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Mosquito
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Mosquitoes are common flying insects in the family Culicidae that are found around the world. There are about 3,500 species. They have a pair of scaled wings, a pair of halteres, a slender body, and six long legs. The females of most mosquito species suck blood (hematophagy) from other animals, which has made them the most deadly disease vector known, killing millions of people over thousands of years and continuing to kill millions per year by the spread of infectious diseases.
uitoes go through four stages in their life cycle: egg, larva, pupa, and adult or imago.
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Mosquitoes are common flying insects in the family Culicidae that are found around the world. There are about 3,500 species. They have a pair of scaled wings, a pair of halteres, a slender body, and six long legs. The females of most mosquito species suck blood (hematophagy) from other animals, which has made them the most deadly disease vector known, killing millions of people over thousands of years and continuing to kill millions per year by the spread of infectious diseases.
Development
Mosquitoes go through four stages in their life cycle: egg, larva, pupa, and adult or imago. The first three stages are aquatic and last 5-14 days, depending on the species and the ambient temperature. 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.
Egg
After drinking blood, adult females can lay a raft of 50-300 eggs per oviposition. Anopheles, Ochlerotatus and Aedes, as well as many other genera, do not make egg rafts but lay eggs singly. Culex, Culiseta, and Anopheles lay eggs directly on the water surface and Anopheles are unique in having floats on either side. Eggs are not resistant to drying and hatch within 2-3 days, although hatching may take up to 2-3 weeks in colder climates. In other mosquitos such as Aedes, the female lays her eggs on damp soil that will be flooded by water, typically near a stagnant pool. However, any open container containing water will suffice for larvae development, even with less than an ounce of water in. Aedes can also breed in running water, so stagnant pools of water are not its only breeding sites. With Haemagogus as the adults tend to live in the canopy of forests, the female lays eggs in between layers of tree bark or in cut bamboo. The eggs adhere to the surface and when submerged by rain water quickly develop into larva.
Larvae
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 8th abdominal segment and therefore must come to the surface frequently. The larvae spend most of their time feeding on algae, bacteria, and other microorganisms in the surface microlayer. They dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion with the mouth brushes.
Larvae develop through 4 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 in Anopheles when viewed from the side. 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. After a few days as a pupa, the dorsal surface of the cephalothorax splits and the adult mosquito emerges.
Adult
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 5 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.
Morphology
Length varies but is rarely greater than , 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 travelling up to in a night.
All mosquitoes have slender bodies with 3 sections: head, thorax and abdomen.
Head
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. 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 proboscis used for feeding, and two sensory palps. The maxillary palps of the males are longer than their proboscises 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.
Thorax
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. One pair are found on the second thoracic segment (the mesothorax). The posterior pair of wings is reduced to halteres, which help to sense orientation and movement, and improve balance by acting similar to gyroscopes. The scutum is the dorsal portion of an insect’s thoracic segment. On the side of the thorax is the scutellum.
Abdomen
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
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.
Mosquitos 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. Mosquitos are adept at infiltration and have been known to find their way into residences via deactivated air conditioning units.
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.
Mosquitoes of the genus Toxorhynchites never drink blood. This genus includes the largest extant mosquitoes, the larvae of which prey on the larvae of other mosquitoes. These mosquito eaters have been used in the past as mosquito control agents, with varying success.
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 complex mixture of secreted proteins. Mosquito saliva affects vascular constriction, blood clotting, platelet aggregation, inflammation, immunity, and angiogenesis. Universally, hematophagous arthropod saliva contains at least one anticlotting, one anti-platelet, and one vasodilatory substance. Mosquito saliva also contains enzymes that aid in sugar feeding and antimicrobial agents to control bacterial growth in the sugar meal. The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins. Despite the great strides in knowledge of these molecules and their role in bloodfeeding achieved recently, scientists still cannot ascribe functions to more than half of the molecules found in arthropod saliva.
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. 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. 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. Unexpectedly, this shift in cytokine expression is observed in splenocytes up to 10 days after mosquito exposure, suggesting that natural feeding of mosquitoes can have a profound, enduring, and systemic effect on the immune response.
T cell populations are decidedly susceptible to the suppressive effect of mosquito saliva, showing enhanced mortality and decreased division rates. Parallel work by Wasserman et al. (2004) demonstrated that T- and B-cell proliferation was inhibited in a dose dependent manner with concentrations as low as 1/7th of the saliva in a single mosquito. Depinay et al. (2005) observed a suppression of antibody-specific T cell responses mediated by mosquito saliva and dependent on mast cells and IL-10 expression.
A recent study suggests that mosquito saliva can also decrease expression of interferon-a/ß during early mosquito-borne virus infection. The contribution of type I interferons (IFN) in recovery from infection with viruses has been demonstrated in vivo by the therapeutic and prophylactic effects of administration of IFN-inducers or IFN, and recent research suggests that mosquito saliva exacerbates West Nile virus infection, as well as other mosquito-transmitted viruses.
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 h after feeding, and subsequently returns to baseline levels by 60 h. 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 h after the blood meal, subsequently falling to baseline levels by 60 h. 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 h after the blood meal, then rises rapidly to a maximum at 30 h 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.
Distribution
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. They can even tolerate snow and temperatures under freezing. In addition, adults can survive throughout winter in suitable microhabitats.
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.
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.
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.
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.
Control
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.
Natural predators
The dragonfly eats mosquitoes 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. Neither bats nor Purple Martins are known to control or even significantly reduce mosquito populations.
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.
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 and recipes exist, most of which are effective against itching, including calamine lotion, baking soda, salt, rubbing alcohol, vinegar, and toothpaste. Also rubbing nail polish (preferably clear) on the bite will stop it from itching most of the time. Ammonia has been clinically demonstrated to be an effective treatment.
Cold or heat treatments, such as using running water, may be effective in reducing the itching sensation but bring relief mainly during their application. Scratching a mosquito bite may also provide temporary relief but will usually serve to irritate and inflame the area further and increase the risk of infection and scarring.
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".
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. Titus died prematurely from unclear causes after only two years in power.
According to Islamic legends Nimrod was punished by Allah by mosquitoes. A mosquito entered his head and the humming sound disturbed him so much that he ordered one of his guards to hit his head with a stick. After few days, the guard became wary and hit him hard on the head, causing it to split, and the mosquito escaped.
Evolution
The Culicinae and Anopheles clades diverged about ~150 million years ago. The Old and New World Anopheles species 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
Identification
- Brunhes, J.; Rhaim, A.; Geoffroy, B. Angel G. Hervy P. Les Moustiques de l'Afrique mediterranéenne 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. ISBN 2-7099-1446-8 Mosquito species can also be identified through their DNA, however this is relatively expensive so it is not commonly performed. See the Use of DNA in forensic entomology.
External links
- chapter in United States Environmental Protection Agency? and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
- - Information on West Nile virus as well as other mosquito- and tick- bourne diseases.
- - A mosquito identification key, along with other helpful information.
- - Links to the online mosquito catalog, keys for mosquito identification, images and information on medically important species and much more.
- Florida and National information on the UF/IFAS Pest Alert Web site
- on the UF/IFAS Featured Creatures Web site
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