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Temperature and humidity. Assman and sling psychrometers were supplemented by maximum and minimum thermometers in a small Stevenson screen. Air temperatures at swarm sites were usually read by the Assman at 4 1/2 ft. above the ground, and on some occasions at 1 in. also. The minima were obtained from the Stevenson screen, set up on or near the roosting site, and the maxima either similarly or from the nearest meteorological station, corrected when necessary for differences of altitude by the dry adiabatic lapse rate (3°C per 1,000 ft.). Temperatures throughout the paper are given in degrees Celsius unless otherwise stated.
Surface wind. Wind-speed was measured by a Sheppard cup anemometer at 2 metres, supplemented at times by a direct-reading hand anemometer.
Wind-direction was observed in Kenya from cigarette smoke, Verey smokepuffs, smoke-generators, or a small sleeve attached to the anemometer. For the later observations in Somalia a paper wind-vane, consisting of a hexagonal tube 5 1/2 in. long and l in. in diameter, was mounted on the Sheppard anemometer, with the casing of the axle carrying the cups passing through holes cut in the upper and lower surfaces of the paper tube respectively 4/5 in. and 3/5 in. from one end, so that the tube was inclined upwards at a slight angle away from the mast and rotated freely without any tendency to rise and foul the cups.
Upper winds. Pilot balloon observations were made during the later observations in Somalia, for the purpose of determining the winds, vertical as well as horizontal, experienced by the higher-flying locusts. The results are presented in Rainey & Waloff, 1951.
Observations on swarms
During our observations the emphasis was in general on the activity of swarms, rather than on the behaviour of individuals, though we are now of the opinion that the latter type of observation is likely to be particularly illuminating.
Observations on the orientation (course) and direction of displacement with respect to the ground (track) of individual locusts, made during the earlier observations by direct comparison of flying locusts and a prismatic compass, were found during the Somalia observations to be greatly facilitated by the use of a modification of the mirror method described by Kennedy (1951). The mirror used was 2 ft. X 1 1/2 ft.., and provided with two pointers pivoted at its centre. As a flying locust passed over the mirror, the observer aligned one pointer parallel to the course and the other to the track of the moving image, and subsequently read off the position of the pointers from a scale of degrees on the mirror.
The method was initially used, in conjunction with simultaneous observations of the wind, for obtaining the air-speed of flying locusts. These data also provided useful material for examining the possible effects of shortperiod fluctuations in wind on orientation, though they may have had some limitations in this respect. It is for example possible that the method may under-estimate the variance of individual courses within a swarm, since the observer is most likely to fail to set the pointers on locusts whose course and track differ widely from the. preceding position of the pointers. A further bias might perhaps be introduced by a tendency to miss the locusts with the higher ground-speeds, i.e., with the more down-wind orientations.
A useful refinement of the method would be the estimation of the height of each locust observed, by reference to a scale of apparent wing-spans mounted on the mirror.
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