Mosquito

The mosquitos are insects which make up the family Culicidae. They have a pair of scaled wings, a pair of halteres, a slender body, and long legs. The females of most mosquito species suck blood (hematophagy) from other animals^[1], which has made them the most deadly disease vectors known to man, killing millions of people over thousands of years and continuing to kill millions per year by the spread of diseases.^[2][3]

Length varies but is rarely greater than 16 mm (0.6 inch)^[4], and weight up to 2.5 mg (0.04 grain). A mosquito can fly for 1 to 4 hours continuously at up to 1–2 km/h^[5] travelling up to 10 km in a night. Most species are nocturnal or crepuscular (dawn or evening) feeders. During the heat of the day most mosquitos rest in a cool place and wait for the evenings. They may still bite if disturbed.^[6]

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Evolution

  Mosquitoes are believed to have evolved around 170 million years ago whilst the first ever known during the Jurassic era (199–144 million years ago) with the earliest known fossils from the Cretaceous era (144–65 million years ago).^[7] They are thought to have evolved in South America, spreading initially to the northern continent Laurasia and re-entering the tropics from the north.^[8] Some ancestral mosquitos were about three times the size of the extant species.^{[citation needed]}

The family Culicidae, a sister group to the Chaoboridae (biting midges), belongs to the order Diptera and contains about 3,500 species in three subfamilies: Anophelinae (3 genera), the Culicinae (at least 37 genera and >80% of all the species) and the Toxorhynchitinae (1 genus). The genera include Anopheles, Culex, Psorophora, Ochlerotatus, Aedes, Sabethes, Wyeomyia, Culiseta, and Haemagoggus. Within the subfamily Anophelinae six subgenera are recognized: Stethomyia, Lophopodomyia, Kerteszia, Nyssorhynchus (all South American), Cellia (Old World only), and Anopheles (worldwide).

Food habits

Both male and female mosquitos are nectar feeders, but the female is also capable of haematophagy (drinking blood). Females do not require blood for survival, but they do need supplemental substances (like protein and iron) for the development and laying of their eggs. Prior to sucking the blood, they inject a mild painkiller, which numbs the host to the pain from the "bite" (Note: mosquitos do not actually bite). The Toxorhynchites species of mosquito never drinks blood.^[9] This genus includes the largest of the extant mosquitos, the larvae of which are predatory on the larvae of other mosquitos. These mosquito eaters have been used in the past as mosquito control agents, with varying success .^[10]

Origin of the name Mosquito

In the Spanish language, the word Mosquito (little fly) dates back to about 1572.^[11] The word was adopted to replace the term "biting flies" to prevent confusion with the house fly. It is derived from the word fly (Latin musca, cf. Skt maksh) and is related to the Italian moschetta and the French moustique. Mosquitoes were originally called "les moucherons" or "les cousins" by French writers, "Stechmücken" or "Schnaken" by Germans, "mygg" and "mygga" by Scandinavians, and "κώνωψ" (konops) by the ancient Greeks. The Scandinavian word is related to the Modern Greek word "μύγα" (myga) for the housefly, and to the English "midge". The Icelandic "mý" mostly stands for biting midges or non-biting chironomids, as there are no mosquitos in Iceland.^[12] Aristotle referred to mosquitoes in 300 B.C. as "empis".

Biology

Anatomy

The mosquito is composed of a head, thorax, and abdomen. The head contains the compound eye and proboscis. The proboscis is a piercing mouthpart used to suck blood from its prey. The mosquito's head is mostly eye. Each eye is made up of many tiny lenses forming a compound eye. This type of eye allows a very big field of vision that easily detects movement. Next is the thorax. The thorax has one pair of wings and one pair of halteres. The thorax also has markings that are used in the identification of the mosquito. Then there is the abdomen or gut, which expands as it ingests its prey's blood. The abdomen also has many markings that are used to identify the mosquito species.

Life cycle and feeding habits

In its life cycle the mosquito undergoes complete metamorphosis, going through four distinct stages: egg, larva, pupa, and adult, first described by the Greek philosopher Aristotle.^[13]

Egg

Female mosquitoes lay their eggs one at a time or together in rafts of a hundred or more eggs on the surface in fresh or any stagnant water. Anopheles and Aedes mosquitoes do not make egg rafts but lay their eggs separately. Culex, Culiseta, and Anopheles lay their eggs on water while Aedes lay their eggs on damp soil that is periodically flooded by water. Most eggs hatch into larvae in about 48 hours. A female mosquito may lay a raft of eggs every third night during its life span if it can find enough blood to develop the eggs.

