Natural Acts: A Sidelong View of Science and Nature - David Quammen (1996)


Sympathy for the Devil

UNDENIABLY THEY HAVE A LOT TO ANSWER FOR: malaria, yellow fever, dengue, encephalitis, and the ominous tiny whine that begins homing around your ear just after you’ve gotten comfortable in the sleeping bag. All these griefs and others are the handiwork of that perfidious family of biting flies known as the Culicidae—the mosquitoes. They assist in the murder of millions of humans each year, carry ghastly illness to millions more, and drive not a few of the rest of us temporarily insane. They are out for blood.

Mosquitoes have been around for 50 million years, which has given them time to figure all the angles. Judged either by sheer numbers or by the scope of their worldwide distribution or by their resistance to enemies and natural catastrophe, they are one of the great success stories on the planet. They come in 2,700 different species. They inhabit almost every land surface, from Arctic tundra to downtown London to equatorial Brazil, from the Sahara to the Himalayas, though best of all they like tropical rainforests, where three quarters of their species reside. Mosquitoes and rain-forests, in fact, go together like gigolos and bridge tournaments, pickpockets and camel markets, insurance salesmen and…But wait, I was talking about insects.

They hatch and grow to maturity in water, any entrapment of quiet water, however transient or funky. A soggy latrine, for instance, suits them fine. The still edge of a crystalline stream is fine. In the flooded footprint of an elephant, you might find a hundred larval mosquitoes. At that stage of life, as inoffensive juveniles, they use facial bristles resembling cranberry rakes to comb such waters for planktonic food; but on attaining adulthood, they are out for blood.

Blood: It isn’t a necessity for individual survival, just a dietary prerequisite of motherhood. Male mosquitoes do not even bite. A guy mosquito lives his short, gentle adult life content, like a swallowtail butterfly, to sip nectar from flowers. As with black widow spiders and mantids, it is only the female that is fearsome. Make of that what larger lessons you dare.

She relies on the blood of vertebrates—mainly warm-blooded ones but also sometimes reptiles and frogs—to finance, metabolically, the development of her eggs.

A female mosquito in a full lifetime will lay about ten separate batches of eggs, roughly 200 in a batch. That’s a large order of ovular mass to be manufactured in one wispy body, and to manage it the female needs a rich source of protein; the sugary juice of flowers will deliver quick energy to wing muscles, but it won’t help her build 2,000 new bodies. So she has evolved a hypodermic proboscis and learned how to steal protein in one of its richest forms, hemoglobin. Among some mosquito species, the female’s first brood will develop before she has tasted blood, but after that she too must have a bellyful for each set of eggs coming to term.

When she drinks, she drinks deeply: The average blood meal amounts to two and a half times the original weight of the insect. Picture Audrey Hepburn sitting down to a steak dinner, getting up from the table weighing 350 pounds, and then flying away. In the Canadian Arctic, where species of the genus Aedes emerge in savage, sky-darkening swarms like nothing seen even in the Amazon and work under pressure of time because of the short summer season, an unprotected human could be bitten 9,000 times per minute. At that rate, a large man would lose half his total blood in two hours. Arctic hares and reindeer move to higher ground or die. And sometimes solid mats of Aedes will continue sucking the cool blood from a carcass.

Evidently the female tracks her way to a blood donor by flying upwind toward a source of warmer air, or toward air that is both warm and moist or that contains an excess of carbon dioxide, or a combination of all three. The experts aren’t sure. Perspiration, involving both higher skin temperature and released moisture, is one good way to attract her attention. In certain villages of Italy there was a folkish belief that to sleep with a pig in the bedroom was to protect oneself from malaria, presumably because the pig, operating at a higher body temperature, would be preferred by mosquitoes. And at the turn of the twentieth century, Professor Giovanni Grassi, then Italy’s foremost zoologist, pointed out that garrulous people seemed to be bitten more than those who kept their mouths shut. The experts aren’t sure, but the Italians are full of ideas.

Guided by CO2 or idle chatter or distaste for pork or whatever, a female mosquito lands on the earlobe of a human, drives her proboscis (actually a thin bundle of tools that includes two tubular stylets for carrying fluid and four serrated ones for cutting) through the skin, and gropes with it until she taps a capillary, and then an elaborate interaction begins. Her saliva flows down one tube into the wound, retarding coagulation of the spilled blood and provoking an allergic reaction that will later be symptomized by itching. A suction pump in her head draws blood up the other tube, a valve closes, another pump pulls the blood back into her gut. And that alternate pumping and valving continues quickly for three orgiastic minutes, until her abdomen is stretched full like a great bloody balloon or a fast human hand ends her maternal career, whichever comes first.

