Mosquitoes:
How
Mosquitoes Work
by Craig C. Freudenrich, Ph.D.
It's summer
time. You're out in your backyard
enjoying the sun and grilling your
dinner. Ouch! You look down at your
arm and see a painful, swelling mosquito
bite. Moments later, you feel another
one bite you. What are these pesky
insects? Why do they bite? Do they
carry diseases? What can you do to
protect yourself?
In this edition
of HowStuffWorks, we'll take a close-up
look at mosquitoes -- how they breed,
how they bite, what diseases they
carry and what you can do to control
them.
The word "mosquito"
is Spanish for "little fly,"
and its use dates back to about 1583
in North America (Europeans referred
to mosquitoes as "gnats").
Mosquitoes belong to the order Diptera,
true flies. Mosquitoes are like flies
in that they have two wings, but unlike
flies, their wings have scales, their
legs are long and the females have
a long mouth part (proboscis) for
piercing skin.
Mosquitoes
are insects that have been around
for over 30 million years. And it
seems that, during those millions
of years, mosquitoes have been honing
their skills so that they are now
experts at finding people to bite.
A mosquito has a battery of sensors
designed to track their prey, including:
Chemical
sensors -- mosquitoes can
sense carbon dioxide and lactic acid
up to 100 feet (36 meters) away. Just
about any mammal or bird gives off
these gases as part of its normal
breathing. Certain chemicals in sweat
also seem to attract mosquitoes (people
who don't sweat don't get nearly as
many mosquito bites).
Visual
sensors -- if you are wearing
clothing that contrasts with the background,
and especially if you move while wearing
that clothing, mosquitoes can see
you and zero in on you. It's a good
bet that anything moving is "alive",
and therefore full of blood, so this
is a good strategy.
Heat
sensors -- Mosquitoes can
detect heat, so they can find warm-blooded
mammals and birds very easily once
they get close enough.
Something with
this many sensors sounds more like
a military aircraft than an insect.
That's why mosquitoes are so good
at finding and biting you! As we'll
see later, one of the only ways to
stop mosquitoes from finding you is
to confuse their chemical receptors
with something like DEET.
Like all insects,
adult mosquitoes have three basic
body parts:
Head
- This is where all the sensors are,
along with the biting apparatus. The
head has two compound eyes, antennae
to sense chemicals and the mouth parts
called the palpus and the proboscis
(only females have the probiscus,
for biting).
Thorax
- This segment is where the two wings
and six legs attach. It contains the
flight muscles, compound heart, some
nerve cell ganglia and trachioles.
Abdomen
- This segment contains the digestive
and excretory organs.
There are over
2,700 species of mosquitoes in the
world, and there are 13 mosquito genera
(plural for "genus") that
live in the United States. Of these
genera, most mosquitoes belong to
three:
Aedes
- These are sometimes called "floodwater"
mosquitoes because flooding is important
for their eggs to hatch. Aedes mosquitoes
have abdomens with pointed tips. They
include such species as the yellow-fever
mosquito (Aedes aegypti) and the Asian
tiger mosquito (Aedes albopictus).
They are strong fliers, capable of
traveling great distances (up to 75
miles/121 km) from their breeding
sites. They persistently bite mammals
(especially humans), mainly at dawn
and in the early evening. Their bites
are painful.
Anopheles
- These tend to breed in bodies of
permanent fresh water. Anopheles mosquitoes
also have abdomens with pointed tips.
They include several species, such
as the common malaria mosquito (Anopheles
quadrimaculatus), that can spread
malaria to humans.
Culex
- These tend to breed in quiet, standing
water. Culex mosquitoes have abdomens
with blunt tips. They include several
species such as the northern house
mosquito (Culex pipiens). They are
weak fliers and tend to live for only
a few weeks during the summer months.
They persistently bite (preferring
birds over humans) and attack at dawn
or after dusk. Their bite is painful.
Some mosquitoes,
such as the cattail mosquito (Coquilettidia
perturbans), are becoming more prevalent
pests as humans invade their habitats.