Larval stage

The hatching eggs turn into larvae that live in the water, coming to the surface to breathe. The first larval stage is known as the first instar. As they grow they shed or moult their skin about four times growing larger after each moulting. After the first molt they are second instars, then third, then fourth. Most larvae use siphon tubes going to the water surface for breathing and hang on or near the water surface. Anopheles larvae do not have a siphon and typically lie parallel to the water surface. The larvae eat micro-organisms and organic matter in the water for food. Mosquito larvae, commonly called "wigglers" or "wrigglers", must live in water from 7 to 14 days depending on the water's temperature. At their last moult they may be up to 1 cm or 1/2 inch long. In each stage they may be eaten by other insects or fish. Mosquito larvae in the genus Toxorhynchites eat other mosquito larvae.

The length of the first three stages (or instars) is dependent on the species and temperature, with lower temperatures increasing the length of the development stage.^[14] Culex tarsalis may complete its life cycle in 14 days at 20 C (68 F) and only ten days at 25 C (77 F). Some species have a life cycle of as little as four days, whereas in other species some adult females can live through the winter, laying their eggs in the spring. Many species of mosquito live their adult stage in roughly two weeks to two months. The larvae are the "wrigglers" found in puddles or water-filled containers. These breathe air through a siphon at the tail end. The pupae, or "tumblers", are nearly as active as the larvae, but breathe through thoracic "horns" attached to the thoracic spiracles. Most larvae feed on micro-organisms, but a few are predatory on other mosquito larvae. Some mosquito larvae, such as those of Wyeomyia live in unusual situations. These mosquito wigglers live either in the water collected in epiphytic bromeliads or inside water stored in carnivorous pitcher plants. Larvae of the genus Deinocerites live in crab holes along the edge of the ocean. On the fourth molt the larva changes into a pupa.

Pupa

The pupae are lighter than water and float on the surface as the mosquito larva metamorphoses (changes) into an adult mosquito in about two days. Pupae do not have mouths and therefore do not feed. This is important to know from a larviciding point of view because most larvicide has to be ingested by the mosquito. A surface oil or mmf (monomolecular film) shoud be applied to the breeding site as a means of suffocating the pupa.

Adult

The newly emerged adult must rest on the surface of the water for a short time to allow itself to dry and all its parts to harden before it can fly. This requires still water: mosquitoes do not breed in fast-moving water.

The total time to go through all four stages depends on the temperature and the type of mosquito, but typically takes 14 days or less in warmer weather. In various species the time varies from 4 to 30 days.

Most mosquito species outside of the tropics overwinter as eggs, but many overwinter as larvae or adults. Mosquitoes of the genus Culex (a vector for St. Louis encephalitis) overwinter as mated adult females.

Most mosquitoes stay fairly close to the ground and do not range too far from where they were born, but may be dispersed long distances by wind. Mosquitoes are not strong flyers making only 1-2 km/h, (1-1.5 mph) therefore an electric fan may suffice as an effective mosquito screen. They feed mostly in the mornings and evenings and occasionally at night; avoiding the heat of the day. During the day they usually find somewhere cool to rest.

  Only female mosquitoes bite animals to get blood needed to produce eggs. Male mosquitoes do not bite, but both the male and female feed on the nectar of flowers for food. In most female mosquitoes, the mouth parts form a long proboscis for piercing the skin of mammals (or in some cases birds or even reptiles and amphibians) to suck their blood. As opposed to a syringe's typically smooth needle, the mosquito proboscis is highly serrated, which leaves a minimal number of points of contact with the skin being pierced — this reduces nerve stimulation to the point where the "bite" is typically not felt at all. (See the Mosquitoes and health section below for an explanation on the swelling). The females require protein for egg development and laying, and since the normal mosquito diet consists of nectar and fruit juice, which has no protein, most females must drink blood to lay eggs. Males differ from females, with mouth parts not suitable for blood-sucking.

The female mosquitoes locate their next blood donor victims primarily through scent. They are extremely sensitive to the carbon dioxide in exhaled breath, as well as several substances found in sweat and various body odours. They are believed to be able to track potential prey for tens of meters. Some people attract more mosquitoes than others, apparently based on how they "smell" to a mosquito. Mosquitoes can also detect heat, so they can find warm-blooded mammals and birds very easily once they get close enough. Repellents like DEET work by disorienting the mosquito as it gets close to its potential next meal but do not kill mosquitoes. Surprisingly this works about 95% of the time.^{[citation needed]}

Male mosquitoes are distinctly smaller than females, with features such as feathered antennae and having no audible sound during flight. Female mosquitoes in flight emit a distinctive high-pitched buzz, which can interrupt sleep.

