Spectral Sensitivity to Light of Two Species of Ticks (Acarina: Ixodidae)

Plots of electroretinogram (ERG) responses of adult American dog ticks, Dermacentor variabilis (Say), and camel ticks, Hyalomma dromedarii Koch, to monochromatic light were unimodal, with peaks at 510 and 470 nm, respectively. Peak eye sensitivities to wavelength were the same at 22 and 29°C, but, for D. variabilis , the ERG responses (millivolts per nanowatt) to wavelengths between 350 and 450 nm were higher at 29°C. Waveforms were characterized by a large negative voltage when the eye was initially exposed to the light stimulus, followed by a correspondingly large positive voltage after the light was turned off. Subsequent exposures of the eye to 0.16-s flashes of light at 0.16-s intervals produced ERG voltages of only about half the amplitude of those produced by the initial exposure. H. dromedarii consistently produced ERG voltage 3- to 10-fold higher than did D. variabilis , and produced faster “on/off” responses. Behavioral bioassays confirmed that both species responded more strongly to blue and green light than to red light. Both the ERG and behavioral bioassay of H. dromedarii showed a positive logarithmic relationship between tick response and radiance. Even red wavelengths were seen when radiance was sufficiently high. With H. dromedarii , the ERG responses to all 15 tested wavelengths between 350 and 650 nm could be visually matched by varying the radiance of 466 nm, indicating that the camel tick possesses a monochromatic receptor system.