Let's examine
how mosquitoes live and breed.
Life
Cycle and Breeding
Like all insects, mosquitoes hatch
from eggs and go through several stages
in their life cycle before becoming
adults. The females lay their eggs
in water and the larva and pupa stages
live entirely in water. When the pupa
change into adults, they leave the
water and become free-flying land
insects. The life cycle of a mosquito
can vary from one to several weeks
depending upon the species (the adult,
mated females of some species can
survive the winter in cool, damp places
until spring, when they will lay their
eggs and die.)
Egg
All mosquitoes lay eggs in water,
which can include large bodies of
water, standing water (like swimming
pools) or areas of collected standing
water (like tree holes or gutters).
Females lay their eggs on the surface
of the water, except for Aedes mosquitoes,
which lay their eggs above water in
protected areas that eventually flood.
The eggs can be laid singly or as
a group that forms a floating raft
of mosquito eggs (see Mosquito Life
Cycle for a picture of an egg raft).
Most eggs can survive the winter and
hatch in the spring.
Larva
Identifying Larvae
You can distinguish the larvae of
various mosquito species. Anopheles
larvae lie parallel to the surface
of the water, while larvae of Aedes
and Culex extend down into the water
(the air tubes of Culex are longer
than those of Aedes).
The mosquito
eggs hatch into larvae or "wigglers,"
which live at the surface of the water
and breathe through an air tube or
siphon. The larvae filter organic
material through their mouth parts
and grow to about 0.5 to 0.75 inches
(1 to 2 cm) long; as they grow, they
shed their skin (molt) several times.
Mosquito larvae can swim and dive
down from the surface when disturbed
(see Mosquito Life Cycle for a Quicktime
movie of free-swimming Asian tiger
mosquito larvae). The larvae live
anywhere from days to several weeks
depending on the water temperature
and mosquito species.
Pupa
The mosquito larvae and pupae are
important food sources for fish in
aquatic ecosystems.
After the fourth molt, mosquito larvae
change into pupae, or "tumblers,"
which live in the water anywhere from
one to four days depending on the
water temperature and species. The
pupae float at the surface and breathe
through two small tubes (trumpets).
Although they do not eat, pupae are
quite active (see Mosquito Life Cycle
for a Quicktime movie of free-swimming
Asian tiger mosquito pupae). At the
end of the pupal stage, the pupae
encase themselves and transform into
adult mosquitoes.
Adult
Inside the pupal case, the pupa transforms
into an adult. The adult uses air
pressure to break the pupal case open,
crawls to a protected area and rests
while its external skeleton hardens,
spreading its wings out to dry. Once
this is complete, it can fly away
and live on the land.
One of the
first things that adult mosquitoes
do is seek a mate, mate and then feed.
Male mosquitoes have short mouth parts
and feed on plant nectar. In contrast,
female mosquitoes have a long proboscis
that they use to bite animals and
humans and feed on their blood (the
blood provides proteins that the females
need to lay eggs). After they feed,
females lay their eggs (they need
a blood meal each time they lay eggs).
Females continue this cycle and live
anywhere from many days to weeks (longer
over the winter); males usually live
only a few days after mating. The
life cycles of mosquitoes vary with
the species and environmental conditions.
Mosquito
Bites
As mentioned before, only female mosquitoes
bite. They are attracted by several
things, including heat (infrared light),
light, perspiration, body odor, lactic
acid and carbon dioxide. The female
lands on your skin and sticks her
proboscis into you (the proboscis
is very sharp and thin, so you may
not feel it going in). Her saliva
contains proteins (anticoagulants)
that prevent your blood from clotting.
She sucks your blood into her abdomen
(about 5 micro liters per serving
for an Aedes aegypti mosquito).
Photo courtesy
Centers for Disease Control &
Prevention, photographer Jim Gathany
This female Anopheles gambiae mosquito
is feeding. You can see the blood
swelling her abdomen.
If she is disturbed,
she will fly away. Otherwise, she
will remain until she has a full abdomen.
