Landing site preferences of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) in the laboratory: avoidance of horizontal features?

Abstract

AbstractLanding site preferences of fed males of Glossina morsitans morsitans Westwood were measured in ca 50-cm-wide arenas in the laboratory. Flies were presented with simple black shapes of 225 cm2 against a white back-ground. Preference ran in the order: circle < square < triangle (whether the targets were black shapes on white or white on black). However, the triangle on its side (i.e. rotated through 90°) or the square presented as a diamond (i. e. rotated 45°) were both greatly preferred over their original orientations, whereas inverting the triangle (by rotating it 180°) greatly decreased its attractiveness. These preferences were retained whether the targets were compared paired or singly. The most obvious correlation was between preference and the relative lack of horizontal features in the target. When the diamond (the most attractive target) was split horizontally by a narrow white band, its attractiveness was reduced by ca 60%, whereas when split by a vertical band the attractiveness was reduced by only ca 25%. Similarly, when an additional narrow black bar was placed horizontally beneath the diamond its attractiveness was cut by 47%, whereas when the bar was placed vertically beside it the attractiveness was cut by only 28%. These reductions occurred even though the targets should have been more attractive because of their greater edge length and area. When the effect was tested with a series of rectangles, the greater the ratio of the vertical:horizontal edge length, the greater the attractiveness. The relative inhibition of landing by the presence of horizontal features in a laboratory-tested landing site thus seems clear, notwithstanding that horizontal resting sites in nature are the most preferred.