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Mosquito

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Species

Mosquitoes are members of a family of nematocerid flies: the Culicidae (from the Latin culex, genitive culicis, meaning "midge" or "gnat"). Superficially, mosquitoes resemble crane flies (family Tipulidae) and chironomid flies (family Chironomidae). In particular, the females of many species of mosquitoes are blood-eating pests and dangerous vectors of diseases, whereas members of the similar-looking Chironomidae and Tipulidae are not. Many species of mosquitoes are not blood eaters and of those that are, many create a "high to low pressure" in the blood to obtain it and do not transmit disease. Also, in the bloodsucking species, only the females suck blood. Furthermore, even among mosquitoes that do carry important diseases, neither all species of mosquitoes, nor all strains of a given species transmit the same kinds of diseases, nor do they all transmit the diseases under the same circumstances; their habits differ. For example, some species attack people in houses, and others prefer to attack people walking in forests. Accordingly, in managing public health, knowing which species, even which strains, of mosquitoes with which one is dealing is important.

Over 3,500 species of mosquitoes have already been described from various parts of the world. Some mosquitoes that bite humans routinely act as vectors for a number of infectious diseases affecting millions of people per year. Others that do not routinely bite humans, but are the vectors for animal diseases, may become disastrous agents for zoonosis of new diseases when their habitats are disturbed, for instance by sudden deforestation

Life cycle

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. 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 of pitcher plants, their larvae feeding on decaying insects that had drowned there or on the associated bacteria; the genus Wyeomyia provides such examples — the harmless Wyeomyia smithii breeds only in the pitchers of Sarracenia purpurea.

However, some of the 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 of Aedes 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.


Eggs and oviposition

Mosquito habits of oviposition, the ways in which they lay their eggs, vary considerably between species, and the morphologies 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. 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 female Culex 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.

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


Larva

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 through propulsion 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 into pupae. At the end of each instar, the larvae molt, shedding their skins to allow for further growth.

Pupa

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 pupa is less active than the larva because it does not feed, whereas the larva feeds constantly.

Adult

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. 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 of sugar. 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.

The length of the adult varies, but is rarely greater than 16 mm (0.6 in), and it weighs up to 2.5 milligrams (0.04 grains). All mosquitoes have slender bodies with three segments: a head, a thorax and an abdomen.

The head is specialized for receiving sensory information and for feeding. It has 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 females. 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.[18] 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-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 to 2 km/h (0.6–1 mph), traveling up to 12 km (7.5 mi) in a night. Males beat their wings between 450 and 600 times per second.

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

Feeding by adults

A mosquito has a variety of ways of finding their prey, including chemical, visual, and heat sensors. Typically, both male and female mosquitoes feed on nectar and plant juices, but in many species the mouthparts of the females are adapted for piercing the skin of animal hosts and sucking their blood as ectoparasites. In many species, the female needs to obtain nutrients from a blood meal before she can produce eggs, whereas in many other species, she can produce more eggs after a blood meal. The feeding preferences of mosquitoes include those with type 0 blood, heavy breathers, those with a lot of skin bacteria, people with a lot of body heat, and the pregnant. Both plant materials and blood are useful sources of energy in the form of sugars, and blood also supplies more concentrated nutrients, such as lipids, but the most important function of blood meals is to obtain proteins as materials for egg production.

The strategy of only females risking their lives on blood sucking is not limited to mosquitoes; it also occurs in some other insect families, such as the Tabanidae. When a female reproduces without such parasitic meals, she is said to practice autogenous reproduction, as in Toxorhynchites; otherwise, the reproduction may be termed anautogenous, as occurs in mosquito species that serve as disease vectors, particularly Anopheles and some of the most important disease vectors in the genus Aedes. In contrast, some mosquitoes, for example, many Culex, are partially anautogenous; they do not need a blood meal for their first cycle of egg production, which they produce autogenously; however, subsequent clutches of eggs are produced anautogenously, at which point their disease vectoring activity becomes operative. With regard to host location, female mosquitoes hunt their blood host by detecting organic substances such as carbon dioxide (CO2) and 1-octen-3-ol produced from the host, and through optical recognition. Mosquitoes prefer some people over others. The preferred victim's sweat simply smells better than others because of the proportions of the carbon dioxide, octenol and other compounds that make up body odor. The most powerful semiochemical that triggers the keen sense of smell of Culex quinquefasciatus is nonanal. Another compound identified in human blood that attracts mosquitoes is sulcatone or 6-methyl-5-hepten-2-one, especially for Aedes aegypti mosquitoes with the odor receptor gene Or4. A large part of the mosquito’s sense of smell, or olfactory system, is devoted to sniffing out blood sources. Of 72 types of odor receptors on its antennae, at least 27 are tuned to detect chemicals found in perspiration. 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.

Most mosquito species are crepuscular (dawn or dusk) feeders. During the heat of the day, most mosquitoes rest in a cool place and wait for the evenings, although they may still bite if disturbed.

Prior to and during blood feeding, blood-sucking mosquitoes inject saliva into the bodies of their source(s) of blood. This saliva serves as an anticoagulant; without it one might expect the female mosquito's proboscis to become clogged with blood clots. The saliva also is the main route by which mosquito physiology offers passenger pathogens access to the hosts' interior. The salivary glands are a major target to most pathogens, whence they find their way into the host via the stream of saliva.

The bump left on the victim's skin after a mosquito bites is called a wheal, which is caused by histamines trying to fight off the protein left by the attacking insect.

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.

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