If you were to cut the sensory nerve
to her abdomen, she would keep sucking
until she burst.
After she has
bitten you, some saliva remains in
the wound. The proteins from the saliva
evoke an immune response from your
body. The area swells (the bump around
the bite area is called a wheal),
and you itch, a response provoked
by the saliva. Eventually, the swelling
goes away, but the itch remains until
your immune cells break down the saliva
proteins.
To treat mosquito
bites, you should wash them with mild
soap and water. Try to avoid scratching
the bite area, even though it itches.
Some anti-itch medicines such as Calamine
lotion or over-the-counter cortisone
creams may relieve the itching. Typically,
you do not need to seek medical attention
(unless you feel dizzy or nauseated,
which may indicate a severe allergic
reaction to the bite).
Myths
Several natural
or man-made products have been touted
as mosquito repellents or effective
in mosquito control. Citronella oil,
which is a product of several types
of plants that can be made into candles
or burned directly, is an effective
mosquito repellant in high concentrations,
but individual citronella-producing
plants do not make enough oil to effectively
repel mosquitoes. Ultraviolet lights,
as used in bug zappers, and ultrasonic
devices are not effective. Also, mosquitoes
are not a significant portion of the
diets of Purple Martin Birds or bats.
No-See'ums
a.k.a. Sand Flies:
Often called
"punkies", "no-see'ums"
or sand flies, the biting midges are
viscous where they occur. They are
often more troublesome than mosquitoes
because they can easily enter dwellings
through ordinary 16-mesh window screen.
The presence of these insects can
decrease property values and severely
hamper use of recreational areas.
Vacationers and campers literally
have been driven away from areas by
these tiny biters. Sand flies can
also cause loss to cattlemen in terms
of annoyance to cattle and transmission
of various nematode diseases.
Sand flies
are members of the insect order Diptera
and undergo a complete development
with egg, larva, pupa, and adult forms.
The adults are less than 1/16-inch
long, dark gray to black in color.
and have one pair of wings, which
are spotted. The sand flies breed
predominantly in salt marshes; however
some species that are found inland
breed in fresh water areas and tree
holes.
Larvae of sand
flies are found in mud, sand, and
derris around the edges of ponds,
springs, lakes, creeks, and in tree
holes or slime covered bark. In the
water they are free swimmers and are
commonly found on floating twigs or
leaf trash. The larvae pupate on floating
debris or at the water's edge. The
adult females, like mosquitoes, require
blood to mature the eggs. Males do
not bite. Sand fly larvae can be found
in marshes the year-round; however,
the period of greatest adult activity
is June to August.
Sand fly activity
is associated with air movement. Adults
of most species seldom bite when there
is air movement. Sand flies are also
sensitive to temperature. Animals
having high body temperature are attractive
to great numbers of female sand flies.
Persons performing hard labor out-of-doors
frequently are severely annoyed by
these insects.
Flies:
A SINGLE FLY
CAN CONTAMINATE FEED/FOOD
WITH ENOUGH BACTERIA TO CAUSE ILLNESS
The fly is
an enemy because it is one of the
biggest disease carriers in existence.
Moist, warm, decaying material protected
by sunlight is favored for fly eggs
to hatch and in only 8 short hours
for fly larvae or maggots to grow.
An uncovered garbage can is an ideal
breeding place. Flies feed and reproduce
on filth, decaying matter, and water.
Flies are transmitters
of disease primarily because they
feed on human and animal wastes. The
dangerous bacteria present in these
wastes stick to the mouth, footpads
and hairs of flies and may then be
deposited in feed intended for animal
consumption. Fly feces, which contain
disease-bearing organisms, can also
contaminate human food. Flies defecate
every four to five minutes. And since
flies have no teeth and must take
their nourishment in liquid form,
they spit on solid food and let the
dissolve before consuming it. Fly
spittle, or vomit, is swarming with
bacteria and contaminates feed and
areas where feed is stored.
A fly can infect
feed/food by merely landing on it.