Mosquitoes and humans

Mosquitoes and health

   Mosquitoes are a vector agent that carries disease-causing viruses and parasites from person to person without catching the disease themselves. Female mosquitoes suck blood from people and other animals as part of their eating and breeding habits. The female mosquito that bites an infected person and then bites an uninfected person might leave traces of virus or parasite from the infected person's blood. The infected blood is injected through, or on, the "dirty" proboscis into the uninfected person's blood and the disease is thus spread from person to person. When a mosquito bites, she also injects saliva and anti-coagulants into the blood which may also contain disease-causing viruses or other parasites. This cycle can be interrupted by killing the mosquitoes, isolating infected people from all mosquitoes while they are infectious or vaccinating the exposed population. All three techniques have been used, often in combination, to control mosquito transmitted diseases. Window screens, introduced in the 1880s, were called "the most humane contribution the 19th century made to the preservation of sanity and good temper."^[15]

Mosquitoes are estimated to transmit disease to more than 70 million people annually in Africa, South America, Central America, Mexico and much of Asia with millions of resulting deaths. In Europe, Russia, Greenland, Canada, the United States, Australia, New Zealand, the UK, Japan and other temperate and developed countries, mosquito bites are now mostly an irritating nuisance; but still cause some deaths each year.^[16] Historically, before mosquito transmitted diseases were brought under control, they caused tens of thousands of deaths in these countries and hundreds of thousands of infections.^[17] Mosquitoes were shown to be the method by which yellow fever and malaria were transmitted from person to person by Walter Reed, William C. Gorgas and associates in the U.S. Army Medical Corps first in Cuba and then around the Panama Canal in the early 1900s.^[18][19] Since then other diseases have been shown to be transmitted the same way.

The mosquito genus Anopheles carries the malaria parasite (see Plasmodium). Worldwide, malaria is a leading cause of premature mortality, particularly in children under the age of five, with around 5.3 million deaths annually, according to the Centers for Disease Control. Most species of mosquito can carry the filariasis worm, a parasite that causes a disfiguring condition (often referred to as elephantiasis) characterized by a great swelling of several parts of the body; worldwide, around 40 million people are living with a filariasis disability. The viral diseases yellow fever and dengue fever are transmitted mostly by Aedes aegypti mosquitoes. Other viral diseases like epidemic polyarthritis, Rift Valley fever, Ross River Fever, St. Louis encephalitis, West Nile virus (WNV), Japanese encephalitis, LaCross encephalitis and several other encephalitis type diseases are carried by several different mosquitoes. Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) occurs in the United States where it causes disease in humans, horses, and some bird species. Because of the high mortality rate, EEE and WEE are regarded as two of the most serious mosquito-borne diseases in the United States. Symptoms range from mild flu-like illness to encephalitis, coma and death.^[20] Viruses carried by arthropods such as mosquitoes or ticks are known collectively as arboviruses. West Nile virus was accidentally introduced into the United States in 1999 and by 2003 had spread to almost every state with over 3,000 cases in 2006.

A mosquito's period of feeding is often undetected; the bite only becomes apparent because of the immune reaction it provokes. When a mosquito bites a human, she injects saliva and anti-coagulants. For any given individual, with the initial bite there is no reaction but with subsequent bites the body's immune system develops antibodies and a bite becomes inflamed and itchy within 24 hours. This is the usual reaction in young children. With more bites, the sensitivity of the human immune system increases, and an itchy red hive appears in minutes where the immune response has broken capillary blood vessels and fluid has collected under the skin. This type of reaction is common in older children and adults. Some adults can become desensitized to mosquitoes and have little or no reaction to their bites, while others can become hyper-sensitive with bites causing blistering, bruising, and large inflammatory reactions.

Mosquito control and integrated mosquito management

Main article: Mosquito control

There are two kinds of mosquito control: large, organized programs to reduce mosquito populations over a wide area, and actions individuals can take to control mosquitoes with respect to themselves and their own property.

