Let's say it's summer time. You're out in your backyard enjoying the sun and grilling your dinner. Ouch! You look down at your arm and see a painful, swelling mosquito bite. Moments later, you feel another one bite you. What are these pesky insects? Why do they bite? Do they carry diseases? What can you do to protect yourself?

Photo courtesy Centers for Disease Control & Prevention, photographer Jim Gathany
A female mosquito (Anopheles gambiae), feeding

In this edition of HowStuffWorks, we'll take a close-up look at mosquitoes -- how they breed, how they bite, what diseases they carry and what you can do to control them.

Up Close
Mosquitoes are insects that have been around for over 30 million years. And it seems that, during those millions of years, mosquitoes have been honing their skills so that they are now experts at finding people to bite. A mosquito has a battery of sensors designed to track their prey, including:

  • Chemical sensors - mosquitoes can sense carbon dioxide and lactic acid up to 100 feet (36 meters) away. Just about any mammal or bird gives off these gases as part of its normal breathing. Certain chemicals in sweat also seem to attract mosquitoes (people who don't sweat don't get nearly as many mosquito bites).
  • Visual sensors - if you are wearing clothing that contrasts with the background, and especially if you move while wearing that clothing, mosquitoes can see you and zero in on you. It's a good bet that anything moving is "alive", and therefore full of blood, so this is a good strategy.
  • Heat sensors - Mosquitoes can detect heat, so they can find warm-blooded mammals and birds very easily once they get close enough.
Little Fly
The word "mosquito" is Spanish for "little fly," and its use dates back to about 1583 in North America (Europeans referred to mosquitoes as "gnats"). Mosquitoes belong to the order Diptera, true flies. Mosquitoes are like flies in that they have two wings, but unlike flies, their wings have scales, their legs are long and the females have a long mouth part (proboscis) for piercing skin.
Something with this many sensors sounds more like a military aircraft than an insect. That's why mosquitoes are so good at finding and biting you! As we'll see later, one of the only ways to stop mosquitoes from finding you is to confuse their chemical receptors with something like DEET.

Like all insects, adult mosquitoes have three basic body parts:

  • Head - This is where all the sensors are, along with the biting apparatus. The head has two compound eyes, antennae to sense chemicals and the mouth parts called the palpus and the proboscis (only females have the proboscis, for biting).
  • Thorax - This segment is where the two wings and six legs attach. It contains the flight muscles, compound heart, some nerve cell ganglia and trachioles.
  • Abdomen - This segment contains the digestive and excretory organs.
So you have a sensor package, a motor package and a fuel processing package -- a perfect design!

The parts of a mosquito

There are over 2,700 species of mosquitoes in the world, and there are 13 mosquito genera (plural for "genus") that live in the United States. Of these genera, most mosquitoes belong to three:

  • Aedes - These are sometimes called "floodwater" mosquitoes because flooding is important for their eggs to hatch. Aedes mosquitoes have abdomens with pointed tips. They include such species as the yellow-fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus). They are strong fliers, capable of travelling great distances (up to 75 miles/121 km) from their breeding sites. They persistently bite mammals (especially humans), mainly at dawn and in the early evening. Their bites are painful.
  • Anopheles - These tend to breed in bodies of permanent fresh water. Anopheles mosquitoes also have abdomens with pointed tips. They include several species, such as the common malaria mosquito (Anopheles quadrimaculatus), that can spread malaria to humans.
  • Culex - These tend to breed in quiet, standing water. Culex mosquitoes have abdomens with blunt tips. They include several species such as the northern house mosquito (Culex pipiens). They are weak fliers and tend to live for only a few weeks during the summer months. They persistently bite (preferring birds over humans) and attack at dawn or after dusk. Their bite is painful.
Some mosquitoes, such as the cattail mosquito (Coquilettidia perturbans), are becoming more prevalent pests as humans invade their habitats.

Let's examine how mosquitoes live and breed.

Life Cycle and Breeding
Like all insects, mosquitoes hatch from eggs and go through several stages in their life cycle before becoming adults. The females lay their eggs in water and the larva and pupa stages live entirely in water. When the pupa change into adults, they leave the water and become free-flying land insects. The life cycle of a mosquito can vary from one to several weeks depending upon the species (the adult, mated females of some species can survive the winter in cool, damp places until spring, when they will lay their eggs and die.)