Flies can enter
a building that has openings not much
larger than the head of a pin.
Although they
rarely travel very far from where
they are hatched, flies may be lured
to the sources of attractive odors.
Flies may also be carried long distances
by air current.
One female
fly can produce thousands of offspring
in a single breeding season. flies
have favorite resting places. They
are especially fond of places protected
from the wind and on edges such as
garbage can rims and electric wires
Flies carry
bacteria that can cause many diseases
known and unknown to man.
One fly can
carry over 33 million disease-causing
microorganisms on the inner and outer
surfaces of its body.
There are over
87,000 different fly species.
The fly is
one of the fastest flying insects.
A fly's vision
is sharp for only 24 to 36 inches.
One successful
hatch can result in well over 2 million
flies.
One season
can breed as much as 25 generations.
Insect Diseases:
Aug 14, 2002
Florida
Reports 1st Human Case Of West Nile
By NEIL JOHNSON (TBO.COM)
TAMPA - Health officials confirmed
the first human case of West Nile
virus in Florida on Tuesday and expanded
a health alert to four more counties,
including Sumter and Orange, bringing
the total to six.
The newest
alert also covers Lake and Marion
counties.
The Sumter
County man who tested positive for
the mosquito-borne virus may have
been bitten in Louisiana, where 85
people have contracted West Nile and
seven have died.
"We don't
know for sure,'' said April Crowley,
spokeswoman for the state Department
of Health.
Earlier this
week, a dead crow found five miles
west of Brooksville in Hernando County
tested positive for West Nile.
Last week,
the Department of Health issued medical
alerts for Escambia and Volusia counties.
The alerts
are issued to advise people the virus
is present in their counties so that
they can avoid mosquito bites, and
also to prompt doctors to be more
aware of potential cases.
The virus previously
occurred in Africa, the Middle East,
western Asia and Europe. It was first
discovered in this country in 1999.
The disease spreads when mosquitoes
bite infected birds and then bite
a person. Humans cannot spread the
disease.
Health officials
test dead birds, horses and chickens
kept in outdoor cages to monitor several
mosquito-borne diseases including
West Nile and eastern equine encephalitis.
Although Sumter
was the only county in the newest
list of counties put under alert with
a human case, birds, horses and chickens
in the other three have tested positive
for West Nile.
In Lake County,
two birds and three horses have been
found with the virus. Marion had six
horses and one bird. Orange had nine
chickens, a horse and two birds, and
Sumter had one horse.
No animal cases
of West Nile have been found in Hillsborough,
but eight chickens have tested positive
for the virus in Pinellas and one
bird in Pasco.
To the south,
Manatee has had a horse and a chicken
with the disease.
Only about
10 percent of people bitten by infected
mosquitoes show any symptoms, which
are similar to the flu. Only about
1 in 200 people develop encephalitis,
a potentially fatal swelling of the
brain.
So far this
year, fewer people have contracted
West Nile in Florida than in 2001.
"Last
year by this time, we had three human
cases. We're seeing a little more
animal cases this year,'' Crowley
said.
June 15, 2002
First West Nile Case of the
Year
Bay News 9
Pinellas County
is dealing with their first West Nile
virus case of the year.
It didn't’t
come as a surprise to Pinellas County
Mosquito Control when a sentinel chicken
tested positive for the virus.
Nancy Page
with mosquito control says, she knew
it was a matter of time. "We
were expecting it eventually."
she says. "We had it come up
last year in November (and) it was
spreading throughout the state this
year, so we knew it would arise in
Pinellas county again this year."
The chicken
came from a St. Petersburg coop, which
is one of the best weapons in detecting
mosquito-born diseases like WEST NILE.
Pinellas County
Mosquito Control has eight chicken
coops across the county that test
the blood from sentinel chickens each
week. Wednesday, one of these samples
tested positive.
WEST NILE is
transmitted from birds to mosquitoes,
who can then pass it on to humans.
