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Mosquito

From Wikipedia, the free encyclopedia
This article is about the insect. For other uses, see Mosquito (disambiguation).
Mosquito
Mosquito 2007-2.jpg
A female mosquito Culiseta longiareolata
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Suborder: Nematocera
Infraorder: Culicomorpha
Superfamily: Culicoidea
Family: Culicidae
Meigen, 1818 [1]
Subfamilies
Diversity
41 genera

Mosquitoes are a family of small, midge-like flies: the Culicidae. Although a few species are harmless or even useful to humanity, most are considered a nuisance because they use their salivation to coat the surface of the skin of living mammals, including humans, to consume their blood, which causes itchy, often times painful red bumps to appear. The females of many species of mosquitoes are blood-eatingpests. In feeding on blood, some of them transmit extremely harmful human andlivestock diseases, such as malaria, yellow fever, and filariasis.[2]

Life cycle[edit]

Anatomy of a Culex larva

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

Like all flies, mosquitoes go through four stages in their lifecycles: egg, larva, pupa, and adult or imago. In most species, adult females lay their eggs in stagnant 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 and does so according to its own ecological adaptations. Some are generalists and are not very fussy. Some breed in lakes, some in temporary puddles. Some breed in marshes, some in salt-marshes. Among those that breed in salt water, some are equally at home in fresh and salt water up to about one-third the concentration of seawater, whereas others must acclimatize themselves to the salinity.[13] Such differences are important because certain ecological preferences keep mosquitoes away from most humans, whereas other preferences bring them right into houses at night.

Some species of mosquitoes prefer to breed in phytotelmata (natural reservoirs on plants), such as rainwater accumulated in holes in tree trunks, or in the leaf-axils of bromeliads. Some specialize in the liquid in pitchers of particular species ofpitcher plants, their larvae feeding on decaying insects that had drowned there or on the associated bacteria; the genusWyeomyia provides such examples — the harmless Wyeomyia smithii breeds only in the pitchers of Sarracenia purpurea.[14]

However, some species of mosquitoes that are adapted to breeding in phytotelmata are dangerous disease vectors. In nature, they might occupy anything from a hollow tree trunk to a cupped leaf. Such species typically take readily to breeding in artificial water containers, such as the odd plastic bucket, flowerpot “saucer”, or discarded bottle or tire. Such casual puddles are important breeding places for some of the most serious disease vectors, such as species of Aedes that transmit dengue and yellow fever. Some with such breeding habits are disproportionately important vectors because they are well-placed to pick up pathogens from humans and pass them on. In contrast, no matter how voracious, mosquitoes that breed and feed mainly in remote wetlands and salt marshes may well remain uninfected, and if they do happen to become infected with a relevant pathogen, might seldom encounter humans to infect, in turn.

The first three stages—egg, larva, and pupa—are largely aquatic. These stages typically last five to 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 ofAedes remain unharmed in diapause if they dry out, and hatch later when they are covered by water.

Eggs hatch to become larvae, which grow until they are able to change into pupae. The adult mosquito emerges from the mature pupa as it floats at the water surface. Bloodsucking mosquitoes, depending on species, gender, and weather conditions, have potential adult lifespans ranging from as short as a week to as long as several months.

Some species can overwinter as adults in diapause.[15][16]

Eggs and oviposition[edit]

Mosquito habits of oviposition, the ways in which they lay their eggs, vary considerably between species, and themorphologies 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 bobbing behavior occurs among some other aquatic insects as well, for example mayflies and dragonflies; it is sometimes called “dapping“. The eggs of Anopheles species are roughly cigar-shaped and have floats down their sides. Females of many common species can lay 100–200 eggs during the course of the adult phase of their lifecycles. Even with high egg and intergenerational mortality, over a period of several weeks, a single successful breeding pair can create a population of thousands.

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

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 may on occasion cluster together on the water, too, but the clusters do not generally look much like compactly glued rafts of eggs.