Photos courtesy Centers for Disease Control & Prevention and USDA/ARS (upper right)
Life cycle of the yellow-fever mosquito (Aedes aegypti):
An egg (upper left) laid on the surface of the water hatches into an aquatic larva (lower left). The larva changes into an aquatic pupa (lower right), which then changes into a free-flying adult (upper right). The adult female bites a human to gather blood for laying eggs.

All mosquitoes lay eggs in water, which can include large bodies of water, standing water (like swimming pools) or areas of collected standing water (like tree holes or gutters). Females lay their eggs on the surface of the water, except for Aedes mosquitoes, which lay their eggs above water in protected areas that eventually flood. The eggs can be laid singly or as a group that forms a floating raft of mosquito eggs (see Mosquito Life Cycle for a picture of an egg raft). Most eggs can survive the winter and hatch in the spring.

Identifying Larvae
You can distinguish the larvae of various mosquito species. Anopheles larvae lie parallel to the surface of the water, while larvae of Aedes and Culex extend down into the water (the air tubes of Culex are longer than those of Aedes).
The mosquito eggs hatch into larvae or "wigglers," which live at the surface of the water and breathe through an air tube or siphon. The larvae filter organic material through their mouth parts and grow to about 0.5 to 0.75 inches (1 to 2 cm) long; as they grow, they shed their skin (molt) several times. Mosquito larvae can swim and dive down from the surface when disturbed (see Mosquito Life Cycle for a Quicktime movie of free-swimming Asian tiger mosquito larvae). The larvae live anywhere from days to several weeks depending on the water temperature and mosquito species.

Photo courtesy Centers for Disease Control & Prevention
Aedes aegypti larvae extend into the water
and have short siphons.

Mosquitoes are Important
The mosquito larvae and pupae are important food sources for fish in aquatic ecosystems.
After the fourth molt, mosquito larvae change into pupae, or "tumblers," which live in the water anywhere from one to four days depending on the water temperature and species. The pupae float at the surface and breathe through two small tubes (trumpets). Although they do not eat, pupae are quite active (see Mosquito Life Cycle for a Quicktime movie of free-swimming Asian tiger mosquito pupae). At the end of the pupal stage, the pupae encase themselves and transform into adult mosquitoes.

Inside the pupal case, the pupa transforms into an adult. The adult uses air pressure to break the pupal case open, crawls to a protected area and rests while its external skeleton hardens, spreading its wings out to dry. Once this is complete, it can fly away and live on the land.

One of the first things that adult mosquitoes do is seek a mate, mate and then feed. Male mosquitoes have short mouth parts and feed on plant nectar. In contrast, female mosquitoes have a long proboscis that they use to bite animals and humans and feed on their blood (the blood provides proteins that the females need to lay eggs). After they feed, females lay their eggs (they need a blood meal each time they lay eggs). Females continue this cycle and live anywhere from many days to weeks (longer over the winter); males usually live only a few days after mating. The life cycles of mosquitoes vary with the species and environmental conditions.

Mosquito Bites
As mentioned before, only female mosquitoes bite. They are attracted by several things, including heat (infrared light), light, perspiration, body odor, lactic acid and carbon dioxide. The female lands on your skin and sticks her proboscis into you (the proboscis is very sharp and thin, so you may not feel it going in). Her saliva contains proteins (anticoagulants) that prevent your blood from clotting. She sucks your blood into her abdomen (about 5 microliters per serving for an Aedes aegypti mosquito).

Photo courtesy Centers for Disease Control & Prevention, photographer Jim Gathany
This female Anopheles gambiae mosquito is feeding. You can see the blood swelling her abdomen.

If she is disturbed, she will fly away. Otherwise, she will remain until she has a full abdomen. If you were to cut the sensory nerve to her abdomen, she would keep sucking until she burst.

After she has bitten you, some saliva remains in the wound. The proteins from the saliva evoke an immune response from your body. The area swells (the bump around the bite area is called a wheal), and you itch, a response provoked by the saliva. Eventually, the swelling goes away, but the itch remains until your immune cells break down the saliva proteins.

To treat mosquito bites, you should wash them with mild soap and water. Try to avoid scratching the bite area, even though it itches. Some anti-itch medicines such as Calamine lotion or over-the-counter cortisone creams may relieve the itching. Typically, you do not need to seek medical attention (unless you feel dizzy or nauseated, which may indicate a severe allergic reaction to the bite).

