Sandflies
by John Walsby
There is one hazard
along New Zealand's coastline that even the Coast Guard is unable
to save or protect us from: small black biting flies. In some
countries they are called black flies but both here and in Australia
we know them as sand flies because they are so common on some
of our sandy beaches.
There are about a dozen
different species in New Zealand, not all of which bite. Those
that do can sometimes be so numerous that they can make a day
at the beach or beside lakes and rivers quite miserable, for they
often land and bite almost as fast as they can be brushed off.
At other times they
are much less of a problem. The weather makes a difference. The
little flies are much more active on warm days when humidity is
high and some people claim that a sudden increase in sandflies
is a sign that it is about to rain.
Sandflies far more
numerous along beaches that are close to running freshwater for
it is in streams and rivers that most breed. The females lay their
eggs on stones or water plants just at or below the waterline.
The legless larvae
that hatch after a few days need well oxygenated water and prefer
flowing streams although some are found in still ponds where there
are good growths of aquatic plants to oxygenate the water. The
larvae feed on minute organisms in the water which they strain
from the passing current with a pair of bristle toothed combs
beside the mouth. To feed most effectively they hang out into
the current to sweep their feeding combs through the water.
To avoid being swept
away the small larvae anchor themselves securely. From the salivary
glands each larva secrete a pad of gelatinous glue to its home
base and clings to this with a circlet of hooks at the rear of
the body. Fully grown larvae pupate underwater in open pouches
woven from silk on to the stone or plant that each is living on.
Most mature from eggs
to adults in about six weeks but water temperature and food availability
can delay development and those that hatch in the late autumn
do not mature until next spring. When pupae are ready to metamorphose
into adult flies they leave their silken holsters and float to
the water surface.
Like mosquitoes it
is only female sandflies that bite us and drink our blood to obtain
the protein-rich food they need to make their eggs. The sandfly's
bite is always more noticeable than that of a mosquito because
a mosquito has mouthparts like a fine hypodermic needle and can
often penetrate our skin without us noticing. Only later when
our body reacts to the anticoagulant injected into the wound to
stop the blood clotting do we notice the irritable soreness.
When female sandflies
feed they must bite into our skin to reach a blood capillary and
this is usually noticed immediately. Unlike many blood sucking
tropical insects sandflies do not carry any serious diseases,
so they are not a major concern to our health in New Zealand though
they can certainly be an irritating and painful nuisance at the
beach.
