Scototaxis and target perception in the camel tick Hyalomma dromedarii.

The camel tick, Hyalomma dromedarii, exhibited positive scototaxis in an arena, e.g. it oriented towards a black or grey target in front of a white background. The degree of the scototactic response varied with the size and the elevation of the target, with its luminance contrast, with its shape and with the speed by which the target was moved: (1) the response to stationary and moving targets increased with increasing target size; (2) presentation of the targets at an elevation of 11 degrees--15 degrees induced the highest response; (3) the response decreased with decreasing luminance contrast of the target; (4) targets with the shape of a disk, a triangle standing on a vertex, a vertical bar or a silhouette of a dromedary caused high responses; a low response was observed when the target was a horizontal bar and there was no response to a striped pattern; (5) the smaller the size of a disk, the faster it had to be moved to elicit an optimum response. The smallest disk which elicited a significant response appeared under a visual angle of 4.8 degrees for a thick at the starting point. The smallest dromedary-shaped silhouette which elicited a significant response corresponded to the silhouette of a real dromedary at a distance of 18 m.