Organized mosquito control programs today draw on the principles of integrated pest management. An integrated mosquito control program typically includes the following measures, all guided by surveillance of mosquito populations and knowledge of the mosquito life cycle:^[21]

• source reduction - the removal of mosquito breeding habitats
• habitat modification - manipulating habitats to reduce breeding
• biocontrol - introducing natural predators of mosquitoes
• larvicide - using pesticides to reduce larval populations
• adulticide - using pesticides to reduce adult populations

Some solutions for malaria control efforts in the third world are: mosquito nets (klamboe), mosquito nets treated with insecticide (often permethrin), and DDT.^[22] Nets are treated with insecticide because mosquitoes can sometimes get past an imperfect net. Insecticide-treated nets (ITN) are estimated to be twice as effective as untreated nets in preventing mosquito bites.^[23] Untreated mosquito nets are less expensive, and they are effective in protecting humans when the nets do not have any holes and are tightly sealed around the edges. Insecticide free nets do not adversely affect the health of natural predators such as dragonflies.

The role of DDT in combating mosquitoes has been the subject of considerable controversy. While some argue that DDT deeply damages biodiversity, others argue that DDT is the most effective weapon in combating mosquitoes and hence malaria. While some of this disagreement is based on differences in the extent to which disease control is valued as opposed to the value of biodiversity, there is also genuine disagreement amongst experts about the costs and benefits of using DDT. Moreover, DDT-resistant mosquitoes have started to increase in numbers, especially in tropics due to mutations, reducing the effectiveness of this chemical.

Mosquito repellents and personal mosquito control

A screened mosquito-proof room or house is one of the best and safest ways to sleep and still get ventilation for cooling. Mosquito netting if properly used and maintained (no holes), provides the maximum possible personal protection against biting insects. When not in use, mosquito nets are compact and light-weight — about 0.5 kg. They can be used under almost any conditions to provide reliable protection against a wide range of biting insects. In many areas of the world, biting insects are not only a nuisance, but also pose a serious health threat. Sleeping under a bednet is highly recommended by the World Health Organization (WHO)^[24] and the U.S. Center for Disease Control (CDC)^[25] if staying in these areas.

One of the best treatments of mosquito repellents is N,N-diethyl-meta-toluamide, commonly known as DEET. It has been used widely since its invention by the U.S. Department of Agriculture in 1945. It is safe to use as directed,and should be removed by soap and water after use. DEET products have been widely used for many years but these products have occasionally been associated with some adverse reactions. DEET concentrations range from a low of about five percent up to 100 percent.

Other mosquito repellents do work but typically not as effectively. Examples include: catnip oil extract, nepetalactone (no known credible tests), citronella 10% solution (84% effective for about 1 hour), or eucalyptus oil extract.^[26] Soy bean oil (in Bite Blocker for Kids) worked for about 1 ½ hours and Repel’s plant-based lemon eucalyptus solution worked for about 3 hours.

Oils of Syzygium aromaticum (clove) and Zanthoxylum limonella (makaen), widely used essential oils for dental caries or flavoring of food in Thailand, were prepared as 10 experimental repellent products in gel or cream form against Aedes aegypti, Culex quinquefasciatus, and Anopheles dirus under laboratory conditions, using the human-arm-in-cage method. Two products that gave the longest-lasting complete protection were selected to examine their repellency against a variety of mosquito species under field conditions. In laboratory tests, 0.1 g of each product was applied to 3x10 cm of exposed area on a volunteer's forearm, while in field trials, 1.0 g was applied to each volunteer's leg (from knee to ankle). In the laboratory, the gel dosage form contained 20% clove oil (Gel B) or 10% clove plus 10% makaen oil mixture (Gel E) were promising plant-based repellents against three mosquito species and gave significantly longer complete protection times of 4-5 hours than all other developing products. Therefore, their efficacy in the field was evaluated. Under field conditions, Gel E showed complete protection for 4 hours and gave 95.7% repellency after 5 hours application, whereas Gel B and 20% deet (di-methyl benzamide) provided only 86.8 and 82.7% repellency after treatment, respectively against Ae. aegypti, daytime-biting mosquitos. For nighttime-biting, the 3 repellents under development yielded equally excellent (average 97.1%) repellency for 5 hours against the predominant Cx. quinquefasciatus and Mansonia uniformis, but they gave 89.0% repellency against Cx. tritaeniorhynchus and Cx. gelidus. This finding demonstrated the effectiveness of Gel B and Gel E products for possible use by low-income rural communities against various mosquito species.

Picaridin, first used in Europe in 2001, has been reported to be effective by Consumer Reports (7% solution)^[27] and the Australian Army (20% solution).^[28] Consumer Report retests in 2006 show that a 7% solution of picaridin now has a protection time of about 0 minutes and a 15% solution was only good for about one hour.^[29] So far DEET is the champion effective repellent against mosquitoes, especially when worn in conjunction with light coloured clothing, long sleeved pants and shirts and a hat.

