Mosquitoes have evolved and survived even thrived, Butler points out because of their ability to choose the best hosts for their blood meals, which they need to lay eggs. They find their hosts, initially, through a keen sense of smell. People can attract mosquitoes from 40 miles away simply by breathing. When a person exhales, their carbon dioxide and other odors mix to produce a plume that travels through the air. The plume acts like a dinner bell to mosquitoes, letting them know a juicy meal is within range.
Mosquitoes fly up the plume in zigzag fashion until they arrive, for example, at a backyard cookout.
Then they localize on eddies of other odors in the air using vision and heat sensing to make a selection. In order to determine the differences among individuals, Butler and McKenzie have been screening materials to determine whether they attract or repel mosquitoes. They built an olfactometer, a machine that measures mosquitoes' preferences for various odors, and connected it to a computer. Small discs of cattle blood and odoriferous gel are covered with a membrane to mimic skin, and then mosquitoes are released into the olfactometer. Butler and McKenzie can then analyze the data to determine which substances the mosquitoes found most attractive.