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Mosquito

The mosquito is a member of the family Culicidae. These insects have a pair of scaled wings, a pair of halteres, a slender body, and long legs. Only the females of most mosquito species suck blood from other animals. Size varies but is rarely greater than 0.6 inch (15 mm). Mosquitoes weigh only about 0.03 to 0.04 grain (2 to 2.5 mg). They can fly at about 0.9 to 1.6 mph (1.5 to 2.5 km/h) and most species are nocturnal.

Mosquitoes are believed to have evolved 170 million years ago during the Jurassic era (206 – 135 million years ago) with the earliest known fossils from the Cretaceous era (144 ““ 65 million years ago). They evolved in the land mass that is now South America, spreading initially to the northern continent Laurasia and re entering the tropics from the north. Ancestral mosquitoes were 3 times the size of the extant species and they are a sister group to the Chaoboridae (biting midges).

The family Culicidae belongs to the order Diptera and contains 3500 species in three subfamilies: Anophelinae (3 genera), the Culicinae (9 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 family Anophelinae six subgenera are recognized: Stethomyia, Lophopodomyia, Kerteszia, Nyssorhynchus (all South American), Cellia (Old World only) and Anopheles (worldwide).

Mosquitos are mainly nectar feeders with only the females requiring a blood meal. In contrast to this rule the Toxorhynchites never drinks blood. This family includes the largest of the extant mosquitoes and their larvae are predatory on the larvae of other mosquitoes. Attempts have been made in the past to use these as mosquito control agents but with variable success.

“Mosquito” is a Spanish word meaning little fly, and its use dates back to about 1583. Before then, they were called “biting flies” in English, but the term “mosquito” was adopted to prevent confusion with the house fly. The word derives from Sanskrit ‘maksh’ (fly) via the Latin word ‘musca’ (fly) and the Italian ‘moschetta’ or Spanish ‘mosquito’ (little fly). The French word is ‘moustique’.

The female mosquito (in almost all species) sucks the blood of mammals, including humans – commonly referred to (incorrectly) as a ‘bite.’ Mosquito bites often swell up hours after happening, causing a red ringed white bump about a centimeter in diameter. This bump can itch for days and over-scratching the bite can cause it to bleed. Mosquito bites can transmit diseases, such as malaria and West Nile Virus, so authorities in many areas take measures to reduce mosquito populations through pesticides or more organic means. An easy way to reduce mosquito populations in a residential area is the removal of standing water (where mosquitoes breed), and the use of repellents, such as citronella candles.

Natural history

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. The females require protein for egg development, and since the normal mosquito diet consists of nectar and fruit juice, which has no protein, most must drink blood to get the necessary protein. Males differ from females, with mouth parts not suitable for blood sucking.

The mosquito undergoes complete metamorphosis, i.e. it goes through four distinct stages in its life cycle: egg, larva, pupa, and adult – a process that was first described by the Greek philosopher Aristotle. The length of the first three stages is species – and temperature-dependent. 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 or up to one month. The larvae are the “wrigglers” or “wigglers” found in puddles or water-filled containers. These breathe atmospheric oxygen through a siphon at the tail end. The pupae are nearly as active as the larvae, but breathe through thoracic “horns” attached to the thoracic spiracles. Most larvae feed on microorganisms, 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.

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

The females of blood sucking species locate their victims primarily through scent. They are extremely sensitive to the carbon dioxide in exhaled breath, as well as several substances found in sweat. Some people seem to attract mosquitoes more than others. Empirical studies of mosquito bites suggest that the risk of being bitten follows an approximately negative binomial distribution. Being male, being overweight, and having type ‘O’ blood may increase the risk of being bitten. Mosquitoes can detect heat, so they can find warm-blooded mammals and birds very easily once they get close enough.

Mosquitoes and health

Some mosquitoes are capable of transmitting protozoan diseases such as malaria, filarial diseases like filariasis, and viral diseases such as yellow fever, dengue, encephalitis, and West Nile virus.

West Nile virus was accidentally introduced into the United States in 1999 and by 2003 had spread to almost every state. Through the transmission of such diseases, it can be argued that mosquitoes have caused more human deaths than any other animal.

When a mosquito first bites a human, she injects saliva and anti-coagulants. When one is first bitten there is no reaction, but after several bites the body’s immune system becomes sensitized and an itchy red mark appears about a day after the bite. This is the usual reaction in young children.

After many 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 could possibly become desensitized to mosquitoes, and not have any reaction to their bites, but others can become hyper-sensitive with the bites causing large, painful, red welts.

