Mosquito (from the Spanish and Portuguese word for little fly) is a common insect in the family Culicidae (from the Latin culex meaning midge or gnat). Mosquitoes resemble crane flies (family Tipulidae) and chironomid flies (family Chironomidae), with which they are sometimes confused by the casual observer.

Mosquitoes go through four stages in their life-cycle: egg, larva, pupa, and adult or imago. Adult females lay their eggs in standing water, which can be a salt-marsh, a lake, a puddle, a natural reservoir on a plant, or an artificial water container such as a plastic bucket. The first three stages are aquatic and last 5–14 days, depending on the species and the ambient temperature; eggs hatch to become larvae, then pupae. The adult mosquito emerges from the pupa as it floats at the water surface. Adults live for 4–8 weeks.

Mosquitoes have mouthparts that are adapted for piercing the skin of plants and animals. While males typically feed on nectar and plant juices, the female needs to obtain nutrients from a "blood meal" before she can produce eggs.

There are about 3,500 species of mosquitoes found throughout the world. In some species of mosquito, the females feed on humans, and are therefore vectors for a number of infectious diseases affecting millions of people per year. Some scientists believe that eradicating mosquitoes would not have serious consequences for any ecosystems.

Mosquito larvae have a well-developed head with mouth brushes used for feeding, a large thorax with no legs and a segmented abdomen.

Larvae breathe through spiracles located on the eighth abdominal segment, or through a siphon, and therefore must come to the surface frequently. The larvae spend most of their time feeding on algae, bacteria, and other micro-organisms in the surface microlayer. They dive below the surface only when disturbed. Larvae swim either through propulsion with the mouth brushes, or by jerky movements of the entire body, giving them the common name of "wigglers" or "wrigglers".

Larvae develop through four stages, or instars, after which they metamorphose into pupae. At the end of each instar, the larvae molt, shedding their skin to allow for further growth.

The pupa is comma-shaped, as in Anopheles when viewed from the side, and is commonly called a "tumbler". The head and thorax are merged into a cephalothorax with the abdomen circling around underneath. As with the larvae, pupae must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on the cephalothorax. However, pupae do not feed during this stage. After a few days, the pupa rises to the water surface, the dorsal surface of the cephalothorax splits and the adult mosquito emerges. The pupa is less active than larva.

The duration from egg to adult varies among species and is strongly influenced by ambient temperature. Mosquitoes can develop from egg to adult in as little as five days, but usually take 40 – 42 days in tropical conditions. The variation of the body size in adult mosquitoes depends on the density of the larval population and food supply within the breeding water. Adult flying mosquitoes frequently rest in a tunnel that they build right below the roots of the grass.

Adult mosquitoes usually mate within a few days after emerging from the pupal stage. In most species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate.

Males live for about a week, feeding on nectar and other sources of sugar. After obtaining a full blood meal, the female will rest for a few days while the blood is digested and eggs are developed. This process depends on the temperature but usually takes 2–3 days in tropical conditions. Once the eggs are fully developed, the female lays them and resumes host seeking. The females are the only ones who actually use blood as a meal, but they feed on nectar as well.

The cycle repeats itself until the female dies. While females can live longer than a month in captivity, most do not live longer than 1–2 weeks in nature. Their lifespan depends on temperature, humidity, and also their ability to successfully obtain a blood meal while avoiding host defenses and predators.

Length of the adult varies but is rarely greater than 16 mm (0.6 in), and weight up to 2.5 milligrams (0.04 grains). All mosquitoes have slender bodies with three sections: head, thorax and abdomen.

