Radar observations of concentrations of insects above a river in Mali, West Africa

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Abstract
1. Radar observations are reported which demonstrate nocturnal accumulations of insects in the altitude range 30–130 m over some sections of the River Niger.2. It is shown that on occasions when individual insect tracks were resolved, the accumulations were maintained by positive insect flight manoeuvres, and were not the result of passive response to air movement.3. The mechanisms by which the insects might perceive the presence of the river are examined and it is deduced that at least some of the insects were able to detect the river boundaries by a visual or radiation sensing mechanism. It is noted that if the insects were using visual perception, their eyes must have been able to function in the absence of moonlight.4. Evidence is presented which suggests that amongst the insects which contributed to the phenomenon were Ephemeroptera, and also larger insects, the latter becoming concentrated over the river in the course of migratory flight across the area.5. Reasons why the insects accumulated over the river are discussed and it is suggested that the most probable strategies included the formation of mating swarms by some insects, and the use of the river by others as a linear feature to facilitate their orientation. It is also suggested that the insects may have been responding to differences in infra‐red radiation from the river and the bank, or to the warmer, moister air to be expected over the river.