How do mosquitoes suck blood?

The nipping's done with a proboscis, a sort of springy with a hollow needle formed by interlocking mouthparts and an outer sheath that rides up when the needle slides through your skin and probes for blood. But hitting the sweet spot isn't so easy -- ask any intern drawing blood from a patient for the first time. Or ask Jose Ribeiro, a medical entomologist at the National Institutes of Allergy and Infectious Diseases, who will gladly give you the gory details. "Less than 5 percent of skin is blood vessel, so the mosquito has to fish. It casts its proboscis back and forth under your skin, sawing through tissue and probing an area ten seconds at a time." After several such "search castings" and no luck, the insect withdraws completely and tries another patch of skin. But if it gets a good probe into one of your small blood vessels, it freezes and sucks from the hemorrhage, pumping in little spitballs of vessel dilators and blood thinners to keep its meal running freely. (An allergic reaction to mosquito drool is what produces those itchy red lumps, if you've wondered.) A mosquito can suck two to three times its weight in blood, no trouble. That's tantamount to a 150-pound human vacuuming up 300 to 450 pounds of food.

At this point, stretch receptors in the mosquito's hugely bloated abdomen, sensing emminent blowout, initiate an urgent message to the brain, saying in effect, "Whoa there, skeeter -- pull out!" It's a pretty mechanical reflex, apparently. You can get mosquitoes to quit feeding by pumping them up -- how can I put this delicately? -- from the other end, with saltwater enemas or air. Marc Klowden, the entomologist at the University of Idaho who did these insufflating experiments, also has videos showing what happens when you prevent the signal from the abdominal receptors from reaching the brain: too much is never enough for these mosquitoes, and they eat until they explode.

Even under normal circumstances, once a mosquito has eaten its fill, "it's so heavy it can barely fly," according to Ribeiro. Stuporous and swollen as a blimp, it looks for a place to lie low and do what anyone would do after going on the mother of all drinking binges -- it excretes like crazy. After a few hours the mosquito has reduced its blood meal by half into a supernutritious slush.

Thankfully, at any given time, in any population, less than half the mosquitoes are biters. That's because, first, only females are hematophagous (Greek for "blood eaters"). Males are sweet nectarloving loving types, peacefully sipping at nature's juice bar. Second, most females feed on blood only when they need the extra protein to finish making their eggs; for routine tine fuel they'll use plant sugars, too. In fact, feeding on blood seems a pretty well orchestrated event, dangerous enough that females have built-in controls to switch it on and off -- they don't seek out victims more than they have to. Remind yourself of this the next time you're being eaten alive. Things could be worse.

Which brings us back to that all-consuming question -- or rather, that question that consumes some of us more than others. How do mosquitoes get wind of their prey, and why are certain people more preferable than others? You know who you are. You're the ones who make backyard barbecues safe -- well, tolerable -- for the rest of us by acting as five bait, the folks who get fed on right through their cotton T-shirts. You're the ones whose mothers dabbed lotion on bites and cheerfully consoled: "You just have sweet blood."

Entomologists are a blunter lot: they labor in the service of science, not in the warm, fuzzy business of sparing our inner child. Mosquitoes, they tell us, use various cues to find food -- color contrast and movement, skin temperature and humidity. But above all, experiments show they're olfactory creatures. Floral scents help steer them to their nectar meals. And breath and body vapors draw them to their animal hosts, including you and me. For mosquitoes our vapor trail is a no-brainer. It says: Fly this way. Make a right. Eat here. Just do it.

Each time you breathe out and blow off carbon dioxide, you're telling mosquitoes (and other nasty biting insects like ticks) that there's a vertebrate, a handy blood container, in the vicinity. Mosquitoes have [CO.cub.2] receptors on little feelers called palpi and can detect a plume of the gas from about 50 feet away. This is bad news: there's a world of bloodsuckers waiting for you to exhale and no way you can hold your breath forever Matters aren't helped, either, by another vertebrate emanation of ours, a volatile chemical called lactic acid, which mosquitoes pick up on their bristly antennae. Humans exude this compound from their hands, from their faces and shoulders -- in fact, from just about every pore of their bodies, in secretions like oil and sweat made by skin glands. Lactic acid escapes from our mouths too, whenever we go in for any heavy-duty exercise. As it happens, some of us are quite a bit mom effusive than others.