In species that lay their eggs in rafts, rafts do not form adventitiously; the femaleCulex 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.[17]

Aedes females generally drop their eggs singly, much as Anopheles do, but not as a rule into water. Instead, they lay their eggs on damp mud or other surfaces near the water’s edge. Such an oviposition site commonly is the wall of a cavity such as a hollow stump or a container such as a bucket or a 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 are not resistant to desiccation straight after oviposition, but must develop to a suitable degree first. Once they have achieved that, however, they can enter diapause for several months if they dry out. Clutches of eggs of the majority of mosquito species hatch as soon as possible, and all the eggs in the clutch hatch at much the same time. In contrast, a batch of Aedes eggs in diapause tends to hatch irregularly over an extended period of time. This makes it much more difficult to control such species than those mosquitoes whose larvae can be killed all together as they hatch. Some Anopheles species do also behave in such a manner, though not to the same degree of sophistication.[18]

Larva[edit]

Mosquito larvae and pupa resting at water surface

The mosquito larva has a well-developed head with mouth brushes used for feeding, a large thorax with no legs, and a segmented abdomen.

Anopheles larva from southern Germany, about 8 mm long

Larvae breathe through spiracles located on their eighth abdominal segments, or through a siphon, so must come to the surface frequently. The larvae spend most of their time feeding on algae, bacteria, and other microbes in the surface microlayer.

Aedes aegypti larva

They dive below the surface only when disturbed. Larvae swim either throughpropulsion with their mouth brushes, or by jerky movements of their entire bodies, giving them the common name of “wigglers” or “wrigglers”.

Larvae develop through four stages, or instars, after which they metamorphose intopupae. At the end of each instar, the larvae molt, shedding their skins to allow for further growth.

Pupa[edit]

Culex larva and pupa

As seen in its lateral aspect, the mosquito pupa is comma-shaped. The head and thorax are merged into a cephalothorax, with the abdomen curving around underneath. The pupa can swim actively by flipping its abdomen, and it is commonly called a “tumbler” because of its swimming action. As with the larva, the pupa of most species must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on their cephalothoraces. However, pupae do not feed during this stage; typically they pass their time hanging from the surface of the water by their respiratory trumpets. If alarmed, say by a passing shadow, they nimbly swim downwards by flipping their abdomens in much the same way as the larvae do. If undisturbed, they soon float up again.

Culex larvae plus one pupa

After a few days or longer, depending on the temperature and other circumstances, the pupa rises to the water surface, the dorsal surface of its cephalothorax splits, and the adult mosquito emerges. The lower activity of the pupa compared to the larva is understandable, bearing in mind that it does not feed, whereas the larva feeds constantly.[17]

Adult[edit]

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 few 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.

Anatomy of an adult mosquito

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 5-7 days, feeding on nectar and other sources ofsugar. 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 two to three 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 one to two weeks in nature. Their lifespans depend on temperature, humidity, and 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),[19] and weight up to 2.5 milligrams (0.04 grains). All mosquitoes have slender bodies with three segments: head, thorax and abdomen.

The head is specialized for receiving sensory information and for feeding. It has eyes and a pair of long, many-segmentedantennae. 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 females.

Adult yellow fever mosquito Aedes aegypti, typical of subfamily Culicinae. Note bushy antennae and longer palpsof male on left vs. females at right.

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.[20] 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.[20]

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 proboscises, 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 1–2 km/h (0.6–1 mph),[21] traveling up to 12 km (7.5 mi) in a night. Males beat their wings between 450 and 600 times per second.[22]

The abdomen is specialized for food digestion and egg development; the abdomen of a mosquito can hold three times its own weight in blood.[23] This segment 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.

Disease[edit]

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.

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

  • Viral diseases, such as yellow fever, dengue fever and chikungunya, transmitted mostly by Aedes aegypti. Dengue fever is the most common cause of fever in travelers returning from the Caribbean, Central America, South America, and South Central Asia. This disease is spread through the bites of infected mosquitoes and cannot be spread person to person.

Severe dengue can be fatal, but with good treatment, less than 1% of patients die from dengue.[61]

Potential transmission of HIV was originally a public health concern, but practical considerations and detailed studies of epidemiological patterns suggest that any transmission of the HIV virus by mosquitoes is at worst extremely unlikely.[64]

Various species of mosquitoes are estimated to transmit various types of disease to more than 700 million people annually in Africa, South America, Central America, Mexico, Russia, and much of Asia, with millions of resultant deaths. At least two million people annually die of these diseases, and the morbidity rates are many times higher still.

