The mosquito's high-pitched whine, buzzing in ears around the globe, signals nights of irritated slapping and days of itching welts. Tiny and ubiquitous, these airborne vampires thrive virtually everywhere on the planet, including the Arctic tundra. They may be the Earth's most annoying animals. They are definitely the deadliest to humans.

Worldwide, mosquitoes are infamous as carriers of some widely lethal diseases including yellow fever, encephalitis, dengue fever and malaria. The World Health Organization estimates that mosquito-borne diseases infect up to 700 million people each year, killing more than 2 million. Malaria alone killed 1.1 million people in 1998 -- 100 times as many as have died since the beginning of the violence in Kosovo.

In the United States, where sophisticated medicine and mosquito-control efforts have made such diseases extremely rare, the insect's principal menace lies in its bite. Which isn't a bite at all.

In fact, it isn't even what mosquitoes normally do for a living. For their ordinary day-to-day existence, both males and females live on fruit and flower nectar. Males consume nothing else and never bite anybody.

Females, however, require a nutritious, protein-rich blood meal to produce viable eggs. So they come equipped with the complex blood-extraction equipment that is so familiar -- after the fact -- to Washingtonians in warm months.

A Bite to Remember

After tracking down a likely target, the mosquito lands gently and begins probing for a blood vessel with its proboscis, the long tube that contains the mouthparts. The proboscis is a marvel of evolution that makes the hypodermic needle look crude. On the outside is the labium, a gutter-shaped lower lip that protects the bundle of delicate mouthparts collectively called a fascicle.

The hungry female presses the labium against the victim, pushing the fascicle into the skin. Two mandibles act like long, microscopic scalpels, cutting through the skin with saw-like tips. A pair of tubes follows just behind the mandibles, gently probing for tiny blood vessels called capillaries.

When it finds a blood vessel, the mosquito dribbles saliva down into the tiny wound. The saliva contains hundreds of handy compounds, including anticoagulant proteins that prevent blood from clotting and other chemicals that dilate capillaries, increasing flow.

It is this saliva that causes everything humans hate about the tiny insects. The itchy red welts that mark the wound are allergic reactions to the foreign proteins, and the organisms that cause disease are squirted into the body along with the saliva.

The whole blood-sucking process is remarkably brief. Finding a blood vessel rarely takes more than 30 seconds. After that, speed becomes crucial. A slow mosquito is usually a dead one, since the human body can take as little as three seconds to react to the saliva injection.

Once the capillary is prepped, pumps behind the mosquito's head draw blood into the stomach, which in most species is reserved for the blood meal. (Nectar and other foods go into a separate crop.) The stomach balloons out, and masochistic backyard scientists can watch it turn crimson. The average draw is about .000004 liters, about one-thousandth of a teaspoon.

In less than two minutes, the mosquito can withdraw its own body weight in blood, pull its proboscis gently out and fly clumsily away. Weighted down by its meal, it goes into what Cyrus Lesser, chief of mosquito control for the Maryland Department of Agriculture, describes as a "controlled crash," looking for a protected spot where it can digest its meal safe from predators.

"They concentrate it," Lesser says. "It's almost like a dehydrated food: They take all the fluid out, and within a few hours they're back to literally the same size they were before."

Why You?

Exactly what attracts mosquitoes is a mystery, but scientists have discovered some of the basics. Body heat and carbon dioxide exhaled by living creatures can draw mosquitoes from as far away as 100 feet. So can the lactic acid and folic acid in your sweat. Mosquitoes that feed during the day may key in on movement and color (or heat, which tends to increase with darker colors). For bug avoidance, white clothing is good; red, brown and black are bad.

So to prevent mosquito bites, all you have to do is stand still, stop breathing, quit sweating and wrap yourself in a sheet. Not surprisingly, many people prefer to use repellents -- which don't exactly repel the creatures. The most familiar is DEET, or diethyl toluamide. Developed after World War II and released in 1954, it interferes with tiny sensory pits and hairs on the mosquito's antennae and body that it uses to sense carbon dioxide.

"Essentially, DEET blocks receptor sites on the mosquito and makes it blind to the host's presence," says Donald Barnard, a USDA researcher in Gainesville, Fla. "There's a lot of natural-product-based repellents on the market, but nothing's as effective as DEET."

