Of these the breeze-flies or gad-flies possess powerful piercing
mouth-parts, with which they torment both stock and human beings. A
well-known species is the long brown clegg (_Hæmatopota pluvialis_),
often met with in woods. In some tropical kinds the jaws are of enormous
length.
Robber-flies are voracious and insatiable forms which prey upon other
insects, even wasps and tiger-beetles being among their victims.
Hover-flies are swift and elegant insects which have already been
mentioned in connection with flowers. Some of them closely resemble bees
in appearance.
The dreaded tsetse fly (_Glossina morsitans_), so fatal to horses in
parts of South Africa, belongs here. Germs of the fly-sickness
(_Nagana_) are introduced into the blood of the victims, Tsetse flies of
other species are responsible for “sleeping sickness,” which makes parts
of tropical Africa uninhabitable.