Mosquito

Mosquitoes are a family of small midge-like flies, the Culicidae. Most species are harmless, even useful, but many are blood-sucking nuisances. Some also transmit several of the most harmful diseases of humans and their livestock. Accordingly, some authorities argue that disease-bearing mosquitoes are the most dangerous animals on earth.^[2]

Introduction

Mosquitoes are members of a family of nematocerid flies: the Culicidae (from the Latin culex, genitive culicis meaning midge or gnat).^[3] The word Mosquito is from the Spanish and Portuguese for little fly.^[4] Superficially, mosquitoes resemble crane flies (family Tipulidae) and chironomid flies (family Chironomidae), and as a result casual observers seldom realise that there are important differences between the members of the respective families and also differences between their habits. In particular, many species of female mosquitoes are blood-sucking pests and dangerous vectors of diseases, whereas members of the similar-looking Chironomidae and Tipulidae are not.

Over 3,500 species of mosquitoes have already been described from various parts of the world.^[5][6] Some mosquitoes that bite humans routinely act as vectors for a number of infectious diseases affecting millions of people per year.^[7][8] Others that do not routinely bite humans, but are the vectors for animal diseases, may become disastrous agents for zoonosis of new diseases when their habitat is disturbed, for instance by sudden deforestation.^[9][10]

While eliminating mosquitoes from the planet may sound extreme, a few scientists have suggested that complete eradication would not have serious ecological consequences.^[11][12] In practice, however, control measures focus on the small group of mosquito species which are vectors of human or livestock disease. Some, such as members of the genus Toxorhynchites, actually are beneficial predators of other mosquitoes.

Life cycle

Anopheles larva from southern Germany, about 8 mm long

 Anatomy of a Culex larvae

Image of pitcher plant mosquito Wyeomyia smithii, showing segmentation and partial anatomy of circulatory system

Like all flies, mosquitoes go through four stages in their life-cycle: egg, larva, pupa, and adult or imago. In most species adult females lay their eggs in standing water; some lay eggs near the water's edge; others attach their eggs to aquatic plants. Each species selects the situation of the water into which it lays its eggs according to its ecological adaptations. Some are generalists and are not very fussy. Some breed in marshes, some in salt-marshes; some in lakes, some in temporary puddles. Some prefer phytotelmata (natural reservoirs on plants) such as rainwater accumulated in holes in tree trunks, or in the leaf-axils of bromeliads. Some specialise in the liquid in pitchers of particular species of pitcher plants, their larvae feeding on decaying insects that had drowned there,, or on the associated bacteria; the harmless genus Wyeomyia provide such examples — Wyeomyia smithii breeds only in the pitchers of Sarracenia purpurea.^[13]

In contrast, artificial water containers such as the odd plastic bucket, flowerpot "saucer", or discarded bottle or tire, are important breeding places for some of the most serious disease vectors such as species of Aedes that transmit dengue and yellow fever.

The first three stages — egg, larva and pupa — are largely aquatic. Typically last 5–14 days, depending on the species and the ambient temperature, but there are important exceptions. Mosquitoes living in regions where some seasons are freezing or waterless, spend part of the year in diapause; they delay their development, typically for months, and carry on with life only when there is enough water or warmth for their needs. For instance, Wyeomyia larvae typically get frozen into solid lumps of ice during winter and only complete their development in spring. The eggs of some species of Aedes remain unharmed in diapause if they dry out, and hatch later when they are covered by water.

Eggs hatch to become larvae, which grow till they are able to change into pupae. The adult mosquito emerges from the mature pupa as it floats at the water surface. Bloodsucking species, depending on type, gender, and weather conditions can live as adults from as little as a week to as long as several months.

