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Locust records are those held in the archives of the ;Centre for Overseas Pest Research. In general, these are reports made by organisations responsible for control in the country concerned. Locust data were. plotted on maps month by month to show the most important populations reported in each degree square (Figs. 5 and; 9). In addition, larger-scale maps were drawn to shove the principal fledging areas and the dates when swarms were reported prior to and during likely periods. of movement across the Red Sea (Figs. 6 and 10). Places mentioned in the text are shown in Fig. 1.

Occurrence records depend on people seeing and importing locusts. Sparsely populated areas are under-reported so it is often difficult to estimate the extent of infested areas. When Egypt is invaded from the east, first reports are often of swarms reaching densely-populated parts of the Nile Valley, after crossing the eastern desert. Thus, the date of the first report may not be the date of first arrival, However, once survey and control teams, are in the field, it is more likely that dates of reports correspond with arrival dates, at least on the Red Sea coast; but a difficulty remains that a subsequent swarm report from the interior could refer either to a new arrival or to a swarm moving inland which had been present but unreported, at a coastal site several days previously.

Although this paper is concerned with the movement of swarms it is also necessary to consider non-swarming populations when discussing the possible source areas for swarms. This is because a few locusts found in an area may indicate that much larger populations were present but went undetected. As locusts are known to change shape when they change their phase from gregarious (existing as swarms) to solitary (locusts not in swarms), it is possible to compare these shapes as a guide to the likely parentage of such locusts. Thus, if a sample has gregarious morphometrics it is likely that the locusts have been part of a swarm. Alternatively, if they have solitarious morphometrics it is more likely that they were part of a low-density population.

Wind records were extracted from daily weather reports published by Egypt (U.A.R. Meteorological Dept. 1955, 1968), Sudan, (Sudan Meteorological Service 1955, 1963-70) and Saudi Arabia (S.A. Meteorological Dept. 1968), from the Northern Hemisphere data tabulations of the U.S. Weather Bureau (U.S. Dept. of Commerce 1968), and from routine daily weather data transmissions received by the British Meteorological Office or requested from local meteorological departments. These wind records were used in two ways: in the preparation of vertical time sections similar to Fig. 2, and on weather maps (see Fig. 8).

Fig, 2 suggests considerable day-to-day variations in the pattern of winds over the Red Sea above about 1 km. A method of displaying such patterns is to draw streamline maps at various levels, each based on winds measured more or less simultaneously. at many stations. Unfortunately, there are often not enough observations to define clearly the streamlines over the area of interest. Moreover, considerable labour is required to construct maps at several levels for several times each day during a whole month. As a compromise, wind soundings up to 3 km from available stations were taken to represent the flow over the northern Red Sea, and the drawing of streamline maps was confined to a few days of particular interest. For all stations there are gaps in the sequence of daily observations.

Vertical wind soundings were arranged in sequence for a three-month period to show day-to-day changes before, during and after the periods of possible crossing. These time sections (e.g. Fig. 2) were used to estimate winds on likely crossing days. Winds having the following ranges of directions were considered capable of assisting westward displacement of locusts 10-140° -: at Hurghada, Luxor and Aswan, 360-90° at Port Sudan and 10-90° at Jiddah (see Fig. 4). Speeds in excess of 3.5 m s^-1 (7 kt) only, were used as it was thought they would be more representative of the flow over the whole width of the Red Sea than lighter winds which tend to be more variable in direction over distances. comparable with the width of the Red Sea. The data are summarised on Figs. 7 and 11. In addition, weather maps were drawn for four likely periods of crossing to show wind patterns in the lowest 1-2 km of the atmosphere.

FIG. 4. - Schematic maps showing examples of observed winds, and inferred streamlines of the flow patterns, on occasions with downwind movement of locusts from Arabia to Egypt.

Daily temperature soundings were also available for Cairo, Aswan, Port Sudan and Khartoum.

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