"There are people on the Eastern Shore who tell you the biggest single use of mosquitoes is to keep the tourists out," Lesser offers with a laugh. "Large areas of the Eastern Shore have been protected from development because the mosquito and biting fly populations are so high." Actually, mosquitoes are flies. They belong to the two-winged order Diptera, or "true flies." Even their name means "little fly" in Spanish. But unlike most of their cousins, the mosquito's role in ecosystems is unclear. "Insectivorous birds and bats usually key in on bigger prey," Lesser says. "The things that most rely on mosquitoes would be viruses and the malaria disease-causing organisms that need mosquitoes to live." A rare orchid in the Midwest seems to rely on the mosquito for pollination, and their sheer numbers make them popular but not essential food for lots of bird species. Otherwise, go figure.

There are more than 3,000 species of mosquito on the planet, and about 170 in the United States. In the Washington area, the most common types are Aedes, Culex, Anopheles (whose cousins carry malaria) and Psorophora.

Buzz Off

On a regional basis, typical control measures include swamp drainage, chemical sprays to kill larvae, bacteria to sicken adults and placement of larvae-eating fish. (Gambusia, a species native to Maryland, is a popular control animal in that state and elsewhere.)

Around the house, you can cut your skeeter count dramatically by eliminating standing water in bird baths, potted plants, small pools, pet dishes and even the odd used tire. (A recent noxious immigrant, the Asian tiger mosquito, came to America via the international scrap-tire trade.) This may be particularly important if you have a dog: Aedes mosquitoes spread canine heartworm.

Andrew Curry, who last wrote for Horizon about Machiavelli, works at U.S. News and World Report.

Killer on the Wing

Most Americans think of mosquitoes as little more than an annoyance. But they are the world's most common disease carrier, or "vector," responsible for transmitting sickness from infected hosts to others. Microscopic parasites such as Plasmodium falciparum, which causes malaria, and the tiny wuchereria bancrofti worm responsible for elephantiasis, live and grow inside mosquitoes. Many viruses -- including those that cause yellow fever, dengue (aka "breakbone") fever and equine encephalitis -- also hitchhike on mosquitoes to move from host to host.

"Mosquitoes aren't just passive transmitters of these viruses and parasites," explains David Brandling-Bennett of the Pan-American Health Organization. "They have complex life cycles, and several stages of development are inside the mosquito. [The parasites] go through different forms and migrate to the salivary glands," where the mosquito injects them into a host. This active development and transmission process is why most scientists believe that mosquitoes cannot transmit the blood-borne AIDS virus, which is presumed to be digested along with the blood meal and thus never reaches the salivary glands.

All of this knowledge is fairly recent. A century ago, experts believed that malaria (from the Italian mala aria, or "bad air") was caused by breathing unwholesome, swampy air. Doctors guessed yellow fever was spread by the vomit or clothes of sick people.

Ignorance helped the spread of disease. Yellow fever, which was common in Africa, killed entire crews of white slave traders on their way to the Americas, leaving the resistant African slaves untouched and introducing the disease to the Caribbean in the 1600s. When France sent an army of 33,000 men to conquer Haiti and occupy the Mississippi Valley in 1802, 29,000 of them succumbed to yellow fever -- prompting Napoleon to sell the disease-ridden Louisiana Territory to the United States for a song in 1803.

It would take another century for scientists to grasp the connection between mosquitoes and tropical disease. In 1897, British scientist Ronald Ross, working at a remote hospital in India, dissected an Anopheles mosquito that had just fed on a malaria patient. In its stomach, he noticed the P. falciparum cells other scientists had spotted in the blood of malaria patients. That night he wrote a short poem, which ended: "I know this little thing/A myriad men will save/O Death, where is thy sting?/Thy victory, O Grave?"

Ross's discovery -- which won him a Nobel Prize in 1902 -- was fleshed out by U.S. Army doctor Walter Reed, who helped demonstrate that malaria and yellow fever pathogens both incubate and mature inside the mosquito. Their work inspired a second Army doctor, William Gorgas, to free Havana (in American hands after 1898's Spanish-American War) from the mosquito Aedes aegypti, which spread yellow fever.

