Order Diptera are the True flies, with around 100,000
known species worldwide. They are sometimes known as
'Two-winged flies', because the forewings are fully formed
and used for flight, whereas the hind wings are reduced to
small knobs on short stems known as halteres. These are used
for stability in flight, acting in a similar way to a
gyroscope, balancing the fly while it flies. There are some
exceptions to this description of Diptera wings, as a few
species such as the Louse-flies (Hippoboscidae) are
parasites with very reduced wings.
wings are usually clear with many veins. The wing vienation
is extremely variable and is often an important
antennae are also important as identification features, at the
sub-order level of classification and also at the species
level. The number, size and shape of the segments and
whether there is an arista (bristle) at the tip are all used
to identify flies.
Diptera have compound eyes (many facets or lenses to the
eye). These are often so large that they meet at the top of
the head, allowing them to detect very small movements over
a very large area. The meeting of the eyes is often an
indication that the fly is a male, although in some groups
the females eyes are like this.
mouth-parts of flies are also variable. With a few
exceptions adult flies feed on liquids. Many species have
mouth-parts consisting of pads used to mop up their food.
They regurgitate digestive fluids to help with this process,
which is part of the reason why house-flies are such
important carriers of disease.