But in the meantime, if she is an individual of the species Anopheles gambiae in Gabon, the protozoa that cause malaria may be streaming into the wound with her saliva, heading immediately off to set up bivouac in the human’s liver. Or if she is Aedes aegypti in Peru, she may be drooling out an advance phalanx of the yellow fever virus. If she is Culex pipiens in Malaysia, long tiny larvae of filarial worms may be squirting from her snout like a stage magician’s spring-work snakes, dispersing to breed in the unfortunate person’s lymph nodes and eventually clog them, causing elephantiasis.

No wonder, then, that in the inverted rogue’s pantheon of those select creatures not only noxious in their essential character but furthermore lacking any imaginable forgiving graces, the Culicidae are generally ranked below even the deer tick, the lake leech, the botfly, the wolverine, and the black toy poodle. The mosquito, says common bias—and on this the experts tend to agree—is an unmitigated and irredeemable pest.

But I don’t see it that way. To begin with, the Culicidae family is not monolithic, and it does have—even from the human perspective—its beneficent representatives. In northern Canada, for instance, Aedes nigripes is an important pollinator of arctic orchids. In Ethiopia, Toxorhynchites brevipalpis as a larva preys voraciously on the larvae of other mosquitoes, malaria carriers, and then metamorphoses into a lovely, huge, iridescent adult that, male or female, drinks only plant juices and would not dream of biting a human.

But even discounting these innocent aberrations, and judging it only by its most notorious infamies, the mosquito is taking a bad rap. It has been victimized, I submit to you, by a strong case of anthropocentric bias. In fact, the little sucker can be viewed, with only a small bit of squinting, as one of the great ecological heroes of planet Earth. If you consider rainforest preservation.

The chief point of blame, with mosquitoes, happens also to be the chief point of merit: They make tropical rainforests, for humans, virtually uninhabitable.

Tropical rainforest constitutes by far the world’s richest and most complex category of terrestrial ecosystem, a boggling entanglement of life-forms and habits and physical conditions and relationships. Those equatorial forests—mainly confined to the Amazon, the Congo basin and its neighboring Central African drainages, the wetter and warmer parts of Indonesia and northern Australia, and parts of mainland Southeast Asia—account for only a small fraction of Earth’s surface but serve as home for an inordinate share of our planet’s total plant and animal species, including about 2,000 kinds of mosquito. But rainforests lately—in case you’ve been stuck in an elevator for twenty years and haven’t heard—are under siege.

They are being clear-cut for cattle ranching, nibbled away for subsistence agriculture, mowed down with bulldozers and pulped for paper, hacked and dried for firewood, milled into chopsticks and cheap plywood, gobbled up hourly for the sake of “development” in all its ambivalent forms. The current rate of loss, by one rough estimate, amounts to eight acres of rainforest gone, poof, since you began reading this sentence. Within a few generations, at that pace, the Amazon will look like New Jersey. Conservation groups are raising a clamor, tossing money at the problem, and making efforts to offer mitigating alternatives, while some governments in the countries at issue take steps for marginal preservation in the form of reserves or national parks. But no one and no thing has done more to delay this catastrophe, over the past 10,000 years, than the mosquito.

The great episode of ecological disequilibrium that we call human history began, so the fossils tell us, in equatorial Africa. Then quickly the focus of intensity shifted elsewhere. What deterred mankind, at least to a large degree, for a very long time, from hacking space for our farms and cities out of the tropical forests? Yellow fever did, and malaria, dengue, filariasis, o-nyong-nyong fever.

Clear the vegetation from the brink of a jungle waterhole, move in with tents and cattle and Jeeps, and Anopheles gambiae, not normally native there, will arrive within a month, bringing malaria. Cut the tall timber from five acres of rainforest, and species of viral-transmitting Aedes—which would otherwise live out their lives in the high forest canopy, passing yellow fever between monkeys—will fall on you and begin biting before your chainsaw has cooled. Nurturing not only more species of snake and bird than anywhere else on Earth, but also more forms of disease-causing microbe and more mosquitoes to carry them, tropical forests are elaborately booby-trapped against disruption.

The resident forest peoples, living at low densities, gradually acquired some immunity to these diseases, and their hunting-and-gathering economies, grounded in relatively simple technology, minimized their exposure to mosquitoes that favored the canopy or disturbed landscape. Meanwhile the occasional white interlopers, the agents of empire, remained vulnerable. West Africa in high colonial days became known as “the white man’s grave.”

So as Europe was being stripped of its virgin woods, and India and China, and the North American heartland, the tropical rainforests largely escaped, lasting into the late twentieth century—with some chance, at least, that they may endure a bit longer. Thanks to what? To a concatenation of accidental and deterministic factors, no doubt, among which should be included this: 10 million generations of jungle-loving, disease-bearing, bloodsucking mosquitoes—the Culicidae, nature’s Vietcong.