Mosquito (from the Spanish meaning little fly [1]) is a common insect in the family Culicidae (from the Latin culex meaning midge or gnat [2]). 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. The adult females lay their eggs in 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. The adult females can live up to a month (or more in captivity) but most probably do not live more than 1–2 weeks in nature. Mosquitoes have mouthparts which are adapted for piercing the skin of plants and animals. They typically feed on nectar and plant juices. In some species, 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. [3] [4]

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Larva

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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 exoskeleton, or skin, to allow for further growth.

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.

Adult

thumb , 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 duration from egg to adult varies considerably 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 10–14 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 grass, shrubbery or other foliage.

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. Females will also feed on sugar sources for energy but usually require a blood meal for the development of eggs. 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.

Length of the adult varies but is rarely greater than ^[5], and weigh up to 2.5 mg (0.04 grain). 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. ^[6] 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. The head also has an elongated, forward-projecting stinger used for feeding, and two sensory palps. The maxillary palps of the males are longer than their stingers 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 up to 1–2 km/h ^[7] travelling up to 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.

Feeding habits of adults

Both male and female mosquitoes are nectar feeders, but the females of many species are also capable of hematophagy (drinking blood). Females do not require blood for their own survival, but they do need supplemental substances such as protein and iron to develop eggs.

In regards to host location, carbon dioxide and organic substances produced from the host, humidity, and optical recognition play important roles. 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. ^[8]

Mosquitoes are crepuscular (dawn or dusk) feeders. During the heat of the day most mosquitoes rest in a cool place and wait for the evenings. They may still bite if disturbed. Mosquitoes are adept at infiltration and have been known to find their way into residences via deactivated air conditioning units. ^[9]

Prior to and during blood feeding, they inject saliva into the bodies of their source(s) of blood. Female mosquitoes hunt their blood host by detecting carbon dioxide (CO₂) and 1-octen-3-ol from a distance.

thumb vector of dengue fever and yellow fever

Mosquitoes of the genus Toxorhynchites never drink blood. ^[10] 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. ^[11]

Saliva

In order for the mosquito to obtain a blood meal it must surmount the vertebrate physiological responses. The mosquito, as with all blood-feeding arthropods, has evolved 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. ^[12] 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 ^[13] and antimicrobial agents to control bacterial growth in the sugar meal. ^[14] The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins. ^[15] 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. ^[16]

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-a release, but not antigen-induced histamine secretion, from activated mast cells. ^[17] 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-? production, while the cytokines IL-4 and IL-5 are unaffected by mosquito saliva. ^[18] 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-? production. ^[19] 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. ^[20] 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 1/7th of the saliva in a single mosquito. ^[21] 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. ^[22] A recent study suggests that mosquito saliva can also decrease expression of interferon-a/ß during early mosquito-borne virus infection. ^[23] 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, ^[24] and recent research suggests that mosquito saliva exacerbates West Nile virus infection, ^[25] as well as other mosquito-transmitted viruses. ^[26]

Egg development and blood digestion

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. ^[27]

Distribution

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While many species are native to tropical and subtropical regions, some such as Aedes have successfully adapted themselves 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. ^{[28] [29]} They can even tolerate snow and temperatures under freezing. In addition, adults can survive throughout winter in suitable microhabitats. ^[30]

Means of dispersal

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.

Disease

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

thumb 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.

The principal mosquito borne diseases are the viral diseases yellow fever and dengue fever, 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 ^[31].

Mosquitoes are estimated to transmit disease to more than 700 million people annually in Africa, South America, Central America, Mexico 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 ^[32]

Control

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There are many methods used for mosquito control. Depending on the situation, source reduction, biocontrol, larviciding (control of larvae), or adulticiding (control of adults) may be used to manage mosquito populations.

These techniques are accomplished using habitat modification, such as removing stagnant water and other breeding areas, pesticide like DDT, natural predators, (eg Dragonflies, larvae-eating fish), and trapping. Garlic Oil concentrate will repel mosquitos for up to 4 weeks.

Natural predators

right are natural predators of mosquitoes.

The dragonfly eats mosquitoes at all stages of development and is quite effective in controlling populations. ^[33] 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. ^[34] Some cyclopoid copepods are predators on 1st instar larvae, killing up to 40 Aedes larvae per day. ^[35] 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, catfish, fathead minnows, the western mosquitofish (Gambusia affinis ), goldfish, guppies, and killifish.

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

Treatment of mosquito bites

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 ineffective home remedies exist, including calamine lotion. Both using a brush to scratch the area surrounding the bite and running hot water (around 49 °C) over it can alleviate itching for several hours by reducing histamine-induced skin blood flow. ^[36] On the other hand, excessive scratching can irritate the bite and break the skin, leading to prolonged recovery and the possibility of infection or scarring.

