The spectral sensitivity in the wavelength range of 340-750 nm was determined by both a behavioral approach based on spontaneous positive phototaxis and the electroretinogram (ERG). 2. Concerning phototaxis the camel tick, Hyalomma dromedarii, showed two sensitivity maxima, one in the UV range (ca. 380 nm) and another in the blue-green range (ca. 500 nm). At higher intensities the relative sensitivity was more pronounced in the UV and at lower intensities more pronounced in the blue-green (reverse Purkinje shift). In the tropical bont tick, Amblyomma variegatum, there was a single sensitivity maximum in the blue range (ca. 480 nm). 3. In the ERG there was a maximum in the blue range (ca. 470 nm) in both species and a weak secondary maximum in the UV in Hyalomma. 4. The absolute sensitivity was very high. The threshold irradiance of phototaxis was as low as 5.2 x 10(6) photons.s-1.cm-2 in Hyalomma and 5.2 x 10(8) photons.s-1.cm-2 in Amblyomma. 5. When the eyes of Hyalomma were covered, the threshold irradiance was still very low, namely 5.2 x 10(8) photons.s-1.cm-2, indicating high absolute sensitivity of the extraretinal photoreceptors. 6. The visual field of the eyes was determined by ERG measurements. In both species the optical axis of each eye, i.e., the center of the visual field, was directed somewhat to the side and above the horizon. In Hyalomma this direction was 35 degrees to the long axis of the animal and 30 degrees above the horizon for natural body posture during walking. In Amblyomma the corresponding angles were 39 degrees and 33 degrees, respectively. The size of the field (at 50% sensitivity) in Hyalomma was relatively small, namely 14 degrees in the horizontal and 25 degrees in the vertical direction, compared with that of Amblyomma with 43 degrees and 49 degrees, respectively. 7. This is the first demonstration in ticks of spectral and absolute sensitivity by the behavioral approach and of the visual field by ERG. The results suggest that tick eyes possess features of both spider eyes and insect ocelli.