The virus first
showed up in the U.S. in New York
in 1999. Two years later, Florida
had its first case. While scientists
don't know a lot about WEST NILE,
aerial spraying and fogging to control
the mosquito population seems to be
the best way to fight it.
Looking for
recorded information on Florida cases
of the West Nile virus? Call the Department
of Health Bureau of Epidemiology's
Arboviral Encephalitis and West Nile
virus hotline at 1-888-880-5782.
Sunday, March
31, 2002
Polk
County Horse Tests Positive For West
Nile Virus
The disease
is spread primarily through infected
mosquitoes.
A horse in
Polk City has been confirmed as this
year's first local casualty of the
West Nile virus.
The disease, carried by mosquitoes,
can make humans ill but it’s
rarely fatal. According to The Ledger,
as many as five other horses have
died in the last month after suffering
symptoms common to the virus. Polk
County health officials say they're
unaware of any other local cases of
disease.
Diseases:
Mosquitoes
can carry many types of diseases that
are caused by bacteria, parasites
or viruses. These diseases include:
•
Malaria - Malaria is caused
by a parasite that is transmitted
by an Anopheles mosquito. The parasite
grows in your bloodstream and can
produce symptoms that develop anywhere
from six to eight days to several
months after infection.The symptoms
include fever, chills, headaches,
muscle aches and general malaise (similar
to flu symptoms). Malaria is a severe
disease that can be fatal, but can
be treated with anti malarial drugs
and prevented by vaccinations. Malaria
is prevalent in tropical or sub-tropical
climates.
•
Yellow Fever - Yellow fever
does not occur in the United States,
but is prevalent in Africa and is
transmitted by the Aedes aegypti mosquito.
Yellow fever produces symptoms similar
to malaria, but also includes nausea,
vomiting and jaundice. Like malaria,
yellow fever can be fatal. There is
no treatment for the disease itself,
only the symptoms. Yellow fever can
be controlled by vaccination and mosquito
control.
•
Encephalitis - Encephalitis
is caused by viruses that are transmitted
by mosquitoes such as the Aedes mosquitoes
or Culiseta mosquitoes. The symptoms
of encephalitis include high fever,
stiff neck, headache, confusion and
laziness/sleepiness. There are several
types of encephalitis that can be
transmitted by mosquitoes, including
St. Louis, Western equine, Eastern
equine, La Crosse and West Nile. West
Nile encephalitis is on the rise in
the eastern United States, which has
raised concerns about mosquito control
(see Overview of West Nile Virus for
more information).
•
Dengue Fever - Dengue fever
is transmitted by the Asian tiger
mosquito, which is native to East
Asia and was found in the United States
in 1985. It is also transmitted by
Aedes aegypti in the tropics. Dengue
fever is caused by a virus that produces
a range of illnesses, from viral flu
to hemorrhagic fever. It is especially
dangerous for children (see Dengue
Fever & Dengue Hemorrhagic Fever
for more information).
The best way to reduce mosquito-borne
diseases is through mosquito control
and personal protection.
WEST
NILE VIRUS HITS FLORIDA
Its
just a short time before the VIRUS
spreads further.
NOW!! is the time to protect your
horses, cattle, poultry and children
from this potentially deadly virus
WEST NILE VIRUS & ENCEPHALITIS
FACT SHEET
What
is West Nile encephalitis?
"Encephalitis"
is an inflammation of the brain and
can be caused by viruses and bacteria
including viruses transmitted by mosquitoes.
West Nile encephalitis is an infection
of the brain caused by West Nile virus.
It is closely related to St. Louis
encephalitis virus found in the United
States.
How
long has West Nile virus been in the
United States and where did it come
from?
It is not known
how long it has been In the U.S. CDC
scientists believe the virus has been
in the eastern U.S. since the early
summer of 1999, possibly longer. West
Nile virus has been commonly found
in humans and birds and other vertebrates
in Africa, Eastern Europe, West Asia,
and the Middle East, but has not previously
been documented in the Western Hemisphere,
It is not known from where the U.S.
virus originated.