The head is specialized for acquiring sensory information and for feeding. The head contains the eyes and a pair of long, many-segmented antennae. The antennae are important for detecting host odors as well as odors of breeding sites where females lay eggs. In all mosquito species, the antennae of the males in comparison to the females are noticeably bushier and contain auditory receptors to detect the characteristic whine of the female. The compound eyes are distinctly separated from one another. Their larvae only possess a pit-eye ocellus. The compound eyes of adults develop in a separate region of the head.^[12] New ommatidia are added in semicircular rows at the rear of the eye; during the first phase of growth, this leads to individual ommatidia being square, but later in development they become hexagonal. The hexagonal pattern will only become visible when the carapace of the stage with square eyes is molted.^[12] The head also has an elongated, forward-projecting "stinger-like" proboscis used for feeding, and two sensory palps. The maxillary palps of the males are longer than their proboscis whereas the females’ maxillary palps are much shorter. (This is typical for representatives of subfamilies.) As with many members of the mosquito family, the female is equipped with an elongated proboscis that she uses to collect blood to feed her eggs.

The thorax is specialized for locomotion. Three pairs of legs and a pair of wings are attached to the thorax. The insect wing is an outgrowth of the exoskeleton. The Anopheles mosquito can fly for up to four hours continuously at 1 to 2 kilometres per hour (0.6–1 mph) travelling up to 12 km (7.5 mi) in a night.

The abdomen is specialized for food digestion and egg development. This segmented body part expands considerably when a female takes a blood meal. The blood is digested over time serving as a source of protein for the production of eggs, which gradually fill the abdomen.

With regard to host location, female mosquitoes hunt their blood host by detecting organic substances such as carbon dioxide (CO₂) and 1-octen-3-ol produced from the host and through optical recognition. Mosquitoes prefer some people over others. The preferential victim's sweat simply smells better than others because of the proportions of the carbon dioxide, octenol and other compounds that make up body odor.^[14] The powerful semiochemical that triggers the mosquito's keen sense of smell is nonanal. A large part of the mosquito’s sense of smell, or olfactory system, is devoted to sniffing out blood sources. Of 72 types of odor receptor on its antennae, at least 27 are tuned to detect chemicals found in perspiration.^[16] In Aedes the search for a host takes place in two phases. First, the mosquito exhibits a nonspecific searching behavior until the perception of host stimulants then it follows a targeted approach.

Most mosquito species are crepuscular (dawn or dusk) feeders. During the heat of the day most mosquitoes rest in a cool place and wait for the evenings, although they may still bite if disturbed. Some species, like Asian tiger mosquito, are known to fly and feed during daytime.

Prior to and during blood feeding, they inject saliva into the bodies of their source(s) of blood. This saliva serves as an anticoagulant: without it, the female mosquito's proboscis would quickly become clogged with blood clots.

Mosquitoes of the genus Toxorhynchites never drink blood. This genus includes the largest extant mosquitoes, the larvae of which prey on the larvae of other mosquitoes. These mosquito eaters have been used in the past as mosquito control agents, with varying success.

In order for the mosquito to obtain a blood meal it must circumvent the vertebrate physiological responses. The mosquito, as with all blood-feeding arthropods, has mechanisms to effectively block the hemostasis system with their saliva, which contains a mixture of secreted proteins. Mosquito saliva negatively affects vascular constriction, blood clotting, platelet aggregation, angiogenesis and immunity and creates inflammation. Universally, hematophagous arthropod saliva contains at least one anticlotting, one anti-platelet, and one vasodilatory substance. Mosquito saliva also contains enzymes that aid in sugar feeding and antimicrobial agents to control bacterial growth in the sugar meal. The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins. Despite the great strides in knowledge of these molecules and their role in bloodfeeding achieved recently, scientists still cannot ascribe functions to more than half of the molecules found in arthropod saliva. One promising application is the development of anti-clotting drugs based on saliva molecules, which might be useful for approaching heart-related disease, because they are more user-friendly blood clotting inhibitors and capillary dilators.

