ic animals in
Central and Southern Africa. In some sections it is almost impossible to
keep any kind of imported animals on account of this disease which is
caused by a parasite (_Trypanosoma brucei_) similar to the one causing
surra. This parasite is to be found in several different kinds of
native animals which seem to be practically immune but are always a
source of danger when other animals are introduced. Two or three species
of tsetse-flies are responsible for the transmission of this disease.

Another group of flies much smaller but more numerous and much more
insistent are the black-flies or buffalo-gnats (Fig. 24). For more than
a century these little flies have been recognized as among the most
serious pests of stock, particularly in the south where, besides the
actual loss by death of many animals yearly, the annoyance is so great
as to sometimes make it impossible to work in the field. Human beings
are often attacked, and as the bite is poisonous and very painful great
suffering may result and cases of deaths from such bites have been
reported.

Belonging to another family, and smaller, but much like the buffalo-gnat
in habits, are the minute little "punkies" or "no-see-ums" which
sometimes occur in great swarms in certain regions where they make life
a burden to man and beast. While it has not been shown that either the
buffalo-gnats or the punkies are responsible for the transmission of any
disease, their habits of feeding on so many different kinds of wild and
domestic animals as well as on man makes it possible for them to act as
carriers of parasites that might under proper conditions become of
serious importance. Then, too, the irritation caused by the bites of
these insects usually causes scratching which may result in abrasions of
the skin that open the way for various harmful germs, particularly those
causing skin diseases.

Coming now to the group containing the house-flies and related forms we
find a number that are of interest on account of the suffering that they
may cause, particularly in their larval stages.

The screw-worm flies (_Chrysomyia macellaria_) are among the most common
and important of these (Fig. 25). These "gray flies," as they are
sometimes called, lay a mass of three or four hundred eggs on the
surface of wounds. The larvae which in a few hours hatch from these make
their way directly into the wound where they feed on the surrounding
tissue until full grown when they wriggl