Other commercial products offered for household mosquito "control" include small electrical mats, mosquito repellent vapor, DEET-impregnated wrist bands, and mosquito coils containing a form of the chemical allethrin. Mosquito-repellent candles containing citronella oil are sold widely in the U.S. All of these have been used with mixed reports of success and failure. Some claim that plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) will act against mosquitoes. However, scientists have determined that these plants are “effective” for a limited time only when the leaves are crushed and applied directly to the skin.^[30]

There are several, widespread, unproven theories about mosquito control such as the assertion that Vitamin B, in particular B1 Thiamine, garlic, ultrasonic devices, incense, can be used to repel or control mosquitoes.^{[31] [32]} Moreover, some manufacturers of "mosquito repelling" ultrasonic devices have been found to be fraudulent,^[33] and their devices were deemed "useless" in tests by the UK Consumer magazine Which?^[34]  

The Dragonfly eats mosquitos at all stages of development and is quite effective in controlling populations. 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. Bats are known carriers of rabies, and neither they nor Purple Martins are known to control or even significantly reduce mosquito populations.^{[citation needed]}

Similarly, bug zappers kill a wide range of flying insects including many beneficial insects that eat mosquitoes as well as some mosquitoes. Bug zappers have not been proven effective at controlling mosquito populations.

Some newer mosquito traps or known mosquito attractants emit a plume of carbon dioxide together with other mosquito attractants such as sugary scents, lactic acid, octenol, warmth, water vapor and sounds. By mimicking a mammal’s scent and outputs, female mosquitoes are drawn toward the trap, where they are typically sucked into a net or holder by an electric fan where they are collected. According to the American Mosquito Control Association,^[35] "these devices will, indeed, trap and kill measurable numbers of mosquitoes," but their effectiveness in any particular case will depend on a number of factors such as the size and species of the mosquito population and the type and location of the breeding habitat. They are useful in specimen collection studies to determine the types of mosquitoes prevalent in an area but are typically far too inefficient to be useful in reducing mosquito populations.

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. These are usually orally or topically applied antihistamines and, for more severe cases, corticosteroids such as hydrocortisone and triamcinolone. Many home remedies are effective against itching, including calamine lotion, baking soda, rubbing alcohol, and vinegar. Ammonia is another ingredient in commercial mosquito bite treatments (e.g. Afterbite). It is also possible to temporarily stop the itching by rubbing a fresh clove of garlic against the bite. Ammonia has been clinically demonstrated to be an effective treatment^[36].

Scratching and cooling are effective but bring relief for only a short time. Another proven remedy for the itch is applying heat directly to the bite. Common methods for applying heat treatment include running hot water from a faucet over the irritated skin or heating a mug of water in a microwave for 1 to 2 minutes and then holding the hot mug against the bite for at least 1 minute. Caution: only apply heat that is tolerable; be sure not to burn or blister the skin. Warning: Microwaving water may cause the water to be superheated, and become dangerous.

Cultural views

According to the “Mosquitoes” chapter in Kwaidan: Stories and Studies of Strange Things, by Lafcadio Hearn (1850–1904), mosquitoes are seen 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]

The Babylonian Talmud (Gittin 56b) asserts that the Roman Emperor Titus was punished by God for having destroyed the Temple in Jerusalem by having a mosquito fly into Titus' nose, picking at his brain, ceaselessly buzzing, driving him crazy and eventually causing his death. No such account appears in any Roman source, but it is quite well known that Titus died prematurely, after only two years in power, from unclear causes.

In contemporary culture, Mosquitoes are sometimes seen as a source of humor. For example, There's a Skeeter on My Peter is a well-known humorous song. Additionally, some individuals jokingly suggest that the mosquito is the state bird of Florida, Minnesota, or Alaska (see list of U.S. state birds for more information on this longstanding joke). Moreover, mosquito jokes constitute a distinct genre of humor.[1][2]

Systematics

• Subfamily Anophelinae

• Anopheles
• Bironella
• Chagasia

• Subfamily Culicinae

• Subfamily Toxorhynchitinae

• Toxorhynchites

Identification

• Brunhes, J.; Rhaim, A.; Geoffroy, B. et al. Les Moustiques de l'Afrique mediterraneene French/English. Interactive identification guide to mosquitoes of North Africa, with database of information on morphology, ecology, epidemiology, and control. Mac/PC Numerous illustrations. IRD/IPT [12640] 2000 CD-ROM

See also

• Insect repellent
• Cymbopogon
• Mosquito net
• DEET
• Anti-itch drug
• Malaria

References