It is now well recognized that the feeding ticks, sandflies, and, more recently, mosquitoes have an ability to modulate the immune response of the animals (hosts) they feed on. The presence of this activity in vector saliva is a reflection of the inherent overlapping and interconnected nature of the host hemostatic and inflammatory/immunological responses and the intrinsic need to prevent these host defenses from disrupting successful feeding. The mechanism for mosquito saliva-induced alteration of the host immune response is unclear, but the data has become increasingly convincing that such an effect occurs. Early work described a factor in saliva that directly suppresses TNF-α release, but not antigen-induced histamine secretion, from activated mast cells. Experiments by Cross et al. (1994) demonstrated that the inclusion of Ae. aegypti mosquito saliva into naïve cultures led to a suppression of interleukin (IL)-2 and IFN-γ production, while the cytokines IL-4 and IL-5 are unaffected by mosquito saliva. Cellular proliferation in response to IL-2 is clearly reduced by prior treatment of cells with SGE. Correspondingly, activated splenocytes isolated from mice fed upon by either Ae. aegypti or Cx. pipiens mosquitoes produce markedly higher levels of IL-4 and IL-10 concurrent with suppressed IFN-γ production. Unexpectedly, this shift in cytokine expression is observed in splenocytes up to 10 days after mosquito exposure, suggesting that natural feeding of mosquitoes can have a profound, enduring, and systemic effect on the immune response.

T cell populations are decidedly susceptible to the suppressive effect of mosquito saliva, showing enhanced mortality and decreased division rates. Parallel work by Wasserman et al. (2004) demonstrated that T- and B-cell proliferation was inhibited in a dose dependent manner with concentrations as low as 1/7 of the saliva in a single mosquito. Depinay et al. (2005) observed a suppression of antibody-specific T cell responses mediated by mosquito saliva and dependent on mast cells and IL-10 expression.

A recent study suggests that mosquito saliva can also decrease expression of interferon−α/β during early mosquito-borne virus infection. The contribution of type I interferons (IFN) in recovery from infection with viruses has been demonstrated in vivo by the therapeutic and prophylactic effects of administration of IFN-inducers or IFN, and recent research suggests that mosquito saliva exacerbates West Nile virus infection, as well as other mosquito-transmitted viruses.

Two important events in the life of female mosquitoes are egg development and blood digestion. After taking a blood meal the midgut of the female synthesizes proteolytic enzymes that hydrolyze the blood proteins into free amino acids. These are used as building blocks for the synthesis of egg yolk proteins.

In the mosquito Anopheles stephensi Liston, trypsin activity is restricted entirely to the posterior midgut lumen. No trypsin activity occurs before the blood meal, but activity increases continuously up to 30 hours after feeding, and subsequently returns to baseline levels by 60 hours. Aminopeptidase is active in the anterior and posterior midgut regions before and after feeding. In the whole midgut, activity rises from a baseline of approximately 3 enzyme units (EU) per midgut to a maximum of 12 EU at 30 hours after the blood meal, subsequently falling to baseline levels by 60 hours. A similar cycle of activity occurs in the posterior midgut and posterior midgut lumen, whereas aminopeptidase in the posterior midgut epithelium decreases in activity during digestion. Aminopeptidase in the anterior midgut is maintained at a constant low level, showing no significant variation with time after feeding. alpha-glucosidase is active in anterior and posterior midguts before and at all times after feeding. In whole midgut homogenates, alpha-glucosidase activity increases slowly up to 18 hours after the blood meal, then rises rapidly to a maximum at 30 hours after the blood meal, whereas the subsequent decline in activity is less predictable. All posterior midgut activity is restricted to the posterior midgut lumen. Depending upon the time after feeding, greater than 25% of the total midgut activity of alpha-glucosidase is located in the anterior midgut. After blood meal ingestion, proteases are active only in the posterior midgut. Trypsin is the major primary hydrolytic protease and is secreted into the posterior midgut lumen without activation in the posterior midgut epithelium. Aminopeptidase activity is also luminal in the posterior midgut, but cellular aminopeptidases are required for peptide processing in both anterior and posterior midguts. Alpha-glucosidase activity is elevated in the posterior midgut after feeding in response to the blood meal, whereas activity in the anterior midgut is consistent with a nectar-processing role for this midgut region.

While many species are native to tropical and subtropical regions, some genera such as Aedes have successfully adapted to cooler regions. In the warm and humid tropical regions, they are active the entire year long; however, in temperate regions they hibernate over winter. Eggs from strains in the temperate zones are more tolerant to the cold than ones from warmer regions. They can even tolerate snow and sub-zero temperatures. In addition, adults can survive throughout winter in suitable microhabitats.

