The Very Simple Psychology of a Very Nasty Pest
Just as the dragonflies began to cut down the appalling hordes of mosquitoes encouraged by the Interior's record snowpack, the magazine Natural History came out with a whole issue devoted to the buzzing bloodsuckers. This, I thought, was good timing; a couple of weeks ago, I couldn't have contemplated the subject rationally, but now I could learn a thing or two.
Among the items I'd never considered was how mosquitoes decide to bite. It's not a thoughtful process; as I learned, a mosquito brain is about the size of the period concluding this sentence---pretty inadequate equipment for making decisions. Instead, they operate on physics and chemistry.
Chemical traces in the air tell a hungry adult female mosquito that an appropriate victim is in the vicinity. If the intended prey is wearing an effective repellent, the sensors on her antennae may be confused, but animals do exude many chemical clues---ask any bloodhound. And a mosquito is a determined seeker. She needs protein for her eggs to mature, and vertebrates (such as people) seem to her like nice big bags of nutritious protein that leak blood when pricked.
The pricking is a complex business, involving a flexible tube made up of six highly specialized mouthparts. Two of them are serrated at the tip and saw back and forth to cut tissue. The tube has extremely sensitive chemical sensors to detect molecules in blood so the mosquito knows when she's hit a pay streak, so to speak. (Drawing blood may seem easy to us, but since only one or two percent of human skin by volume contains blood, it's more challenging to a tiny insect.)
The itching and swelling a mosquito bite offers isn't from the sawing; it's from our own body chemistry reacting to proteins in the mosquito's saliva, which serves an an anticoagulant. She injects it continuously while probing for or drinking blood. The injected saliva is what transmits mosquito-borne diseases. (Grim but true: over the ages, millions of people have been sickened by mosquito drool.)
A successful mosquito will suck up about one-millionth of a gallon of blood. That doesn't sound like much, but it may amount to four times her original weight. Her abdomen distends enormously from the bulk of this huge meal. Stretch receptors in her bulging abdomen send full-tank signals to her dot-sized brain, which tells her to disengage and get away. This is a physical mechanism, and simply reflexive. Determined and possibly vengeful scientists observed that when the connection between their stretch receptors and brain is cut, mosquitoes would feed until they burst.
A well-fed mosquito lies low for the two days or so it takes her eggs to mature. This decision may seem wise---why risk death by swatting when you've already won the vital protein? But again, it's the wisdom of the evolutionary ages operating through chemistry. While her eggs are maturing, the mosquito's system produces a hormone that turns off the chemical sensors in her antennae. She won't be tempted to fly to another victim because she can't tell one is near.
However, once she lays that batch of eggs, the hormonal switch goes back to seek-and-suck, and she'll return to buzzing around the bedroom, saliva and saws at the ready. During her month-long adulthood, she can nail someone a dozen times---assuming the human swat reflex doesn't get her first.
Many of the authors in the mosquito issue seemed to have a sneaking respect for their subjects: "Consider the outcome if you were to approach an elephant with a syringe," wrote one. I couldn't go that far. I'm cheering for the dragonflies. May they live long and eat well.
