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Mosquitoes are small, midge-like flies which compose the family Culicidae. Although a few species are harmless or even useful to humanity, the females of most species are ectoparasites, whose tube-like mouthparts (called a proboscis) pierce the hosts' skin to suck the blood. The word "mosquito" (formed by mosca and diminutive ito) is from the Spanish or Portuguese for "little fly". Thousands of species feed on the blood of various kinds of hosts, mainly vertebrates, including mammals, birds, reptiles, amphibians, and even some kinds of fish. Some mosquitoes also attack invertebrates, mainly arthropods. Though the loss of blood is seldom of any importance to the victim, the saliva of the mosquito often causes an irritating rash that is a serious nuisance. Much more serious though, are the roles of many species of mosquitoes as vectors of diseases. In passing from host to host, some transmit extremely harmful infections such as malaria, yellow fever, west nile virus and filariasis.
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.
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.
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.
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
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.
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.
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.
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 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.
Adult
yellow fever mosquito Aedes aegypti, typical of subfamily Culicinae. Note bushy
antennae and longer palps of 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. 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.
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 O 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.
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.
Here an Anopheles stephensi female is gorged with blood and
beginning to pass unwanted liquid fractions of the blood to make room in her gut
for more of the solid nutrients.
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. 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. 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.
Many, if not all, blood-sucking species of mosquitoes
are fairly selective feeders that specialise in particular host species, though
they often relax their selectivity when they experience severe competition for
food, defensive activity on the part of the hosts, or starvation. Some species
feed selectively on monkeys, while others prefer particular kinds of birds, but
they become less selective as conditions become more difficult. For example,
Culiseta melanura sucks the blood of passerine birds for preference and such
birds are typically the main reservoir of the Eastern equine encephalitis virus
in North America. Early in the season while mosquito numbers are low, they
concentrate on passerine hosts, but as mosquito numbers rise and the birds are
forced to defend themselves more vigorously, the mosquitoes become less
selective in attacking their avian hosts. Soon the mosquitoes begin attacking
mammals more readily, thereby becoming the major vector of the virus, and
causing epidemics of the disease, most conspicuously in humans and horses.
Even more dramatically, in most of its range in North America, the main
vector for the Western equine encephalitis virus is Culex tarsalis, because it
is known to feed variously on mammals, birds, reptiles, and amphibians. Even
fish may be attacked by some mosquito species if they expose themselves above
water level, as mudskippers do.
It has long been known that some species
of blood-sucking flies, such as many of the Ceratopogonidae, will attack large,
live insects and suck their haemolymph and that others, such as the so-called
"jackal flies" (Milichiidae), will attack the recently dead prey of say, crab
spiders (Thomisidae), but in the late 1960s it was reported that some species of
anautogenous mosquitoes would feed on the haemolymph of caterpillars. Other
observations include mosquitoes feeding on cicadas, and mantids. More recently
it has been shown that malaria transmitting mosquitoes will actively seek out
some species of caterpillars and feed on their haemolymph, and do so to their
apparent physical detriment.
Mosquito mouthparts are very specialized, 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
sugary fluids. Accordingly, their mouthparts do not require the same degree of
specialization 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 are both 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.
For the mosquito to obtain a blood meal, it must
circumvent the vertebrate's 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, such as clotting inhibitors and capillary
dilators, that could be useful for cardiovascular disease.
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 that the inclusion of Ae. aegypti mosquito
saliva into nave 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.
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 Hemiptera, 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 three 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.
In the sense of the entire family Culicidae,
mosquitoes are cosmopolitan; in every land region except for Antarctica and a
few islands, mainly in polar or subpolar climates, at least some species of
mosquito will be present. Iceland is an unusual example of such an island, being
essentially free of mosquitoes. In warm and humid tropical regions, various
mosquito species are active for the entire year, but in temperate and cold
regions they hibernate or enter diapause. Arctic or subarctic mosquitoes, like
some other arctic midges in families such as Simuliidae and Ceratopogonidae may
be active for only a few weeks annually as melt-water pools form on the
permafrost. During that time, though, they emerge in huge numbers in some
regions and may take up to 300 ml of blood per day from each animal in a caribou
herd.
The absence of mosquitoes from Iceland and similar regions is
probably because of quirks of their climate, which differs in some respects from
mainland regions. At the start of the uninterrupted continental winter of
Greenland and the northern regions of Eurasia and America, the pupa enters
diapause under the ice that covers sufficiently deep water. The imago ecloses
only after the ice breaks in late spring. In Iceland however, the weather is
less predictable. In mid-winter it frequently warms up suddenly, causing the ice
to break, but then to freeze again after a few days. By that time the mosquitoes
will have emerged from their pupae, but the new freeze sets in before they can
complete their life cycle. Any anautogenous adult mosquito would need a host to
supply a blood meal before it could lay viable eggs; it would need time to mate,
mature the eggs and oviposit in suitable wetlands. These requirements would not
be realistic in Iceland and in fact the absence of mosquitoes from such subpolar
islands is in line with the islands' low biodiversity; Iceland has fewer than
1500 described species of insects, many of them probably accidentally introduced
by human agency. In Iceland most ectoparasitic insects live in sheltered
conditions or actually on mammals; examples include lice, fleas and bedbugs, in
whose living conditions freezing is no concern, and most of which were
introduced inadvertently by humans.
