| Skeeters! The Horrible
Truth About Mosquitoes
By Andrew Curry
Special to the Washington
Post
August 11, 1999, Horizon
Section
The mosquito's
high-pitched whine, buzzing in ears around the globe, signals nights of
irritated slapping and days of itching welts. Tiny and ubiquitous, these
airborne vampires thrive virtually everywhere on the planet, including
the Arctic tundra. They may be the Earth's most annoying animals. They
are definitely the deadliest to humans.
Worldwide, mosquitoes are
infamous as carriers of some widely lethal diseases including yellow fever,
encephalitis, dengue fever and malaria. The World Health Organization estimates
that mosquito-borne diseases infect up to 700 million people each year,
killing more than 2 million. Malaria alone killed 1.1 million people in
1998 -- 100 times as many as have died since the beginning of the violence
in Kosovo.
In the United States, where
sophisticated medicine and mosquito-control efforts have made such diseases
extremely rare, the insect's principal menace lies in its bite. Which isn't
a bite at all.
In fact, it isn't even what
mosquitoes normally do for a living. For their
ordinary day-to-day existence,
both males and females live on fruit and flower nectar. Males consume nothing
else and never bite anybody.
Females, however, require
a nutritious, protein-rich blood meal to produce viable eggs. So they come
equipped with the complex blood-extraction equipment that is so familiar
-- after the fact -- to Washingtonians in warm months.
A Bite to Remember
After tracking down a likely
target, the mosquito lands gently and begins probing for a blood vessel
with its proboscis, the long tube that contains the mouthparts. The proboscis
is a marvel of evolution that makes the hypodermic needle look crude. On
the outside is the labium, a gutter-shaped lower lip that protects the
bundle of delicate mouthparts collectively called a fascicle.
The hungry female presses
the labium against the victim, pushing the
fascicle into the skin.
Two mandibles act like long, microscopic scalpels, cutting through the
skin with saw-like tips. A pair of tubes follows just behind the mandibles,
gently probing for tiny blood vessels called capillaries.
When it finds a blood vessel,
the mosquito dribbles saliva down into the tiny wound. The saliva contains
hundreds of handy compounds, including anticoagulant proteins that prevent
blood from clotting and other chemicals that dilate capillaries, increasing
flow.
It is this saliva that causes
everything humans hate about the tiny
insects. The itchy red welts
that mark the wound are allergic reactions to the foreign proteins, and
the organisms that cause disease are squirted into the body along with
the saliva.
The whole blood-sucking process
is remarkably brief. Finding a blood vessel rarely takes more than 30 seconds.
After that, speed becomes crucial. A slow mosquito is usually a dead one,
since the human body can take as little as three seconds to react to the
saliva injection.
Once the capillary is prepped,
pumps behind the mosquito's head draw blood into the stomach, which in
most species is reserved for the blood meal. (Nectar and other foods go
into a separate crop.) The stomach balloons out, and masochistic backyard
scientists can watch it turn crimson. The average draw is about .000004
liters, about one-thousandth of a teaspoon.
In less than two minutes,
the mosquito can withdraw its own body weight in blood, pull its proboscis
gently out and fly clumsily away. Weighted down by its meal, it goes into
what Cyrus Lesser, chief of mosquito control for the Maryland Department
of Agriculture, describes as a "controlled crash," looking for a protected
spot where it can digest its meal safe from predators.
"They concentrate it," Lesser
says. "It's almost like a dehydrated food: They take all the fluid out,
and within a few hours they're back to
literally the same size
they were before."
Why You?
Exactly what attracts mosquitoes
is a mystery, but scientists have
discovered some of the basics.
Body heat and carbon dioxide exhaled by living creatures can draw mosquitoes
from as far away as 100 feet. So can the lactic acid and folic acid in
your sweat. Mosquitoes that feed during the day may key in on movement
and color (or heat, which tends to increase with darker colors). For bug
avoidance, white clothing is good; red, brown and black are bad.
So to prevent mosquito bites,
all you have to do is stand still, stop
breathing, quit sweating
and wrap yourself in a sheet. Not surprisingly, many people prefer to use
repellents -- which don't exactly repel the creatures. The most familiar
is DEET, or diethyl toluamide. Developed after World War II and released
in 1954, it interferes with tiny sensory pits and hairs on the mosquito's
antennae and body that it uses to sense carbon dioxide.
"Essentially, DEET blocks
receptor sites on the mosquito and makes it blind to the host's presence,"
says Donald Barnard, a USDA researcher in Gainesville, Fla. "There's a
lot of natural-product-based repellents on the market, but nothing's as
effective as DEET."
"There are people on the
Eastern Shore who tell you the biggest single use of mosquitoes is to keep
the tourists out," Lesser offers with a laugh. "Large areas of the Eastern
Shore have been protected from development because the mosquito and biting
fly populations are so high." Actually, mosquitoes are flies. They belong
to the two-winged order Diptera, or "true flies." Even their name means
"little fly" in Spanish. But unlike most of their cousins, the mosquito's
role in ecosystems is unclear. "Insectivorous birds and bats usually key
in on bigger prey," Lesser says. "The things that most rely on mosquitoes
would be viruses and the malaria disease-causing organisms that need mosquitoes
to live." A rare orchid in the Midwest seems to rely on the mosquito for
pollination, and their sheer
numbers make them popular
but not essential food for lots of bird species. Otherwise, go figure.
There are more than 3,000
species of mosquito on the planet, and about 170 in the United States.
In the Washington area, the most common types are Aedes, Culex, Anopheles
(whose cousins carry malaria) and Psorophora.
Buzz Off
On a regional basis, typical
control measures include swamp drainage, chemical sprays to kill larvae,
bacteria to sicken adults and placement of larvae-eating fish. (Gambusia,
a species native to Maryland, is a popular control animal in that state
and elsewhere.)
Around the house, you can
cut your skeeter count dramatically by
eliminating standing water
in bird baths, potted plants, small pools, pet dishes and even the odd
used tire. (A recent noxious immigrant, the Asian tiger mosquito, came
to America via the international scrap-tire trade.) This may be particularly
important if you have a dog: Aedes mosquitoes spread canine heartworm.
There's more! Check
out "Skeeters" Part II, "Killers on the Wing,"
and Part III, "The Circle of Life."
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