by Matthew Chmielewski, Biology graduate student, Clark University
Photos courtesy of U.S. Centers for Disease Control and Prevention
Illustrations courtesy of the American Mosquito Control Association.
Mosquitoes have been a problem for people all over the world as long as humans have existed. This stems partly from the fact that the 3000 or so known species of mosquitoes have found places to live and roles to fill in almost every climate and ecosystem available. They have evolved many tricks over thousands of years to make them very good at what they do. These tricks include specialized mouthparts for blood feeding, incredible sensory perception, and most amazingly, the development from an aquatic life stage through to an airborne home.
A mosquito's life can be divided typically into 4 major life stages. These are the egg, the larva (plural= larvae), the pupa (plural= pupae), and the adult. Each of these stages has a body that is specially designed for the type of activities it will be doing. The ability for the mosquito to go from one to the other is an amazing transformation, comparably complicated to metamorphosis in butterflies. We will look at all the stages in depth, one by one to complete the life cycle of the typical mosquito.
Many species will lay their eggs on top of the water, either singly or in little raft shapes. The eggs and rafts can be shaped differently depending on the species of mosquito involved. Those laid singly are many times shaped like a small canoe, with rounded up ends and floats on the sides, to keep the egg on the surface. Those laid in rafts lie vertically and stick together due to surface attraction. The raft altogether becomes shaped like a wedge with upturned ends, like a canoe.
Other species, like the treehole breeder that the Labor Day Mosquito Count is focusing on, lay their eggs along the edges of water on the walls of the container. The eggs, which can endure dry conditions, can then sit around until rain fills the container more and they are submerged. At this point the eggs are in the water and they can hatch.
Eggs have been known to stand up to many harsh situations and survive through them. Some mosquito eggs are laid by adults that live in desert areas and can survive for months without any water. When the rain does come they are quick to hatch and grow up before the water pools formed in the wet season dry up again. Other eggs can withstand being frozen in cold and temperate lands. These eggs hatch when the weather becomes warm again in the late spring.
To prevent all the larvae from dying if something should happen to a pool of water, some eggs are laid in such a way that they are spread out. That way there is a better chance of some of each group of eggs surviving. This also makes it so that the larvae aren't forced into competing with their siblings. Some eggs that are laid in groups get around this problem by hatching at different times. If many eggs have already hatched, the remaining ones will wait until those larvae are gone until they begin to hatch again. Some of the work done in this lab has shown that the larvae must touch the eggs in order to prevent hatching, and that the eggs don't hatch when the larvae have removed bacteria from the eggs' shells.
2: LarvaLarvae are the most active aquatic form of the mosquito. They are a large part of the life cycle of the mosquito, and therefore are well adapted to their role. Since the adults can no longer grow, success as a larva will determine how large the adults can be. Adult size limits how many eggs can be produced, so things that change larval success will affect the birth rate in the next generation. This stage can be considered just as important, in a way, as the adult stage. While this is true, to humans, the adult is the worrisome stage and the larvae are many times overlooked.
Mosquito larvae are little long and hairy wormlike creatures. They of course don't have hair as monkeys, dogs, and humans do (all over our bodies), but instead have a more limited amount of spines that stick out all over the place. These tiny spines help the larvae feel around them and also to float in the water.
The ability to float is important because mosquito larvae need air just as we do. They have developed a specialized structure called a breathing siphon. This is kind of like a long straw sticking out of the mosquito that goes up to the surface of the water and breathes the air that is up there. Some mosquitoes have long breathing siphons, and others have shorter ones. Those with longer ones usually feed deeper in the water, while those with shorter siphons feed mainly at the surface.
Mosquito larvae are usually filter-feeders. They have fan-like mouth brushes that sweep water at their mouths from in front of them. In this method they filter out small particles of decaying material and microorganisms, which they then eat. Some larger larvae are known to eat other mosquito larvae. These larvae get lots of protein by eating other species, and even members of their own species. As these do not bite people when they turn into adults, and they control populations of biting mosquitoes, the predatory larvae have been called allies to humans in the fight against mosquitoes.
Larvae swim by wiggling their bodies back and forth. They do so to move up and down the water column to feed, and to avoid danger. When mosquito larvae sense a sudden change in light intensity they immediately swim towards the bottom of the water container they are in to avoid being eaten from up above. It is very easy to observe this when approaching any container containing larvae within.
Some larvae have a specialized breathing siphon that allows them to pierce the roots and stems of water plants to get air. These larvae use little saw-like structures to get at air pockets within the plants. They can then hook onto the plant to eat and breathe while totally submerged under the water.
