Knowing something about the lifecycle of a mosquito can empower you to prevent them from reproducing in your backyard. A mosquito goes through "complete" metamorphosis, which means it is an egg, larva, pupa, and finally an adult. Depending on the species, climate, season, availability of blood, and temperature, a mosquito will take anywhere from a week to a month to complete its metamorphosis.
Every mosquito starts out as one tiny egg amongst hundreds that comprise a "raft" of floating ova. After a large blood meal, the adult female lays this batch of eggs in stagnant water. Appropriate water sources could be a sewer, ditch, puddle, birdbath, wheelbarrow, rain gutter, untreated swimming pool, water fountain, or even ocean tide pools. The white eggs incubate for 1-6 days before hatching into squirming larvae.
Larvae look like thin worms that twitch constantly just below the surface of the water. They enjoy the light, heat, and air close to the surface. Now the mosquitoes eat bits of food such as rotting leaves, animals, or sewage to give them strength to molt four times. Each time they molt, they shed their small, tight layer of skin and emerge slightly larger.
Finally, after another week, the larvae become pupae with their fourth molting. Pupae have larger heads and a small tail, so they're nicknamed tumblers. They also stay underwater, yet breathe air. Small tubules, called siphons, reach from their lungs to break the surface of the water like miniature snorkels. They develop wings without eating any more food.
A few days later, the larvae swim to the top of the water and balance on their feet to stay in open air. Here, they dry out their wings and harden their exoskeleton in preparation for flight. They're mature adults. They'll live one to three more weeks, completely above water. Females live 2-3 times longer than males because they must eat more blood in order to lay eggs.
Female mosquitoes suck the blood of humans, cows, sheep, horses, birds, and even amphibians in order to build up enough nutrients to lay eggs. Males, though, only drink plant nectar. The female can lay several batches of eggs, 10-14 days apart, before she dies. Then the cycle repeats itself.