Three different ways of getting to the water
Mosquitoes lay eggs which hatch into larvae or "wrigglers" that live for a while in water, then emerge as new adult mosquitoes. Though mosquitoes live on land, they've evolved three different ways to get their eggs in the water, so the larvae can hatch where they need to be. The video shows a Culex mosquito laying her eggs to create an egg "raft" and, later, the larvae hatching from the eggs.
Mosquitoes of the genus Culex lay hundreds of eggs connected together to form a raft which floats on the surface of still water in a place like a pond, swimming pool, ditch, discarded tire or even a sewage cesspool. When the eggs hatch about a day later, the larvae wriggle free from the bottom of the eggs, swimming directly down into the water. They look like strange alien creatures, as you can see in the video.
To lay the eggs, the mosquito first lands on the surface of the water and takes a drink. Scientists think she's testing it, to make sure it's suitable for her larvae, and she may also be filling her abdomen so she can more easily push the eggs out through her oviduct. Though scientists aren't sure exactly what the mosquito is looking for, they know she likes water where other mosquitoes have already laid rafts and where other larvae are successfully living. When she is satisfied that she has found a good place, she deposits eggs one at a time, close together, so they stick into a raft of a hundred or more.
The more blood the female mosquito drinks, the more eggs she can lay. Scientists have found that for every milligram of human blood, she can lay an average of about 40 eggs. A milligram of blood is tiny, smaller than a grain of sand, so even one good bite can give a mosquito enough nourishment to lay hundreds of eggs at once. The eggs are so small, though, that a single raft of 200 to 300 eggs is only 1/4 inch long. Mosquitoes usually lay the eggs at night and the rafts are so inconspicuous, you may never notice them.
Mosquitoes of the genus Anopheles also lay a hundred or more eggs at once, but they lay them separately, rather than connecting together, though the eggs will sometimes drift together in a loose mat. Each of the scattered eggs rests on the surface of the water like a tiny boat, because it has a "float" attached on either side. This page from the CDC, which describes the life cycle of the malaria mosquito, has a good drawing of the eggs and their floats.
The floats are "air-filled chambers formed from the outer layer of the egg, the exochorion," according to Mosquitoes and Their Control by Norbert Becker. The eggs are so small you can barely see them without a magnifying glass. The photo on this page of an Anopheles mosquito laying her eggs shows the mosquito larger than life. You can imagine how small the eggs are compared to a real-life mosquito.
After two or three days, the floating eggs hatch and the wriggling larvae drop into the water. Both Anopheles and Culex mosquitoes have solved the problem of how to get their larvae into the water the same way, by finding a good pool or puddle of water and floating the eggs directly on top of it, but Aedes mosquitoes do it differently.
Waiting for Water
Mosquitoes of the genus Aedes lay their eggs on soil where the eggs wait like seeds, until spring rain washes them into shallow puddles or pools, or wet weather causes ponds to flood and rise to reach them. When conditions are too dry and cold, the eggs can lie dormant for months, hatching within hours or days when warm wet weather returns.
Aedes mosquitoes need to choose where to lay their eggs carefully. Scientists have determined that the moisture of the soil, the type of soil and the plants nearby all help them decide where to deposit their eggs, but they also choose locations near where other mosquitoes have laid their eggs, resulting in more eggs near the edges of pools which regularly rise and fall in rain and drought. This knowledge can help scientists develop better ways to control them, since they can encourage most mosquitoes in an area to lay in the same place, then use an insecticide that targets their larvae.
Since larvae may not survive if they hatch on an unseasonably warm winter day, too early in the spring, they may enter a condition called diapause. After a long cold period, they may require two weeks or more of warm temperatures before they'll hatch, or they may wait for longer spring days, to be sure warm weather has finally arrived for good.
Aedes mosquitoes don't need to survive the winter as adults, since their eggs can wait out the cold months and hatch only when conditions are right in spring. Other kinds, whose eggs float on the water and hatch quickly, must hibernate as adults.
Winter Eggs and Summer Eggs
There are a few kinds of mosquitoes, like Anopheles walkeri, which lay two kinds of eggs, ones that hatch quickly in the summer, and special "winter eggs" that they lay in the fall.
Scientists discovered that the winter eggs, which are larger than the summer ones, won't hatch unless they have been exposed to freezing temperatures, then warmed up again, to signal that winter has come and gone. The winter eggs appear larger than the summer ones, because they're protected by "enlarged floats that nearly cover the dorsal surface of the exochorion" or outer shell, according to an article by Wayne J. Crans of Rutgers University.
All these different ways help mosquitoes continue for another generation, but studying them helps people figure out new and better ways to prevent mosquitoes from multiplying.
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Public domain photos courtesy of the CDC.