Commanding a huge force of fumigators and building inspectors, Gorgas rid the city of standing-water breeding sites. Within months, yellow fever in the Cuban capital -- a hotbed of the disease for almost three centuries -- plummeted, dropping from 1,400 cases in 1900 to 37 in 1901. In 1904, Gorgas repeated his efforts in Panama, making possible the construction of the Panama Canal through more than 40 miles of dense tropical jungle.

Thereafter, optimistic scientists lavished attention on the tiny mosquito. When Ross made his discovery, "he assumed he had found the way to eliminate malaria," Brandling-Bennett says.

This logic prompted massive, world-wide mosquito eradication campaigns in the 1950s and '60s. The World Health Organization helped local governments use DDT and other insecticides to kill adult mosquitoes. One authority, noting the spectacular success of the intense spraying, wrote in 1955: "This is the DDT era of malariology. For the first time it is economically feasible for nations, however undeveloped and whatever the climate, to banish malaria completely from their borders."

The efforts succeeded in eradicating malaria in some areas, such as Europe and North America. But in most places mosquito populations were reduced but not eliminated. When interest in control programs declined and funds dried up, mosquitoes made a comeback. Liberal spraying also made many insect populations resistant to common insecticides. Today, authorities focus on selectively controlling populations, not eradicating them.

The Circle of Life

Few of us regard the mosquito as an aquatic creature. But all mosquito species need water to breed; and three of the insect's four life stages (observed and recorded at least as early as Aristotle 2,300 years ago) take place in or under water.

A stomach full of blood signals the female to start producing eggs. She mates just once, storing enough sperm in special pouches to fertilize several batches of eggs in her lifetime, laying as often as every three days. Each batch requires a new blood meal. (Some species -- notably the giant Toxorynchites -- do not suck blood at all, getting the protein they need by eating other larvae during early stages of development.)

Almost all species, including the locally popular Anopheles and Culex, lay their eggs directly in water, singly or stuck together in floating rafts. Others, such as Aedes and Psorophora, find a fairly dry spot that will soon be flooded, triggering the hatch.

Within a few days (depending on weather and species), the eggs hatch into larvae. They swim around, feeding on microorganisms and organic matter suspended in the water, but breathe through miniature snorkels that protrude above the surface. That's why spreading oil on ponds was once a common control technique.

As the larvae get bigger, they molt, or shed their skin, exactly four times. The fourth molt -- usually four days to a week after the larvae hatch -- produces the pupa, a quiescent stage in which the mosquito does not eat, but floats just below the surface, breathing through tubes. After a couple of days, the pupal skin splits and the adult emerges, pops to the surface, dries off and flies away.

Within hours, males are using their bushy antennae to listen for the distinctive whine of the female's wings, a signal that she is ready to mate. Males die soon after mating; females set off in search of blood. Some can fly several miles, and mosquitoes have been captured more than 100 miles out to sea.

Mosquitoes thrive at 80 degrees Fahrenheit, get sluggish at 60 degrees and zonk out entirely below 50, which is why you don't see many of them around here after October. They hibernate over the winter, but can pop up in suitably warm pools of melted snow.

"In the springtime or fall of the year, [female mosquitoes can live] over a month because it's cooler and damper," says Cyrus Lesser, chief of mosquito control for the Maryland Department of Agriculture. "The thing that really shortens a mosquito's life is dry, hot weather. They just lose moisture and literally dehydrate." Some species have evolved to survive in cold climates, surviving frozen winters by hibernating as adults or as eggs. The short Arctic summers then bring mosquitoes out all at once, in swarms seen in few other places. Scientists researching mosquitoes in northern latitudes have recorded nearly 300 bites per minute on an exposed forearm. Mosquito control staff here also use the forearm method to tally mosquito density.

CAPTION: When a mosquito lands, it presses the proboscis against the skin. The outer covering of the proboscis, the labium, bends back and the bundle-like fascicle pierces the flesh. When the fascicle locates a capillary, the mosquito dribbles saliva into the wound to keep the blood from clotting and increase its flow. (This graphic was not available)

CAPTION: The Circle of Life: Depending on species and temperature, it takes an average of three weeks for a mosquito to proceed through the four development stages from egg to adult. (This graphic was not available)