Ernst Hess

Warm and cold receptors of two sensilla on the foreleg tarsi of the tropical bont tickAmblyomma variegatum

SummaryA pair of antagonistic thermal receptors has been identified in each of two long, tapering, poreless setae located distally on the foreleg tarsi of the tropicalbont tick,Amblyomma variegatum (Fig. 1). One, the cold receptor, responds to a rapiddrop in temperature (T) with a sudden rise in impulse frequency (F). The other, a warm receptor, responds to a rapidrise inT with a sudden rise inF (Figs. 2, 4). These two units are unusual for sharing their seta with two other units which are mechanosensitive. The four are distinguishable on the basis of spike amplitude and form (Fig. 3). Hence the thermal sensitivity displayed is hardly attributable to the pair of cells with tubular bodies but rather to the two extending up into the seta (for structure, see Hess and Vlimant 1982, 1983 a).As based on the first 100 ms of the response, differential sensitivity to rapidT change is −16.1± 10.4 (imp/s)/°C for cold units, 17.6 ± 9.5 (imp/s)/°C for warm (Table 1). As progressively larger segments of the spike train are employed to determineF, differential sensitivity of the warm unit drops off much more quickly than that of the cold (Table 2, Figs. 5, 6). In the cold unit resolving power (the difference in rapid temperature change discriminable with 90% probability by a pair of responses of a single unit at average sensitivity) continues to increase as the segment of the spike train determiningF is lengthened (from 0.58 °C for 100 ms segments to 0.41 °C for 1,100 ms segments). Resolving power of the warm unit, on the other hand, tends to decrease as longer segments are employed (from 0.52 °C for the first 100 ms to 0.80 °C for the first 1,100 ms). These relationships provoke the question of whether the spike trains may be evaluated in the CNS in different fashions.

Spectral sensitivity, absolute threshold, and visual field of two tick species,Hyalomma dromedarii andAmblyomma variegatum

Summary1.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 inHyalomma.4.The absolute sensitivity was very high. The threshold irradiance of phototaxis was as low as 5.2×106 photons·s−1·cm−2 inHyalomma and 5.2×108 photons·s−1·cm−2 inAmblyomma.5.When the eyes ofHyalomma were covered, the threshold irradiance was still very low, namely 5.2×108 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. InHyalomma this direction was 35° to the long axis of the animal and 30° above the horizon for natural body posture during walking. InAmblyomma the corresponding angles were 39° and 33°, respectively. The size of the field (at 50% sensitivity) inHyalomma was relatively small, namely 14° in the horizontal and 25° in the vertical direction, compared with that ofAmblyomma with 43° and 49°, 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.