Some other aquatic Diptera, such as
Simuliidae, do survive in Iceland, but their habits and adaptations differ from
those of mosquitoes; Simuliidae for example, though they, like mosquitoes, are
bloodsuckers, generally inhabit stones under running water that does not readily
freeze and which is totally unsuited to mosquitoes; mosquitoes are generally not
adapted to running water.
Eggs of species of mosquitoes from the
temperate zones are more tolerant of cold than the eggs of species indigenous to
warmer regions. Many even tolerate subzero temperatures. In addition, adults of
some species can survive the winter by taking shelter in suitable microhabitats
such as buildings or hollow trees.
Worldwide introduction of various mosquito species over large distances into regions where they are not indigenous has occurred through human agencies, primarily on sea routes, in which the eggs, larvae, and pupae inhabiting water-filled used tires and cut flowers are transported. However, apart from sea transport, mosquitoes have been effectively carried by personal vehicles, delivery trucks, trains, and aircraft. Man-made areas such as storm water retention basins, or storm drains also provide sprawling sanctuaries. Sufficient quarantine measures have proven difficult to implement. In addition, outdoor pool areas make a perfect place for them to grow.
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.
The
parasitic diseases collectively called malaria, caused by various species of
Plasmodium, carried by mosquitoes of the genus Anopheles
Lymphatic filariasis
(the main cause of elephantiasis) which can be spread by a wide variety of
mosquito species
West Nile virus is a concern in the United States, but there
are no reliable statistics on worldwide cases.
Eastern equine encephalitis
virus is a concern in the eastern United States.
Tularemia, a bacterial
disease caused by Francisella tularensis, is variously transmitted, including by
biting flies. Culex and Culiseta are vectors of tularemia, as well as arbovirus
infections such as West Nile virus.
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.
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:
Vector
control aimed at mosquito control or eradication
Disease prevention, using
prophylactic drugs and developing vaccines
Prevention of mosquito bites, with
insecticides, nets, and repellents
Since most such diseases are carried
by "elderly" female mosquitoes, some scientists have suggested focusing on these
to avoid the evolution of resistance.
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" 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 in (such as changing the salinity of the water if
ecologically viable). 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, 55
gallon drums, dilapidated swimming pools, old bird basins, large puddles,
coconut shells, or any outside object that may hold rain water.
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/ecological 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.
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 the sides and top, and without a hole. The size of the 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.
Biological control or "biocontrol" is the use of natural enemies to manage mosquito populations. There are several types of biological control methods including the direct introduction of non-ecologically invasive parasites, pathogens, vegetation, and predators (aquatic and non-aquatic) to target mosquitoes.
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
may sometimes be a viable introduction into ponds to assist in control. Some
cyprinids (carps and minnows) and tilapia also consume mosquito larvae. Many
other types of fish consume mosquito larvae, including bass, bluegills,
piranhas, Arctic char, salmon, trout, catfish, fathead minnows and goldfish.
Some cyclopoid copepods are predators on first-instar larvae, killing up to
40 Aedes larvae per day.
Other predators include dragonfly nymphs, which
consume mosquito larvae in the breeding waters, adult dragonflies, which eat
adult mosquitoes, and some species of lizard and gecko. Biocontrol agents that
have had lesser degrees of success include the predatory mosquito Toxorhynchites
and predatory crustaceans such as copepods of the genus Mesocyclops, nematodes
and fungi. 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
digestive systems. 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. as can nematodes. Though important at times,
their effectiveness varies with circumstances.
Introducing large numbers of sterile males is another
approach to reducing mosquito numbers.
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. Larvae of the non-biting Toxorhynchites mosquitoes
are also 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.
Bacillus
thuringiensis israelensis has also been used to control them as a biological
agent.
Insect repellents are applied on skin and give
short-term 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 phthalate,
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 being bitten by a mosquito.
Many scientists have suggested that complete eradication of mosquitoes would not have serious ecological consequences
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. A study published by the Wageningen University and Research Centre in the Netherlands suggests that sufficiently high temperatures are able to denature IgG.
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
Burmese amber that is 90 to 100 million years old. Two mosquito fossils have
been found that show very little morphological change in modern mosquitoes
against their counterpart from 46 million years ago.
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(opti) and S(avanah)
molecular forms. Consequently, some pesticides that work on the M form no longer
work on the S form.
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. The two main subfamilies are the Anophelinae and Culicinae, with
their genera as shown in the subsection below. The distinction is of great
practical importance because the two subfamilies tend to differ in their
significance as vectors of different classes of diseases. Roughly speaking,
arboviral diseases such as yellow fever and dengue fever tend to be transmitted
by Culicine species, not necessarily in the genus Culex. Some transmit various
species of avian malaria, but it is not clear that they ever transmit any form
of human malaria. Some species do however transmit various forms of filariasis,
much as many Simuliidae do.
Anopheline mosquitoes, again not necessarily
in the genus Anopheles, sometimes bear pathogenic arboviruses, but it is not yet
clear that they ever transmit them as effective vectors. However, all the most
important vectors of human malaria are Anopheline.
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