34 



of November and the beginning of December, about the time when 

 the wheat is being sown, but are not present in their largest numbers 

 until the months of February and March. Shortly after this 

 they disappear and are not noticed again until the following 

 winter. 



" They are at their worst during the hours of sunset and sunrise, 

 when it is impossible to walk anywhere with comfort without a 

 veil or smoky torch to keep them away. During the heat of the 

 day many of them rest among the vegetation ready to attack 

 anything that approaches their haunts. 



" At night they are harmless. 



" Their habit of appearing when the wheat is sown, disappearing 

 when the wheat is harvested, and apparently originating among 

 the vegetation, has given rise, among the natives, to the erroneous 

 idea that their presence is entirely due to the wheat. 



" Nimitti are not usually a pest in Dongola Province south of 

 Debba, though a stray swarm sometimes reaches Merowe and the 

 district. 



" A glance at the map of Dongola Province will show that at 

 Debba the Nile makes a pronounced bend. 



" As the river falls in the autumn, the rocks in the cataracts 

 immediately north of Abu Fatma — the Third Cataract — become 

 exposed. The cataracts then constitute ideal breeding places for 

 these tiny sandflies, which shortly afterwards emerge in 

 myi'iads. 



" During the winter the wind blows steadily from the north and 

 the Nimitti are consequently borne southwards to Debba, and then, 

 owing to the altered course of the river, on into the desert. In the 

 spring the wind changes and blows from the south, carr3ring away 

 with it most of the Nimitti. As the summer advances, the river 

 rises, the rocks in the cataracts become covered with water, and 

 suitable breeding places are then few and far between. 



" The occasional swarms that occur in Merowe and the district 

 are usually to be accounted for by temporary changes in the 

 direction of the wind. Sometimes, however, as in the spring of this 

 year, when the river \^s exceptionally low, it is possible that rocks 



