We welcome a thunderstorm after sweltering days. The air clears, the temperature goes down. And, damn, the mosquitoes hatch.
Biology professors like to ask what animal kills the most people. Their poor students humiliate themselves by calling out grizzly bear, tiger, cobra, even hippo. The right answer, of course, is the female mosquito—no fur, no fangs, just a hypodermic needle on the wing. She’s less than a quarter-inch long, has six legs, and is the most efficient transmitter of disease in the animal kingdom. She uses scent to find us, attracted by the lactic acid and other ingredients in perspiration. She also senses the carbon dioxide in our exhalations and follows the slipstream back to our faces. The more you sweat and pant as you shoo her away, the more attractive you become.
She’s not revolting to look at, but elegant, small, sleek, long-legged, and fragile. We might be willing to give her a milliliter of blood, even with the itchy welt, if we didn’t worry about what she might give back. The worst of the many pathogens a mosquito may carry is malaria, which kills more than 1 million people a year, two-thirds of those in sub-Saharan Africa and most being children under 5.
There’s no sense trying to rehabilitate the reputation of such a creature; nobody loves a mosquito, and nobody loves a mosquito hugger. However it’s unfair to malign all 2,600 described species of mosquito when it’s just 80 or so—3 percent—that drink human blood. Among those 2,520 relatively blameless kinds of mosquitoes, there’s even one we’d like to see in greater numbers: Toxorhynchites, the mosquito that eats other mosquitoes. As larvae, the Toxorhynchites wrigglers eat their cousins, then turn on their siblings, and often keep attacking until only one is left. This drama takes place in small amounts of water in hollows in trees or similar small puddles. The tree-hole mosquitoes, including the disease-bearing Aedes, have adapted to using discarded tires as breeding grounds. As anyone who has tried knows, it’s very hard to drain water out of a tire.
Even the blood-consuming mosquitoes don’t need it for every meal. In fact they suck most of their energy from flowers and plants and are useful as pollinators. The male mosquito, innocent except for his role in producing more females, is happy to subsist on nothing but nectar and plant fluids. One kind of mosquito with no interest in us is the most important pollinator of the rather pretty blunt-leaved orchid that grows in bogs of boreal forests. Another kind pollinates the Monkey-Face orchid, an endangered Appalachian species.
Why can’t all mosquitoes be vegetarian? Eons ago, a primitive (perhaps far-sighted) mosquito may have mistaken a mammal for a plant and taken an accidental bite that led to a taste for blood. Now the females of those dangerous 80 species have evolved, like ticks, to use blood for producing eggs. The determined buzzing we hear outside (or inside) the tent is about the survival of the race. Mammalian blood carries a rich mix of protein, iron, fats, and sugar that triggers a mosquito’s ovaries. In as little as 90 seconds, she can take in as much as three times her weight in blood.
To accomplish this feat, the female mosquito makes use of her proboscis. The simple scissors of her midge ancestors distended and developed over the generations into an efficient skin piercer and blood imbiber. The proboscis has two tubes surrounded by pairs of cutting blades. When she lands to feed, the cutting edges glide against each other, like an electric carving knife, and split the skin. As she probes to nick a tiny blood vessel, her salivary tube pumps an anti-coagulant that prevents her narrow intake tube from clogging. The proteins in her saliva evoke an immune response—our swelling and itching. Any disease organisms she carries flow out through those salivary glands. In a diabolical evolutionary twist, the malaria-causing parasites that multiply in the gut of Anopheles interfere with the lobe that manufactures anti-coagulant. Their hostess then has to visit more victims to drink the same amount of blood, and the plasmodium parasite prospers.
The first mosquitoes were here more than 200 million years ago, probably sipping on the nectar of the new flowering plants or the blood of dinosaurs. (In the movie Jurassic Park, dino-DNA was extracted from a mosquito sealed in amber.) How delighted they must have been some 190 million years later when we came along, almost fur-free and comparatively soft-skinned. Lucy and her kin in East Africa almost certainly suffered fevers from mosquito-borne pathogens.
Then, as now, mosquitoes developed in standing water. And all too quickly: The blood-sucking insect can grow from an egg to an adult in just five days—and the eggs are plentiful. The malaria mosquito lays several hundred of them, one by one; other species drop rafts at a time. Their nursery-cum-swimming-pool can be as small as a discarded paper cup or a jar top, and it can be very polluted—pure sewage will do. A bristly mosquito larva, about 8 millimeters long, resembles an aquatic wirehaired dachshund or, if you prefer, a hairy maggot. Its head hangs down with its body suspended from the water surface by a breathing tube. As the snorkel takes in air, brushes filter the water for protozoans and bacteria.
A New Zealand mosquito has a mating habit that is the very definition of rapacious. Once the larvae have developed into comma-shaped pupae, adult males start to hang around, waiting for new females to hatch. As soon as one emerges, a male pushes in beside her and mates before her wings are dry enough to help her escape. A more common, and more voluntary, mosquito mating ritual has the males gathering in a cloud. The females choose to fly into the party.
Our living allies in mosquito control are primarily the fish that eat the larvae stage. Here we can give thanks for piranhas and the aptly named Western Mosquitofish. Dragonfly larvae eat mosquito larvae and dragonfly adults eat mosquito adults. Bats, for their part, have gotten more good press than they deserve. It turns out mosquitoes constitute less than 1 percent of the bat diet. Same goes for the purple martin, though these lovely swallows are nice to have around.
Even if it were possible for bats, birds, and pesticides to eradicate them all—which it isn’t—wholesale mosquito slaughter would not be a great idea. Their huge numbers of larvae feed small fish, and those small fish feed big fish, and those big fish are the primary source of protein for much of the developing world.
Naturally, we take an anthropocentric view of mosquitoes. They matter because they are Man’s Deadliest Foes. Maybe it’s worth thinking of life from the insect’s point of view. It’s no picnic being a female mosquito, for the three to six weeks she lives on this earth. Drinking blood isn’t easy; the longer the little phlebotomist has to poke for a blood vessel, the greater her chance of getting squashed. And after all, she didn’t choose to carry all those deadly parasites. Where does she pick them up? From us.
We’ve spent the past 50 years trying to find a malaria vaccine for humans, so we don’t give malaria to mosquitoes or contract it from them. It may make more sense to help the bugs out instead. We’ve recently sequenced the genomes of two of the most dangerous kinds of mosquitoes. Instead of using this knowledge to wipe them out more efficiently, what if we used it to strengthen their immune systems? We could live with the welt and the itch as long as we knew we weren’t going to get the fevers.