MOSQUITO

Many a night I've lain awake to the sound of a high pitched buzzing, getting nearer and nearer. I'd lure the little bugger in by pretending not to move, then when sure she (the most dangerous mosquitoes) was almost impossible to miss, I'd strike. Clap! Gone. But that was just one of them!

Knowing they'd eat me alive, I would arm myself with a towel, turn on the lights and launch an attack. It's a bit like playing squash. I would not rest until every last one was squished. Well, it's either them or me. Check out the youtube at the foot of this page to see if you think I'm mad.

Mosquitoes are a family of small, midge-like flies: the Culicidae. Although a few species are harmless or even useful to humanity, most are a nuisance because they consume blood from living vertebrates, including humans. The females of many species of mosquitoes are blood eating pests. In feeding on blood, some of them transmit extremely harmful human and livestock diseases, such as malaria. Some authorities argue accordingly that mosquitoes are the most dangerous animals on Earth.

Mosquitoes are members of a family of nematocerid flies: the Culicidae (from the Latin culex, genitive culicis meaning "midge" or "gnat"). The word mosquito (formed by mosca and diminutive ito) is from the Spanish or Portuguese for "little fly". Superficially, mosquitoes resemble crane flies (family Tipulidae) and chironomid flies (family Chironomidae); as a result, casual observers seldom realise the important differences between the members of the respective families. 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 many of those that do create a "high to low pressure" in the blood to obtain it 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 or 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 habitat is disturbed, for instance by sudden deforestation.

Several scientists have suggested complete eradication of mosquitoes would not have serious ecological consequences.


LIFE CYCLE

Like all flies, mosquitoes go through four stages in their life cycle: 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 acclimatise themselves to the saltiness. 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 specialise 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 species of mosquito 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 5–14 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, that 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 little as a week to as long as several months.

Some species can overwinter as adults in diapause.


FEEDING

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. 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.

For females to risk their lives on blood sucking while males abstain is not a strategy limited to the 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 practise 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. 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. Some species, such as the Asian tiger mosquito, are known to fly and feed during daytime.

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. Not surprisingly the salivary glands are a major target to most pathogens, whence they find their way into the host via the stream of saliva.

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.


MOUTHPARTS

Mosquito mouthparts are very specialised, particularly those of the females, which in most species are adapted to piercing skin and then sucking blood. Apart from bloodsucking, the females generally also drink assorted fluids rich in dissolved sugar, such as nectar and honeydew, to obtain the energy they need. For this, their blood-sucking mouthparts are perfectly adequate. In contrast, male mosquitoes are not bloodsuckers; they only drink such sugary fluids as they can find. Accordingly, their mouthparts do not require the same degree of specialisation as those of females.

Externally, the most obvious feeding structure of the mosquito is the proboscis. More specifically, the visible part of the proboscis is the labium, which forms the sheath enclosing the rest of the mouthparts. When the mosquito first lands on a potential host, her mouthparts will be enclosed entirely in this sheath, and she will touch the tip of the labium to the skin in various places. Sometimes, she will begin to bite almost straight away, while other times, she will prod around, apparently looking for a suitable place. Occasionally, she will wander for a considerable time, and eventually fly away without biting. Presumably, this probing is a search for a place with easily accessible blood vessels, but the exact mechanism is not known. It is known that there are two taste receptors at the tip of the labium, which may well play a role.

The female mosquito does not insert her labium into the skin; it bends back into a bow when the mosquito begins to bite. The tip of the labium remains in contact with the skin of the victim, acting as a guide for the other mouthparts. In total, there are six mouthparts besides the labium: two mandibles, two maxillae, the hypopharynx, and the labrum.

The mandibles and the maxillae are used for piercing the skin. The mandibles are pointed, while the maxillae end in flat, toothed "blades". To force these into the skin, the mosquito moves its head backwards and forwards. On one movement, the maxillae are moved as far forward as possible. On the opposite movement, the mandibles are pushed deeper into the skin by levering against the maxillae. The maxillae do not slip back because the toothed blades grip the skin.

