ABSTRACT
During the winter months, the banks of the Nile near Khartoum abound with small non-biting midges, Tanytarsus lewisi Freeman (Diptera, Chironomidae), which emerge from the surface of the river shortly after sunset and fly around in the immediate neighbourhood for about 
hr. In an endeavour to relate variations in the incidence of these insects (‘nimitti’) to seasonal and climatic factors, collections were made every evening for some considerable time; the insects trapped for this purpose provided the source of material used for the experimental work described below.