Some species can overwinter as adults in diapause.^[14][15]

Eggs and oviposition

Mosquito habits of oviposition, the ways in which they lay their eggs, vary considerably between species, and the morphologies of the eggs vary accordingly. The simplest procedure is that followed by many species of Anopheles: like many other gracile species of aquatic insects, females just fly over the water, bobbing up and down to the water surface and dropping eggs more or less singly. The eggs are roughly cigar-shaped and have floats down the sides

Some other species, for example members of the genus Mansonia, lay their eggs in arrays, attached usually to the under-surfaces of waterlily pads. Their close relatives, the genus Coquillettidia lay their eggs similarly, but not attached to plants. Instead the eggs form layers called "rafts" that float on the water. This is a common mode of oviposition, and most species of Culex are known for the habit, which also occurs in some other genera such as Culiseta and Uranotaenia. Anopheles eggs often cluster together on the water too, but they do not generally look much like compactly glued rafts of eggs. The rafts do not form accidentally; the female Culex settles carefully on still water with her hind legs crossed, and as she lays the eggs one by one, she twitches to arrange them into a head-down array that sticks together to form the raft.^[16]

An egg raft of a Culex species. It is partly broken, showing individual egg shapes.

Aedes generally drop their eggs singly too, but not as a rule into water. Instead they lay their eggs on damp mud or other surfaces near the water's edge. This commonly is the wall of a cavity such as a hollow stump or a container such as a bucket or discarded vehicle tire. The eggs generally do not hatch until they are flooded, and they may have to withstand considerable desiccation before that happens. They cannot do so straight after hatching, but must develop to a suitable degree first. Once that has happened however, they can enter diapause for several months. and unlike most mosquitoes that hatch as soon as possible, a batch of Aedes eggs in diapause tend to hatch irregularly. This makes it much more difficult to control such pests, as compared to the species that can be killed all together as they hatch. Some Anopheles species also behave in such a manner, though not to the same degree of sophistication.^[17]

Larva

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.

Culex sp larvae plus one pupa

 

 

 

Pupa

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 curving 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 the larva.^{[citation needed]}

Adult

 

Anatomy of an adult mosquito.

Adults of the yellow fever mosquito Aedes aegypti, a typical member of the subfamily Culicinae. The male on the left, females on the right. Note the bushy antennae and longer palps in the male.

The period of development from egg to adult varies among species and is strongly influenced by ambient temperature. Some species of mosquitoes can develop from egg to adult in as little as five days, but a more typical period of development in tropical conditions would be some 40 days or more for most species. 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 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 typically 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 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),^[18] 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 receiving sensory information and for feeding. It has 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.^[19] 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.^[19] 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. In typical bloodsucking species the female has an elongated proboscis.

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)^[20] travelling up to 12 kilometres (7.5 mi) in a night. Males beat their wings between 450 and 600 times per second.^[21]

The abdomen is specialized for food digestion and egg development, the abdomen of a mosquito can hold three times its own weight in blood.^[22] 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.

Feeding by adults

Aedes aegypti, a common vector of dengue fever and yellow fever

Typically both male and female mosquitoes feed on nectar and plant juices, but in many species the mouthparts of the females are adapted for piercing the skin of animal hosts and sucking their blood as ectoparasites. In many species the female needs to obtain nutrients from a "blood meal" before she can produce eggs, whereas in many other species she can produce more eggs if she can obtain a blood meal. Both plant materials and blood are useful sources of energy in the form of sugars, and blood also supplies more concentrated nutrients such as lipids, but the most important function of blood meals is to obtain proteins as materials for egg production.

Such blood-sucking by females in support of their reproduction, at the risk of their lives, is not limited to the mosquitoes; it also is found in other insect families such as the Tabanidae. When a female reproduces without such parasitic meals, she is said to practise autogenous reproduction, as in Toxorhynchites; otherwise the reproduction may be termed anautogenous, as occurs in mosquito species that serve as disease vectors, particularly Anopheles and some of the most important disease vectors in the genus Aedes. In contrast, some mosquitoes, for example, many Culex, are partially anautogenous; they do not need a blood meal for their first cycle of egg production, which they produce autogenously; however, subsequent clutches of eggs are produced anautogenously, at which point their disease vectoring activity becomes operative.^[23]

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 preferred 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.^[24] The powerful semiochemical that triggers the mosquito's keen sense of smell is nonanal.^[25] 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.^[26] 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.^[27]

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.^[28] Some species, like the Asian tiger mosquito, are known to fly and feed during daytime.^{[citation needed]}

Both male and female are nectar feeders.