The hypopharynx and the labrum both are hollow. Saliva with anticoagulant is pumped down the hypopharynx to prevent clotting, and blood is drawn up the labrum.

To understand the mosquito mouthparts, it is helpful to draw a comparison with an insect that chews food, such as a dragonfly. A dragonfly has two mandibles, which are used for chewing, and two maxillae, which are used to hold the food in place as it is chewed. The labium forms the floor of the dragonfly's mouth, the labrum forms the top, while the hypopharynx is inside the mouth and is used in swallowing. Conceptually, then, the mosquito's proboscis is an adaptation of the mouthparts that occur in other insects. The labium still lies beneath the other mouthparts, but also enfolds them, and it has been extended into a proboscis. The maxillae still "grip" the "food" while the mandibles "bite" it. The top of the mouth, the labrum, has developed into a channeled blade the length of the proboscis, with a cross-section like an inverted "U". Finally, the hypopharynx has extended into a tube that can deliver saliva at the end of the proboscis. Its upper surface is somewhat flattened so, when pressed against it, the labrum forms a closed tube for conveying blood from the victim.

SALIVA

For the mosquito to obtain a blood meal, it must circumvent the vertebrate physiological responses. The mosquito, as with all blood-feeding arthropods, has mechanisms to effectively block the hemostasis system with their saliva, which contains a mixture of secreted proteins. Mosquito saliva negatively affects vascular constriction, blood clotting, platelet aggregation, angiogenesis and immunity, and creates inflammation. Universally, hematophagous arthropod saliva contains at least one anticlotting, one antiplatelet, 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. One promising application is the development of anticlotting drugs based on saliva molecules, which might be useful for approaching heart-related diseases, because they are more user-friendly blood clotting inhibitors and capillary dilators.

It is now well recognized that feeding ticks, sandflies, and, more recently, mosquitoes, have an ability to modulate the immune response of the animals (hosts) on which they feed. 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 have become increasingly convincing that such an effect occurs. Early work described a factor in saliva that directly suppresses TNF-α release, but not antigen-induced histamine secretion, from activated mast cells.

Experiments by Cross et al. (1994) demonstrated the inclusion of Ae. aegypti mosquito saliva into naïve cultures led to a suppression of interleukin (IL)-2 and IFN-γ 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-γ production. Unexpectedly, this shift in cytokine expression is observed in splenocytes up to 10 days after mosquito exposure, suggesting 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 increased 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 1/7 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 mosquito saliva can also decrease expression of interferon−α/β 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 mosquito saliva exacerbates West Nile virus infection, as well as other mosquito-transmitted viruses.


EGG DEVELOPMENT and BLOOD DIGESTION

Female mosquitoes use two very different food sources. They need sugar for energy, which is taken from sources such as nectar, and they need blood as a source of protein for egg development. Because biting is risky and hosts may be difficult to find, mosquitoes take as much blood as possible when they have the opportunity. This, however, creates another problem. Digesting that volume of blood takes a while, and the mosquito will require energy from sugar in the meantime.

To avoid this problem, mosquitoes have a digestive system which can store both food types, and give access to both as they are needed. When the mosquito drinks a sugar solution, it is directed to a crop. The crop can release sugar into the stomach as it is required. At the same time, the stomach never becomes full of sugar solution, which would prevent the mosquito taking a blood meal if it had the chance.

