_Buffalo gnats_, which are often called sand-flies, are insects which breed in running water and amongst cataracts and rapids. They are formidable biters although they are not known to cause any disease. The same protective measures are indicated in their case as in that of owl midges. _House Flies._—There are several species of these. They are definitely known to distribute certain harmful bacteria and protozoa and to contaminate food by carrying the organisms on their bodies, wings and legs, or depositing them by regurgitation or in their droppings. Hence it is very important to protect all food and drink from flies and to destroy these insects wherever possible. The subject of their destruction is too large a question to be dealt with here, so all that need be said is that the traveller should provide himself with wire mesh fly covers to screen food and also with pieces of mesh or calico weighted with beads, which can be utilized for covering vessels containing milk or other liquids. It is worth noting that ordinary fish netting hung over a tent door or used to screen the windows or doors of a house will effectively exclude flies, despite its large mesh. _Tsetse Flies._—These are considered under Sleeping Sickness (_see_ page 234). _The Congo Floor-maggot Fly._—It is the maggot or larva of this fly which is important, as in many parts of Africa it infests the floors of native huts and it is a blood-sucker feeding at night. So far as is known it does not convey any disease but it is an unpleasant visitor, and if its numbers are great they may extract comparatively large quantities of blood from their unconscious victims. _Prophylaxis._—General cleanliness and enforcement of sanitary measures, the use of high beds, the scrutiny of sleeping mats and blankets in which eggs or larvæ may be concealed. Infected huts can be rendered habitable by firing the ground or by removing the surface soil, which can then be disinfected.