Mosquitoes are also very irritating as they sometimes tend to fly around the ears of humans, their wingbeats coming within audible range of the human ear. This is especially more pronounced during the monsoon season in tropical climates, when the incidence of mosquitoes increases greatly. The buzzing noise wakes up people who would otherwise have not been disturbed by the mosquito bite alone. This has prompted many to stuff their ears with cotton to shut out the buzzing of the mosquitoes.

It is interesting to note that the tone produced by wings of most mosquitoes is coincident with that of a high G. This was discovered in 1889 by Alexander Graham Bell. Bell noticed that an electric generator behind a tent party was invisible beneath a carpet of male mosquitoes all attracted to the tone produced by a motor within the machine. Bell reasoned that this tone was identical to that of the female mosquito’s wings.

Mosquito control

There are many methods used for mosquito control. Some target the larval stage, while others are used to kill or repel adults. Much of modern mosquito control is no longer dependent on dangerous pesticides but specialized organisms that eat mosquitoes, or infect them with a disease that kills them. Such methods can even be used in Conservation Areas, like the “Forsyth refuge” and the Seaview Marriott Golf Resort, where some major mosquito control is performed and monitored using “killifish” and juvenile eels. The success is documented with most advanced underwater microscopes like the ecoSCOPE. However, outbreaks of human mosquito-borne diseases may still result in fogging with chemicals that are less toxic than those used in the past.

A few varieties of the natural soil bacteria Bacillus thuringiensis, especially Bt israelensis (Bti), are used to interfere in the digestion systems of larvae. It can be dispersed by hand or dropped by helicopter in large areas. Unfortunately Bti is no longer effective after the larvae turn into pupae, because they stop eating. At this point larviciding oils, such as Golden Bear, can be used which increase the water tension until the pupae and larvae cannot break the surface to obtain air and therefore drown. A chemical commonly used in the United States is methoprene, considered slightly toxic to larger animals, which mimics and interferes with natural growth hormones in mosquito larvae, preventing development. Methoprene is frequently distributed in time-release briquette form in breeding areas. Malathion has also been sprayed in metropolitan areas like New York City to decrease the mosquito population and prevent the spread of West Nile Virus.

Dragonflies, also known as mosquito hawks, are excellent control agents. Dragonfly naiads consume mosquito larvae in the breeding waters, and adult dragonflies eat adult mosquitoes, particularly the day flying Asian tiger mosquitoes. Fogging for adult mosquitoes can backfire and increase long term populations if it removes dragonflies and other natural controls. Lizards are also useful predators which eat mosquitoes indoors.

Mosquito repellants generally contain one of the following active ingredients: DEET, Catnip oil extract, nepetalactone, citronella, or eucalyptus oil extract. Often the best “repellant” is a fan or gentle breeze as mosquitoes do not like moving air.

The most effective solutions for malaria control efforts in the third world are: mosquito nets (klamboe), insecticide-laced mosquito nets, and DDT. Plain mosquito nets are cheap and are completely effective in protecting humans within the net. Also they do not adversely affect the health of natural predators such as dragonflies, and do not require sophisticated public health capacity on the part of the government. 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.

Other popular methods of household mosquito control are the use of small electrical mats plugged into a socket, mosquito repellent vapor and mosquito coil all having a form of allethrin – a chemical commonly used to combat mosquitoes and other pests in general. Mosquito repellant candles containing Citronella oil is another method to keep mosquitoes at bay. Some more lesser known methods use the cultivation of plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) which act against mosquitoes. However scientists have added that these plants are effective only when the leaves are crushed and used.

A newer approach to killing mosquitoes in a non-toxic way is to use a device that burns propane, thus generating carbon dioxide, warmth, and water vapor. These three elements, often coupled with a chemical attractant heated in this process, draws the mosquitoes toward the propane flame, where they are then sucked into a net or holder where they collect. Given that the typical mosquito’s lifespan ranges from days to weeks, simply trapping them long enough allows them to die without any further effort. One such product is marketed by the Coleman Company.

One of the unconventional methods of mosquito control is the use of electromagnetic, ultrasound devices designed to confuse mosquitoes. It has found use in a variety of devices including a downloadable software which is claimed to thwart the insects using PC speakers. However the effectiveness is questionable since computer speakers aren’t designed for high frequency sounds (15-20 KHz range).

Bats are an extremely effective form of natural mosquito control. One brown bat can consume 1200 mosquitoes in an hour. It’s easy to build a bat box to encourage bats to live nearby. One averaged size bat box can provide a home for dozens of bats. Care must be taken, however, to ensure no humans are bitten by the bats, as bats are the number-one vector for rabies, an always-fatal disease in the absence of immediate vaccination. The purple martin swallow (Progne subis) is not, however, a prodigious consumer of mosquitoes as is so often claimed by companies that manufacture martin housing.

Mosquito


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