There are approximately 2,500 mosquito species in the world, but a mere fraction of them feed on human blood. Of this fraction, only the females are vampires, as they require blood to nourish their eggs. When she's ready to lay these eggs, which usually number in the hundreds, the female typically does so on a small, still body of water. In some mosquito species, she creates little rafts for the eggs. They float until they hatch as tiny larvae a few days later. Like butterflies, they eventually turn into pupae, which ultimately metamorphose into the insects we know.