| The Circle of Life
By Andrew Curry
Special to the Washington
Post
August 11, 1999, Horizon
Section
Few of us regard
the mosquito as an aquatic creature. But all mosquito species need water
to breed; and three of the insect's four life stages (observed and recorded
at least as early as Aristotle 2,300 years ago) take place in or under
water.
A stomach full
of blood signals the female to start producing eggs. She mates just once,
storing enough sperm in special pouches to fertilize several batches of
eggs in her lifetime, laying as often as every three days. Each batch requires
a new blood meal. (Some species -- notably the giant Toxorynchites -- do
not suck blood at all, getting the protein they need by eating other larvae
during early stages of development.)
Almost all species,
including the locally popular Anopheles and Culex, lay their eggs directly
in water, singly or stuck together in floating rafts. Others, such as Aedes
and Psorophora, find a fairly dry spot that will soon be flooded, triggering
the hatch.
Within a few
days (depending on weather and species), the eggs hatch into larvae. They
swim around, feeding on microorganisms and organic matter suspended in
the water, but breathe through miniature snorkels that protrude above the
surface. That's why spreading oil on ponds was once a common control technique.
As the larvae
get bigger, they molt, or shed their skin, exactly four
times. The fourth molt --
usually four days to a week after the larvae
hatch -- produces the pupa,
a quiescent stage in which the mosquito does not eat, but floats just below
the surface, breathing through tubes. After a couple of days, the pupal
skin splits and the adult emerges, pops to the surface, dries off and flies
away.
Within hours,
males are using their bushy antennae to listen for the
distinctive whine of the
female's wings, a signal that she is ready to
mate. Males die soon after
mating; females set off in search of blood. Some can fly several miles,
and mosquitoes have been captured more than 100 miles out to sea.
Mosquitoes thrive
at 80 degrees Fahrenheit, get sluggish at 60 degrees and zonk out entirely
below 50, which is why you don't see many of them around here after October.
They hibernate over the winter, but can pop up in suitably warm pools of
melted snow.
"In the springtime
or fall of the year, [female mosquitoes can live] over a month because
it's cooler and damper," says Cyrus Lesser, chief of mosquito control for
the Maryland Department of Agriculture. "The thing that really shortens
a mosquito's life is dry, hot weather. They just lose moisture and literally
dehydrate." Some species have evolved to survive in cold climates, surviving
frozen winters by hibernating as adults or as eggs. The short Arctic summers
then bring mosquitoes out all at once, in swarms seen in few other places.
Scientists researching mosquitoes in northern latitudes have recorded nearly
300 bites per minute on an exposed forearm. Mosquito control staff here
also use the forearm method to tally mosquito density.
There's more!
Check out "Skeeters" Part I, "The Horrible
Truth About Mosquitoes," and Part II, "Killers
on the Wing."
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