10-12-2012, 01:08 PM
Mosquitos and other biting Diptera
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Biology
The biting Diptera are two-winged flying insects that suck blood from humans and
animals. In many parts of the world their biting is a considerable nuisance. More
importantly, they are carriers of a number of diseases, mostly in the tropics,
causing illness and death on a large scale.
The most important group of biting Diptera is the mosquitos, which have a
long, slender body and long, needle-shaped, piercing mouthparts. Others include
the blackflies, phlebotomine sandflies, tsetse flies, biting midges, horseflies
(tabanids) and stable flies, which generally have shorter biting mouthparts and
more robust bod
Mosquitos
Mosquitos differ from the other biting Diptera in having a long slender body, long
legs and long needle-shaped mouthparts (Fig. 1.1a). The wings sometimes have
discernible patterns of scales. The adult insects measure between 2 mm and
12.5 mm in length.
Some species bite in the morning or evening and at night; others feed during
the day. Species may bite indoors or out of doors.
Blackflies
Blackflies are stout-bodied, about 1–5 mm long, and are usually black, although
orange and yellow species exist. They have relatively large eyes. The legs are short,
and the wings are short, broad and colourless (Fig. 1.1b).
Biting midges
Biting midges are about 1.5 mm long. They bite at any time of day or night, but
most commonly in the late afternoon and the early part of the night. Because of
their short mouthparts (Fig. 1.1c) they are not very successful in biting through
clothing; they are often observed in swarms around the head, biting the face. Other
exposed parts of the body may also be attacked. Most species only feed out of
doors. They can be a severe nuisance and because of their small size they can easily
pass through standard mesh mosquito nets.
Phlebotomine sandflies
Sandflies are about 1.5–4 mm long. They have a hairy appearance, conspicuous
black eyes and long, stilt-like legs (Fig. 1.1d). They have a characteristic hopping
flight with many short flights and landings. In contrast to all other biting Diptera,
the wings are held erect over the body when at rest. Sandflies usually bite after
dark, but may bite in daytime during cloudy weather in forests. Most species feed
outdoors but a few feed indoors. Because of their short mouthparts they cannot
bite through clothing.
Mosquitos
Mosquitos are important vectors of several tropical diseases, including malaria,
filariases, and numerous viral diseases, such as dengue, Japanese encephalitis and
yellow fever. In countries with a temperate climate they are more important as
nuisance pests than as vectors.
There are about 3000 species of mosquito, of which about 100 are vectors of
human diseases. Control measures are generally directed against only one or a few
of the most important species and can be aimed at the adults or the larvae.
Life cycle
Mosquitos have four distinct stages in their life cycle: egg, larva, pupa and adult
(Fig. 1.2). The females usually mate only once but produce eggs at intervals
throughout their life. In order to be able to do so most female mosquitos require
a blood-meal (Fig. 1.3). Males do not suck blood but feed on plant juices.
The digestion of a blood-meal and the simultaneous development of eggs take 2–3 days in the tropics but longer in temperate zones. The gravid females search
for suitable places to deposit their eggs, after which another blood-meal is taken
and another batch of eggs is laid. This process is repeated until the mosquito
dies.
Depending on the species, a female lays between 30 and 300 eggs at a time.
Many species lay their eggs directly on the surface of water, either singly (Anopheles)
or stuck together in floating rafts (e.g. Culex). In the tropics, the eggs usually
hatch within 2–3 days. Some species (e.g. Aedes) lay their eggs just above the water
line or on wet mud; these eggs hatch only when flooded with water. If left dry they
can remain viable for many weeks.