The larva of a mosquito goes through four stages called instars. These are successively bigger stages that lead towards the pupa stage from the egg. These four instars are each categorized by the growth developments going on within the larvae that are progressing toward the adult stage. Each stage sets up more and more pockets of cells that will later grow and change to become parts of the adult body.
3: PupaAt the end of the fourth larval instar, the skin of the larva splits and the pupa comes out. It is shaped like a comma, has no mouth, and swims by a sort of kicking motion unlike the wriggling of the larva. It has two trumpet shaped breathing tubes on the top of its head rather than the one on its rear as in the larval stage. Like larvae, the pupae are also sensitive to light changes, swimming to the bottom if anything moves over the water.
If pupae become stranded due to the drying out of their water pool, they are still able to turn into mosquitoes, if they are not first eaten or exposed to too dry of conditions. They can also kick and bounce around until they come to a new pool of water, if there is one nearby.
If the conditions are right, it only takes a few days for the pupa to transform into an adult. The insides of the pupa change while it rests on the surface, parts breaking down while new parts form. After the change fully takes place, the adult is ready to break free. During the last few minutes the pupa loses the ability to swim around and can only float on the surface. Soon after the top part of the pupa splits open and the adult mosquito squeezes out of the pupal skin. It is able to do so because it is still soft and will harden within the next hour. The adult climbs out of the pupa and stands on the surface. In a few minutes it is able to make its first flight to a nearby tree or wall to rest while it finishes developing.
4: AdultThe adult mosquito is the form that most people fear. It is the form that makes those itchy bumps we get. It is the form that can spread disease. While the other forms are not very well known, the adult mosquito is a creature that is known to almost everyone in the entire world. It does, however have a range of activity that is probably not known to most people.
One of the first important things to cover is that only the females feed off of blood. The females use the blood to nourish and develop eggs. Otherwise both males and females feed on honey-dew, nectar from flowers, and juice from berries and fruit. It is important to note again that those mosquitoes that eat other larvae in their larval stage tend to not need any blood-meal to nourish eggs. These eating habits are a bit elusive, as nectar feeding usually happens early mornings and late evenings. In the lab, it is easy to show this need for a carbohydrate source by providing caged adults with a flask of sugar-water solution, made available to them with a paper towel or cotton wick, and depriving another group of the same. They need a carbohydrate meal every couple of days to survive.
To find a blood meal, however, is vital if a female is to lay eggs. Some females can get by without a blood meal for their first batch of eggs, even if they weren't predators in their larval stages, but to most it is essential. They have many biological "tricks" they employ when approaching their intended victim.
The most obvious one, of course, is vision, but this tends to be limited to short range location at best. Many mosquitoes approach their blood-hosts at night, so vision may be even less helpful then. The second trick is heat sense. Using the many receptors they have on their antennae, the mosquitoes are attracted to heat, which allows them to follow the movement trails of animals upwind of them. The mosquitoes turn and follow the warm air to their target. This is combined with smell also via the antennae receptors. The combination allows them to follow the warm, moist trail filled with scent that we leave behind as we move. Mosquitoes are attracted to carbon dioxide, as well as other chemicals such as lactic acid. If we are physically exercising, this can increase the chance of mosquitoes finding us. Lastly, mosquitoes can use sound vibrations as locators, but this really becomes most important for males.
For females to lay eggs, there needs to be both a male and a female. The female needs a blood meal, and the male needs to give his DNA to the female. The males find females using sound. Their antennae are all bristly with little hairs that act to receive sound. When a female flies by, they can hear her and locate exactly where she is by her sound. Once they meet each other the female can take the male's DNA and go lay her eggs. Quite often, she will already have some ready for laying before mating, but she can store sperm for fertilizing her eggs later. Sometimes the males will hang around in a large group, often times noticeable on late summer afternoons. These males dance around in a group getting all excited about meeting the females and then grab any that go by. It's sort of like a dance club where males all go to dance and get energized to go dating the female mosquitoes.
After the female has met the male and gotten the blood she needs, her eggs finish developing. She finds an appropriate spot on the edge of the water, in the water, or where water will be when it rains (in a very dry situation), depending on the species of mosquito. She adds the male DNA to each egg as she lays them. Once she finishes with this, she flies off and tries to store up some more energy, get another blood meal, and do the egg laying process over again. The second time she doesn't need any more males, she is set essentially for her life after the first time. The egg, on the other hand, begins the whole journey through all the life cycles, repeating the same processes that the mother just had to go through.