In the Temperate
Zone of the world (i.e., between latitudes
23.5 and 66.5 degrees north and south),
West Nile encephalitis cases occur
primarily in the late summer or early
Fall. In the southern climates where
temperatures are milder, West Nile
virus ran be transmitted year round.
What
Is the incubation period in humans
(or time from infection to onset of
disease
symptoms) for West Nile encephalitis?
Usually 5 to
15 days.
What
are the symptoms of West Nile encephalitis?
Mild infections
have symptoms that include fever,
headache, and body aches, often with
skin rash and swollen lymph glands.
More severe infection may be marked
by headache, high fever, neck stiffness,
stupor, disorientation, coma, tremors,
convulsions, muscle weakness, paralysis,
and rarely death. If you believe you
have West Nile, seek medical care
as soon as possible.
Is
a woman's pregnancy at risk if she
gets West Nile encephalitis?
There is NO
documented evidence that a pregnancy
is at risk due to infection with West
Nile
virus.
How
do people get West Nile encephalitis?
The bite of
a mosquito that is infected with West
Nile virus is all it takes to transmit
the infection. Mosquitoes become infected
when they feed on infected birds.
After an incubation period of 1 to
2 weeks, infected female mosquitoes
can then transmit West Nile virus
to humans and animals while biting.
The mosquitoes and the bites they
cause do not differ from those bites
of an uninfected mosquito.
Can
you get West Nile encephalitis from
an infected person or an infected
bird?
NO. West Nile
encephalitis is NOT transmitted from
person-to-person. For example, you
cannot get West Nile virus from touching
or kissing a person who has the disease,
or from a health care worker who has
treated someone with the disease.
There is NO
evidence that a person can get the
virus from handling live or dead infected
birds. However, avoid contact with
any dead bird or animal, by using
gloves or plastic bags to place it
in a garbage can.
Can
you get West Nile virus directly from
other insects or animals?
Ticks infected
with West Nile virus have been found
in Asia and Africa. Their role in
the transmission and maintenance of
the virus is uncertain. However, there
is no information to suggest that
ticks transmitted West Nile virus
to patients in the New York area outbreak.
Infected mosquitoes are the primary
source for West Nile virus and have
been found to cause the recent outbreak
in the New York City metropolitan
area.
Although other
animals (e.g., horses and dogs) may
become infected with West Nile Virus,
there have been no reports of the
disease being transmitted from the
infected animal to humans or other
animals. Contact a Veterinarian if
you have questions about pets or other
animals.
How
is West Nile encephalitis treated?
There is NO
specific therapy or vaccine for West
Nile infection. In more severe cases,
intensive supportive therapy is indicated
which includes hospitalization.
Who
is at risk for getting West Nile encephalitis?
All residents
of areas where virus activity had
been identified are at risk of getting
West Nile encephalitis; persons greater
than 50 years of age have the highest
risk of severe disease.
What
can I do to reduce my risk of becoming
infected with West Nile virus?
Check and repair
window and door screens.
Remove containers that collect standing
water such as cans, bottles, buckets,
toys and old tires.
Remove standing water from kiddy pools,
pool covers, flowerpots and plant
containers.
Drill holes in the bottoms of recycling
containers and other receptacles that
are left outdoors.
Check and repair window and door screens.
Remove containers that collect standing
water such as cans, bottles, buckets,
toys and old tires.
Remove standing water from kiddy pools,
pool covers, flowerpots and plant
containers.
Drill holes in the bottoms of recycling
containers and other receptacles that
are left outdoors.
Minimize time spent outdoors during
the dawn and dusk hours when mosquitoes
are out in greater numbers.
If you are outdoors in the evening
hours, wear long-sleeved shirts and
long pants.
Apply an appropriate insect repellent.
If you choose to do this, read and
follow carefully the manufacturer's
DIRECTIONS FOR USE, as printed on
the product. Repellents may irritate,
the eyes and mouth, so avoid applying
repellent to the hands of children.
Insect repellents should not be applied
to very young children (less than
3 years old).