Blood is directed straight into the mosquito's stomach. In species that feed on mammalian or avian blood, hosts whose blood pressure is high, the mosquito feeds selectively from active blood vessels where the pressure assists in filling the gut rapidly. If, instead of slapping a feeding mosquito, one stretches one's skin so that it grips the proboscis and the mosquito cannot withdraw it, the pressure will distend the gut until it breaks and the mosquito dies. In the unmolested mosquito however, the mosquito will withdraw, and as the gut fills up the stomach lining secretes a peritrophic membrane that surrounds the blood. This membrane keeps the blood separate from anything else in the stomach. However, like certain other insects that survive on dilute, purely liquid diets, notably many of the Homoptera, many adult mosquitoes must excrete unwanted aqueous fractions even as they feed. As long as they are not disturbed, this permits mosquitoes to continue feeding until they have accumulated a full meal of nutrient solids. As a result, a mosquito replete with blood can continue to absorb sugar, even as the blood meal is slowly digested over a period of several days. Once blood is in the stomach, 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 hours after feeding, and subsequently returns to baseline levels by 60 hours. 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 hours after the blood meal, subsequently falling to baseline levels by 60 hours. 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 hours after the blood meal, then rises rapidly to a maximum at 30 hours 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 on 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. Aminoptidase 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.

NATURAL PREDATORS

Dragonfly and damselfly nymphs and various other aquatic insect predators eat mosquitoes at all stages of development and dense populations can be useful in reducing mosquito problems. Various small fishes, such as species of Galaxias and members of the Poeciliidae, such as Gambusia (so-called mosquitofish) and guppies (Poecilia), eat mosquito larvae and sometimes may be worth introducing into ponds to assist in control. Many other types of fish are also known to consume mosquito larvae, including bass, bluegills, piranhas, Arctic char, salmon, trout, catfish, fathead minnows, goldfish, and killifish.

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.

Some cyclopoid copepods are predators on first-instar larvae, killing up to 40 Aedes larvae per day. Larval Toxorhynchites mosquitoes are known as 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). However, Toxorhynchites can consume all types of prey, organic debris, or even exhibit cannibalistic behavior.

Other natural predators and parasitoids include fungi and nematodes. Though important at times, their effectiveness varies with circumstances.

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


TREATING 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 and III) and delayed hypersensitivity reactions (type IV) to mosquito bites.

Both reactions result in itching, redness and swelling. Immediate reactions develop within a few minutes of the bite and last for a few hours. Delayed reactions take around a day to develop, and last for up to a week. Several anti-itch medications are commercially available, including those taken orally, such as Benadryl, or topically applied antihistamines and, for more severe cases, corticosteroids, such as hydrocortisone and triamcinolone. Using a brush to scratch the area surrounding the bite (but not the bite itself) and running hot water (around 49 °C or 120 °F) over it can alleviate itching for several hours by reducing histamine-induced skin blood flow. Tea tree oil has been shown to be an effective anti-inflammatory, reducing itching.


REPELLANTS

Insect repellents are applied on skin and give short time protection against mosquito bites. The chemical DEET repels some mosquitoes and other insects. Some CDC-recommended repellents are picaridin, Eucalyptus oil (PMD) and IR3535. Others are indalone, dimethyl pthalate, dimethyl carbate, ethyl hexanediol. Plants can also repel mosquitoes, such as citronella, catnip, marigolds, ageratum and horsemint to name a few.


EVOLUTION

The oldest known mosquito with an anatomy similar to modern species was found in 79-million-year-old Canadian amber from the Cretaceous. An older sister species with more primitive features was found in amber that is 90 to 100 million years old.

Genetic analyses indicate the Culicinae and Anophelinae clades may have diverged about 150 million years ago. The Old and New World Anopheles species are believed to have subsequently diverged about 95 million years ago.

The mosquito Anopheles gambiae is currently undergoing speciation into the M and S molecular forms. This means some pesticides that work on the M form will not work anymore on the S form.


TAXONOMY of the CULICDAE

Over 3,500 species of the Culicidae have already been described. They are generally divided into two subfamilies which in turn comprise some 43 genera. These figures are subject to continual change, as more species are discovered, and as DNA studies compel rearrangement of the taxonomy of the family.

MILITARY VEHICLES

'Mantis' is also the name of an ROV used in connection with the Dragonfly mine hunter ship, that operates with a partly submerged hull, so just like the dragonfly insect, it interfaces with air and water during its life cycle. The Dragonfly is suitable for military (NATO) peacekeeping operations such as mine hunting and persistent surveillance, where economic considerations dictate flexible autonomous survey, rather than brute force battleship presence. 

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