soup of tissue fluids produced by the host. There the larva passes through three more "instars," or developmental stages, growing larger all the while. At the end of the third instar, the larva wriggles out of its warble, falls to the ground, and burrows into the soil to pupate. Some weeks later an adult fly emerges from the soil to seek a mate, and the cycle is repeated. Because most egg porters are not picky about whose blood they sip, the larvae of Dermatobia hominis can end up on almost any warm-blooded animal--from a squirrel to a monkey to (as the name implies) a human being. Double Eek!

My fellow scientists on the island regaled me with dramatic tales of intensely painful bot fly larvae growing in inaccessible places, in disgusting places, in very private places. According to these battle-scarred veterans, the best way to get rid of a larva is to plaster a thick piece of bacon on your skin above the breathing hole of a larva's warble. In desperation (since bot fly larvae have to breathe), the larva crawls out of its warble and up into the bacon. Then you whip off the bacon with the larva trapped inside. Poor howler monkeys, up in the trees with no bacon, nor even with the manual dexterity to force larvae out of the warbles by hand--my heart went out to them!

I went on with my field study, and months went by. Thankfully, I acquired no bot fly larvae, and neither did anyone else on the island. In fact, none of the other monkey species on the island--capuchins, spider monkeys, tamarins--were infested with bot fly larvae either, even though during some months virtually every howler monkey I saw bore multiple warbles. The other biologists noticed the same thing. As it turned out, none of these scientist-raconteurs had ever gotten a Dermatobia larva on Barro Colorado Island; all their exciting stories were based on experiences elsewhere in the neotropics. Perhaps these larvae were not that same notorious pest after all.

A veterinarian friend in Panama named Nathan B. Gale, the director of the Veterinary Public Health Laboratory, took an interest in the problem. Sick or wounded wild animals were occasionally brought to his clinic for treatment, and when a howler monkey arrived one day, he removed its bot fly larvae, put them in a preservative, and mailed them to an entomologist friend at Washington State University in Pullman, the late E. Paul Catts. Catts recognized that they were larvae of an entirely different species, Alouattamyia baeri, the howler-monkey bot fly. That was a big surprise, but also a big relief: the reason only howler monkeys were afflicted with the larvae was that the bot fly is host-specific.

Catts had written an extensive review describing the members of Cuterebridae, the New World family to which both Dermatobia and Alouattamyia belong. From Catts's review it was clear that Dermatobia is a maverick. Other species in the family tend to associate closely with just one mammalian host, typically a rodent or rabbit. In general, they also place their eggs not on egg porters but rather in areas of habitat likely to be visited by the host. A rodent bot fly, for instance, might leave its eggs on grass or twigs near the trail of its specific rodent host. When the rodent passes by, the heat from its body alerts the larvae, which emerge instantly from their eggs and attach themselves to the animal's whiskers or fur.