The story of the mosquito.

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.