Mosquitoes are adept at infiltration and have been known to find their way into residences via deactivated air conditioning units.^{[citation needed]}

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.^[29] 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.^[30]

[Mouthparts

Mosquito mouthparts are very specialised, particularly those of the females, which in most species are adapted to piercing skin and then sucking blood. Apart from bloodsucking, the females generally also drink assorted fluids rich in dissolved sugar, such as nectar and honeydew, to obtain the energy they need. For this their blood-sucking mouthparts are perfectly adequate. In contrast, male mosquitoes are not bloodsuckers; they only drink such sugary fluids as they can find. Accordingly their mouthparts do not require the same degree of specialisation as those of females.^[31]

Externally the most obvious feeding structure of the mosquito is the proboscis. More specifically the visible part of the proboscis is the labium, which forms the sheath enclosing the rest of the mouthparts. When the mosquito first lands on a potential host, her mouthparts will be enclosed entirely in this sheath, and she will touch the tip of the labium to the skin in various places. Sometimes she will begin to bite almost straight away, while other times she will prod around, apparently looking for a suitable place. Occasionally she will wander for a considerable time, and eventually fly away without biting. Presumably this probing is a search for a place where there are easily accessible blood vessels, but the exact mechanism is not known. It is known that there are two taste receptors at the tip of the labium, which may well play a role.^[32]

The female mosquito does not insert her labium into the skin; it bends back into a bow when the mosquito begins to bite. The tip of the labium remains in contact with the skin of the victim, acting as a guide for the other mouthparts. In total there are six mouthparts besides the labium: two mandibles, two maxillae, the hypopharynx, and the labrum.

The mandibles and the maxillae are used for piercing the skin. The mandibles are pointed, while the maxillae end in flat, toothed "blades". To force these into the skin, the mosquito moves its head backwards and forwards. On one movement, the maxillae are moved as far forward as possible. On the opposite movement, the mandibles are pushed deeper into the skin by levering against the maxillae. The maxillae do not slip back because the toothed blades grip the skin.

The hypopharynx and the labrum both are hollow. Saliva with anticoagulant is pumped down the hypopharynx to prevent clotting, and blood is drawn up the labrum.

To understand the mosquito mouthparts, it is helpful to draw a comparison with an insect that chews food, such as a dragonfly. A dragonfly has two mandibles, which are used for chewing, and two maxillae, which are used to hold the food in place as it is chewed. The labium forms the floor of the dragonfly's mouth, the labrum forms the top, while the hypopharynx is inside the mouth and is used in swallowing. Conceptually, then, the mosquito's proboscis is an adaptation of the mouthparts that occur in other insects. The labium still lies beneath the other mouthparts, but also enfolds them, and it has been extended into a proboscis. The maxillae still "grip" the "food" while the mandibles "bite" it. The top of the mouth, the labrum, has developed into a channeled blade the length of the proboscis, with a cross-section like an inverted "U". Finally, the hypopharynx has extended into a tube that can deliver saliva at the end of the proboscis. Its upper surface is somewhat flattened so that, when pressed against it, the labrum forms a closed tube for conveying blood from the victim.^[33]

Saliva

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.^[34] 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^[35] and antimicrobial agents to control bacterial growth in the sugar meal.^[36] The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins.^[37] 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.^[37] 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.^[38]

It is now well recognized that feeding ticks, sandflies, and, more recently, mosquitoes have an ability to modulate the immune response of the animals (hosts) they feed on.^[34] 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.^[39] 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.^[40] Cellular proliferation in response to IL-2 is clearly reduced by prior treatment of cells with SGE.^[40] 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.^[41] 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.^[41]

T cell populations are decidedly susceptible to the suppressive effect of mosquito saliva, showing enhanced mortality and decreased division rates.^[42] 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.^[43] 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.^[44]

A recent study suggests that mosquito saliva can also decrease expression of interferon−α/β during early mosquito-borne virus infection.^[45] 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,^[46] and recent research suggests that mosquito saliva exacerbates West Nile virus infection,^[47] as well as other mosquito-transmitted viruses.^[48]

Egg development and blood digestion

A Mosquito feeding on blood

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. Aminoptidase 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.^[49]

Distribution

Female Ochlerotatus notoscriptus feeding on a human arm, Tasmania, Australia

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.^[50][51] They can even tolerate snow and sub-zero temperatures. In addition, adults can survive throughout winter in suitable microhabitats.^[52]

Means of dispersal

Over large distances, worldwide introduction of various mosquito species into regions where they are not indigenous has occurred through human agencies, primarily on sea routes, in which the eggs, larvae, and pupae inhabiting water-filled used tires and cut flowers are transported. However, apart from sea transport, mosquitoes have been effectively carried by personal vehicles, delivery trucks, and trains and aircraft. Quarantine measures have proved difficult to apply sufficiently consistently.

