Hyalomma truncatum (Acari; Ixodidae): evidence for the inability of adult ticks to discriminate between colours.

Behavioural investigations into the perception and differentiation of coloured objects by unfed adult Hyalomma truncatum ticks revealed that silhouettes of blue, green, red and yellow colour, under illumination by a sun-simulating waveband spectrum, are perceived by the ticks and responded to equally by a directed response. Two green or dark grey rectangles each with a luminance contrast ratio of 5:1 against the white wall of the test arena in combination with an overlapping, equally sized dark grey or green target were consistently reached by ticks in a ratio of 2:1. Since the outer targets were occupied by the double number of ticks compared with the central silhouette this shows that the response is independent of the colour of the object. Investigations into target perception under monochromatic radiation of different wavelength ranges which were evenly adjusted in their irradiances revealed that ticks responded equally to a black target irradiated by blue, green, yellow and red light of wavelengths 428-472, 517-563, 549-591 and 606-654 nm, respectively. These results indicate the lack of true colour vision in H. truncatum.