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High Altitude Migration of Oscinella frit L. (Diptera: Chloropidae)
C. G. Johnson, L. R. Taylor and T. R. E. Southwood
Journal of Animal Ecology
Vol. 31, No. 2 (Jun., 1962), pp. 373-383
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/2148
Page Count: 11
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1. The vertical distribution of frit fly (O. frit L., Diptera, Chloropidae) between 9 and 1000 ft (2.7 to 305 m) above ground level is described for each hour of the day on 5 days in August 1955. 2. Flight began at 05-06 h G.M.T. The density height profile begins steeply, becomes less steep as upward transport increases, and reaches its minimum slope between 11 and 13 h, when about 90% of the airborne population was between 20 and several thousand feet. This coincided with the maximum numbers in the air when, it was estimated, on average about 92 000 O. frit were airborne up to 3000 ft over an area 106 sq. ft (283 X 103 sq. m). Most had descended by 15-16 h and flight ceased after 19 h. 3. This rhythmic process of flight and dispersal via the upper air is repeated day after day, though not necessarily by the same individuals; this, with the evidence of phototactic exodus from the breeding site, indicates that the distributive process in O. frit (at least of the August generation in Britain) resembles that of aphids in which aerial currents play so large a part. Aphid and frit-fly vertical profiles are compared. 4. The density curve at 9 ft coincides with the curve for total population during the day, except at the peak where the integrated curve is slightly displaced. This suggests that the average duration of flight is about 1 h or a little more at the peak. The median height of flight (above which half the population is distributed) reached a maximum of 1300 ft at 11-12 h and the mean wind-speed at this height was 17 m.p.h.; the average drift expected at this time would, therefore, be rather more than 15-20 miles. During the day, the number descending to earth during the period of the observations was estimated to average about 3/sq. yd (sq. m)/day. 5. We conclude that O. frit is a migrant, and that dispersal through the upper air by winds is an evolved adaptation.
Journal of Animal Ecology © 1962 British Ecological Society