Over large distances the worldwide distribution is carried out primarily through sea routes, in which the eggs, larvae, and pupae in combination with water-filled used tires and cut flowers are transported around. As with sea transport, the transport of mosquitoes in personal vehicles, delivery trucks, and trains plays an important role.

Mosquitoes are a vector agent that carries disease-causing viruses and parasites from person to person without catching the disease themselves.

The principal mosquito borne diseases are the viral diseases yellow fever, dengue fever and Chikungunya, transmitted mostly by the Aedes aegypti, and malaria carried by the genus Anopheles. Though originally a public health concern, HIV is now thought to be almost impossible for mosquitoes to transmit.

Mosquitoes are estimated to transmit disease to more than 700 million people annually in Africa, South America, Central America, Mexico, Russia and much of Asia with millions of resulting deaths. At least 2 million people annually die of these diseases.

Methods used to prevent the spread of disease, or to protect individuals in areas where disease is endemic include Vector control aimed at mosquito eradication, disease prevention, using prophylactic drugs and developing vaccines and prevention of mosquito bites, with insecticides, nets and repellents. Since most such diseases are carried by "elderly" females, scientists have suggested focusing on these to avoid the evolution of resistance.

There are many methods used for mosquito control. Depending on the situation, source reduction (e.g., removing stagnant water), biocontrol (e.g. importing natural predators such as dragonflies), trapping, and/or insecticides to kill larvae or adults may be used.

The dragonfly nymph eats mosquitoes at all stages of development and is quite effective in controlling populations. Gambusia, also called Mosquitofish, eat mosquito larvae and can be introduced into ponds. Although bats and Purple Martins can be prodigious consumers of insects, many of which are pests, less than 1% of their diet typically consists of mosquitoes. Neither bats nor Purple Martins are known to control or even significantly reduce mosquito populations.

Some cyclopoid copepods are predators on first instar larvae, killing up to 40 Aedes larvae per day. Larval Toxorhynchites mosquitoes are known as natural predators of other Culicidae. Each larva can eat an average of 10 to 20 mosquito larvae per day. During its entire development, a Toxorhynchites larva can consume an equivalent of 5,000 larvae of the first instar (L1) or 300 fourth instar larvae (L4) (Steffan & Evenhuis, 1981; Focks, 1982). However, Toxorhynchites can consume all types of prey, organic debris (Steffan & Evenhuis, 1981), or even exhibit cannibalistic behavior. A number of fish are also known to consume mosquito larvae, including bass, bluegill, piranha, catfish, fathead minnows, the western mosquitofish (Gambusia affinis), goldfish, guppies, and killifish.

Bacillus thuringiensis israelensis has also been used to control them as a biological agent.

Visible, irritating bites are due to an immune response from the binding of IgG and IgE antibodies to antigens in the mosquito's saliva. Some of the sensitizing antigens are common to all mosquito species, whereas others are specific to certain species. There are both immediate hypersensitivity reactions (Types I & III) and delayed hypersensitivity reactions (Type IV) to mosquito bites (see Clements, 2000).

There are several commercially available anti-itch medications, including those taken orally, such as Benadryl, or topically applied antihistamines and, for more severe cases, corticosteroids such as hydrocortisone and triamcinolone. Many effective home remedies exist, including calamine lotion and vinegar. A paste of meat tenderizer containing papain and water breaks down the proteins in the mosquito saliva. By using a brush to scratch the area surrounding the bite and running hot water (around 49 °C or 120 °F) over it can alleviate itching for several hours by reducing histamine-induced skin blood flow. Plain household sudsy ammonia is also a good treatment, ammonia being the main ingredient in Tender's AfterBite remedy, especially as a first wash option if applied immediately after multibite exposure. A simple, yet effective remedy can be achieved by applying a piece of adhesive tape over the affected area, or by sucking it with a drinking straw; creating enough counter tension on the surface of the skin to alleviate the itch. Tea tree oil has been shown to be an effective anti-inflammatory, reducing itching.