The human immunodeficiency virus (HIV) that causes AIDS cannot survive in a mosquito, and therefore cannot be transmitted from one person to another through mosquito bites.
Mosquitoes can carry many types of diseases that are caused by bacteria, parasites or viruses. These diseases include:

  • Malaria - Malaria is caused by a parasite that is transmitted by an Anopheles mosquito. The parasite grows in your bloodstream and can produce symptoms that develop anywhere from six to eight days to several months after infection.The symptoms include fever, chills, headaches, muscle aches and general malaise (similar to flu symptoms). Malaria is a severe disease that can be fatal, but can be treated with antimalarial drugs and prevented by vaccinations. Malaria is prevalent in tropical or sub-tropical climates.

  • Yellow Fever - Yellow fever does not occur in the United States, but is prevalent in Africa and is transmitted by the Aedes aegypti mosquito. Yellow fever produces symptoms similar to malaria, but also includes nausea, vomiting and jaundice. Like malaria, yellow fever can be fatal. There is no treatment for the disease itself, only the symptoms. Yellow fever can be controlled by vaccination and mosquito control.

  • Encephalitis - Encephalitis is caused by viruses that are transmitted by mosquitoes such as the Aedes mosquitoes or Culiseta mosquitoes. The symptoms of encephalitis include high fever, stiff neck, headache, confusion and laziness/sleepiness. There are several types of encephalitis that can be transmitted by mosquitos, including St. Louis, Western equine, Eastern equine, La Crosse and West Nile. West Nile encephalitis is on the rise in the eastern United States, which has raised concerns about mosquito control (see Overview of West Nile Virus for more information).

  • Dengue Fever - Dengue fever is transmitted by the Asian tiger mosquito, which is native to East Asia and was found in the United States in 1985. It is also transmitted by Aedes aegypti in the tropics. Dengue fever is caused by a virus that produces a range of illnesses, from viral flu to hemorrhagic fever. It is especially dangerous for children (see Dengue Fever & Dengue Hemorrhagic Fever for more information).
The best way to reduce mosquito-borne diseases is through mosquito control and personal protection.

Protection and Control
You can do a few things to reduce the number of mosquito bites that you get while enjoying the outdoors. First, wear clothing that covers most of your body, if temperatures permit. Second, use a mosquito repellant that contains NN-diethyl-meta-toluamide (DEET) at a concentration of 7.5 percent to 100 percent. Lower concentrations are sufficient for most outdoor protection, and a 15-percent concentration is recommended for children. Avon's Skin-So-Soft is a weak, short-lasting (less than 20 minutes) mosquito repellant. Permethrin, an effective pesticide, is for use on clothing only (Never apply it to your skin, it is a neurotoxin!). To learn more about mosquito repellents, see the EPA's How to Use Insect Repellents Safely.

Beyond mosquito repellents and clothing, you can try to control the mosquito population. Mosquitos need water to breed and will use any source of standing water.

Photos courtesy Centers for Disease Control & Prevention
Mosquitoes can breed anywhere that water collects, such as in buckets, old tires, rain barrels and cemetery urns.
(Click on each image to see a larger version.)

So, to reduce the mosquito population, eliminate sources of standing water in your yard. Empty watering cans, remove old tires and cover rain barrels.

Photos courtesy Centers for Disease Control & Prevention
By removing old tires and covering rain barrels, you reduce potential mosquito breeding sites.

If you have a lily pond in your garden, stock it with some fish that will eat the mosquito larvae. Some petroleum oils can be added to water to form a thin surface layer that suffocates the mosquito eggs; however, many of these oils will also suffocate any fish living in the water.

To prevent mosquitoes from entering your home, make sure that all of your window screens are intact.

Finally, there are many commercial pesticides available to kill mosquito larvae and mosquito adults. Many communities conduct large-scale spraying of pesticides containing mallothione to control mosquito populations during the spring and summer, especially in attempts to reduce the spread of West Nile encephalitis.

Photo courtesy Centers for Disease Control & Prevention
A municipal worker in Miami, FL, sprays ditches with insecticides to kill mosquito larvae.

Several natural or man-made products have been touted as mosquito repellents or effective in mosquito control. Citronella oil, which is a product of several types of plants that can be made into candles or burned directly, is an effective mosquito repellant in high concentrations, but individual citronella-producing plants do not make enough oil to effectively repel mosquitoes. Ultraviolet lights (as used in bug zappers) and ultrasonic devices are not effective. Also, mosquitoes are not a significant portion of the diets of Purple Martin Birds or bats.

For more information on mosquitoes and what you can do about them, see the links on the next page.

Lots More Information!

Related HowStuffWorks Articles

More Great Links

Florida Medical Entomology Laboratory