Methods used to prevent the spread of disease, or to protect individuals in areas where disease is endemic, include:

Since most such diseases are carried by “elderly” female mosquitoes, some scientists have suggested focusing on these to avoid the evolution of resistance.[65]

Control[edit]

Main article: Mosquito control

Many methods are used for mosquito control. Depending on the situation, the most important usually include:

  • 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
  • exclusion (mosquito nets and window screening)

Source reduction[edit]

World War II era pamphlet aimed to discourage creation of stagnant water

“Source reduction” means elimination of breeding places of mosquitoes. It includes engineering measures such as filling, leveling and drainage of breeding places, and water management (such as intermittent irrigation). Source reduction can also be done by making water unsuitable for mosquitoes to breed, for example, by changing salinity of water. Some specific measures are:

  • For Culex: abolition of domestic and peridomestic sources of water suitable for breeding, for example removal and disposal of sewage and other waste water
  • For Aedes: eliminating incidental containers such as discarded tins, crockery, pots, broken bottles, and coconut shells
  • For Anopheles: abolish breeding places by filling or drainage
  • For Mansonia: removal of aquatic plants manually or by application of herbicides

Details of the biology of different species of mosquitoes differ too widely for any limited set of rules to be sufficient in all circumstances. However, the foregoing are the most economical and practical measures for most purposes. The importance of peridomestic control arises largely because most species of mosquitoes rarely travel more than a few hundred meters unless the wind is favorable.

Exclusion[edit]

In combination with scrupulous attention to control of breeding areas, window screens and mosquito nets are the most effective measures for residential areas. Insecticide-impregnated mosquito nets are particularly effective because they selectively kill those insects that attack humans, without affecting the general ecology of the area.

An ideal mosquito net is white in color (to allow easy detection of mosquitoes), rectangular, netted on sides and top, without a hole. The size of opening in net should not exceed 1.2 mm (0.05 in) in diameter, or about 23 holes per square centimeter (150 per square inch). Window screens should have copper or bronze gauze with 16 wires per inch.

Biocontrol[edit]

Gambusia affinis (Mosquitofish), a natural mosquito predator.

Biological control or “biocontrol” is the use of natural enemies to manage mosquito populations. There are several types of biological control including the direct introduction of parasites, pathogens and predators to target mosquitoes.

Fish[edit]

Various small fishes, such as species of Galaxias and members of the Poeciliidae, such as Gambusia (so-called mosquitofish), guppies (Poecilia), and Banded killifish(Fundulus diaphanus), eat mosquito larvae and sometimes may be worth introducing into ponds to assist in control.[66] Some cyprinids (carps and minnows) and tilapia also consume mosquito larvae.[67] Many other types of fish consume mosquito larvae, including bass, bluegills, piranhas, Arctic char, salmon, trout,catfish, fathead minnows and goldfish.[68]

Some cyclopoid copepods are predators on first-instar larvae, killing up to 40 Aedes larvae per day.[69]

Others[edit]

Other predators include dragonfly naiads, which consume mosquito larvae in the breeding waters, adult dragonflies, which eat adult mosquitoes and some species of lizard and gecko.[70] Biocontrol agents that have had lesser degrees of success include the predator mosquito Toxorhynchites and predator crustaceansMesocyclops copepods,[71] nematodes andfungi.[72] Predators such as birds, bats, lizards and frogs, have been used, but their effectiveness is only anecdotal.

Dead spores of the soil bacterium Bacillus thuringiensis, especially Bt israelensis (BTI) interfere with larval digestivesystems. It can be dispersed by hand or dropped by helicopter in large areas.

Two species of fungi can kill adult mosquitoes: Metarhizium anisopliae and Beauveria bassiana.[73][74] as can nematodes.[75] Though important at times, their effectiveness varies with circumstances.

Reproduction[edit]

Introducing large numbers of sterile males is another approach to reducing mosquito numbers.[76]

Experimental genetic methods including cytoplasmic incompatibility, chromosomal translocations, sex distortion and gene replacement have been explored. They are cheaper and not subject to vector resistance.[77] Larvae of the non-bitingToxorhynchites mosquitoes also are natural predators of other Culicidae. Each larva can eat 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).[78][79] However, Toxorhynchites can consume all types of prey, organic debris, or even exhibit cannibalistic behavior.

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

Repellents[edit]

Main article: Insect repellent

Insect repellents are applied on skin and give short-term protection against mosquito bites. The chemical DEET repels some mosquitoes and other insects.[80] Some CDC-recommended repellents are picaridin, eucalyptus oil (PMD) andIR3535.[81] Others are indalone, dimethyl pthalate, dimethyl carbate, and ethyl hexanediol.

There are also electronic insect repellent devices which produce ultrasounds that were developed to keep away insects (and mosquitoes). However, no scientific research based on the EPA’s and many universities’ studies has ever sought evidence that these devices prevent a human from getting bitten by a mosquito bite.[82]