Cultural views

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According to the “Mosquitoes” chapter in Kwaidan: Stories and Studies of Strange Things , by Lafcadio Hearn (1850–1904), mosquitoes are seen in Japanese popular belief as reincarnations of the dead, condemned by the errors of their former lives to the condition of Jiki-ketsu-gaki , or "blood-drinking pretas". ^[37]

Evolution

The Culicinae and Anopheles clades are believed to have diverged about 150 million years ago. ^[38] The Old and New World Anopheles species are believed to have subsequently diverged about 95 million years ago.

Systematics

There are approximately 3,500 species of mosquitoes grouped into 41 genera. Human malaria is transmitted only by females of the genus Anopheles . Of the approximately 430 Anopheles species, while over 100 are known to be able to transmit malaria to humans only 30–40 commonly do so in nature. Since breeding and biting habit differ considerably between species, species identification is important for control programmes.

See also

• Insect repellent
• Cymbopogon
• Anti-itch drug
• Octenol

References

1. Mosquito at dictionary.com.
2. Culex at dictionary.com.
3. Africa's Malaria Death Toll Still "Outrageously High"
4. "Mosquito-Borne Diseases" – The American Mosquito Control Association. Retrieved 2008-10-14.
5. Mosquito
6. Evolution of eye development in arthropods: Phylogenetic aspects
7. Flight performance of the malaria vectors Anopheles gambiae and Anopheles atroparvus
8. R.G. Estrada-Franco & G.B. Craig (1995) Biology, disease relationship and control of Aedes albopictus. Pan American Health Organization, Washington DC: Technical Paper No. 42.
9. Rest boxes as mosquito surveillance tools
10. The carnivores, Toxorhynchites
11. http://www.pestscience.com/PDF/BNIra56.PDF
12. Role of arthropod saliva in blood feeding: sialome and post-sialome perspectives
13. The salivary glands of the vector mosquito, Aedes aegypti, express a novel member of the amylase gene family
14. Bacteriolytic factor in the salivary glands of Aedes aegypti
15. Toward a description of the sialome of the adult female mosquito Aedes aegypti
16. Dr. Nigel Beebe, University of Technology, Sidney, Australia
17. Extracts of mosquito salivary gland inhibit tumour necrosis factor alpha release from mast cells
18. Differential modulation of murine cellular immune responses by salivary gland extract of Aedes aegypti
19. Mosquito feeding modulates Th1 and Th2 cytokines in flavivirus susceptible mice: an effect mimicked by injection of sialokinins, but not demonstrated in flavivirus resistant mice
20. Differential modulation of murine host immune response by salivary gland extracts from the mosquitoes Aedes aegypti and Culex quinquefasciatus
21. Saliva of the Yellow Fever mosquito, Aedes aegypti, modulates murine lymphocyte function
22. Mast cell-dependent down-regulation of antigen-specific immune responses by mosquito bites
23. Aedes aegypti salivary gland extracts modulate anti-viral and TH1/TH2 cytokine responses to sindbis virus infection
24. Protection against Japanese encephalitis virus in mice and hamsters by treatment with carboxymethylacridanone, a potent interferon inducer
25. Potentiation of West Nile encephalitis by mosquito feeding
26. The enhancement of arbovirus transmission and disease by mosquito saliva is associated with modulation of the host immune response
27. Blood digestion in the mosquito, ''Anopheles stephensi'' Liston (Diptera: Culicidae): activity and distribution of trypsin, aminopeptidase, and alpha-glucosidase in the midgut
28. Overwintering survival of ''Aedes albopictus'' (Diptera: Culicidae) eggs in Indiana
29. ''Aedes albopictus'' (Diptera: Culicidae) eggs: field survivorship during northern Indiana winters
30. Cold acclimation and overwintering of female ''Aedes albopictus'' in Roma
31. Can I get HIV from mosquitoes? CDC 20-October-2006
32. Resistance is Useless The Economist 8-April-2009
33. Laboratory studies on the predatory potential of dragon-fly nymphs on mosquito larvae
34. Mosquitoes and mosquito repellents: a clinician's guide
35. Cyclopoid copepods
36. Noxious Heat and Scratching Decrease Histamine-Induced Itch and Skin Blood Flow
37. Hearn, Lafcadio. ''Kwaidan: Stories and Studies of Strange Things''. Dover Publications, Inc., 1968 (ISBN 0-486-21901-1)
38. The salivary gland transcriptome of the neotropical malaria vector Anopheles darlingi is thought to reveal accelerated evolution of genes relevant to hematophagy