Disease

Anopheles albimanus mosquito feeding on a human arm. This mosquito is a vector of malaria and mosquito control is a very effective way of reducing the incidence of malaria.

Main articles: mosquito-borne disease and life-threatening disease

Mosquitoes can act as a vector for many disease-causing viruses and parasites. Infected mosquitoes carry these organisms from person to person without exhibiting symptoms themselves. Mosquito-borne diseases include:

• the viral diseases yellow fever, dengue fever and Chikungunya, transmitted mostly by Aedes aegypti;

• the parasitic disease malaria, carried by mosquitoes of the genus Anopheles;

• lymphatic filariasis (the main cause of elephantiasis) which can be spread by a wide variety of mosquito species;^[53]

• West Nile virus is a concern in the United States, but there are no reliable statistics on worldwide cases.

Though originally a public health concern, HIV is now thought to be almost impossible for mosquitoes to transmit.^[54]

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.^[55]

Control

Larvae in stagnant water

Main article: Mosquito control

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.

Natural predators

The dragonfly nymph eats mosquitoes at all stages of development and is quite effective in controlling populations.^[56] Gambusia, also called Mosquitofish, eat mosquito larvae and can be introduced into ponds^[57] . 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.^[58]

Some cyclopoid copepods are predators on first instar larvae, killing up to 40 Aedes larvae per day.^[59] 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, Arctic char, salmon, trout, catfish, fathead minnows, the western mosquitofish (Gambusia affinis), goldfish, guppies, and killifish.

Other natural predators and parasitoids include fungi^[60] and nematodes^[61] Though important at times, their effectiveness varies with circumstances.

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

Mosquito bites and treatment

Mosquito bite wounds

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. 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.^[62] Tea tree oil has been shown to be an effective anti-inflammatory, reducing itching.^[63]

Repellents

Main article: Insect repellent

Depending on the regions around the world, specific types of insect repellents are made. The commercial chemical DEET also repels some mosquitoes and other insects.^[64] Other CDC-recommended repellents are Picaridin, Oil of Eucalyptus (PMD) and IR3535.^[65]

Evolution

The oldest known mosquito with an anatomy similar to modern species was found in 79-million-year-old Canadian amber from the Cretaceous.^[66] An older sister species with more primitive features was found in amber that is 90 to 100 million years old.^[67]

Genetic analyses indicate that the Culicinae and Anophelinae clades may have diverged about 150 million years ago.^[68] The Old and New World Anopheles species are believed to have subsequently diverged about 95 million years ago.^[68]

The mosquito Anopheles gambiae is currently undergoing speciation into the M and S molecular forms. This means that some pesticides that work on the M form, won't work anymore on the S form.^[69]

Taxonomy of the Culicidae

Over 3,500 species of the Culicidae have already been described.^[70] They are generally divided into two subfamilies which in turn comprise some 43 genera. These figures are subject to continual change as more species are discovered, and as DNA studies compel rearrangement of the taxonomy of the family. The two main subfamilies are the Anophelinae and Culicinae, with their genera as shown in the subsection below.^[71]

Subfamilies and genera

Anophelinae

• Anopheles
• Bironella
• Chagasia

Culicinae

• Aedeomyia
• Aedes
• Armigeres
• Ayurakitia
• Borachinda
• Coquillettidia
• Culex
• Culiseta
• Deinocerites
• Eretmapodites
• Ficalbia
• Galindomyia
• Haemagogus
• Heizmannia
• Hodgesia
• Isostomyia
• Johnbelkinia
• Kimia
• Limatus
• Lutzia
• Malaya
• Mansonia
• Maorigoeldia
• Mimomyia
• Onirion
• Opifex
• Orthopodomyia
• Psorophora
• Runchomyia
• Sabethes
• Shannoniana
• Topomyia
• Toxorhynchites
• Trichoprosopon
• Tripteroides
• Udaya
• Uranotaenia
• Verrallina

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