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Electrophysiology of the visual system in the cricket Gryllus firmus (Orthoptera: Gryllidae): Spectral sensitivity of the compound eyes
J. Insecr Phwol. Printed in Great
Vol. 31. No. 5. pp. 353-357. Britain. All rights reserved
1985 Copyright
0022-191Oi85 $3.00 + 0.00 I(‘ 1985 Pergamon Press Ltd
ELECTROPHYSIOLOGY OF THE VISUAL SYSTEM IN THE CRICKET GRYLLUS FIRMUS (ORTHOPTERA: GRYLLIDAE): SPECTRAL SENSITIVITY OF THE COMPOUND EYES ABNER B.
LALL,
ECBERT
T.
LORD*
and C. OVID TROUTHt
McCollum-Pratt Institute and Department of Biology, The Johns Hopkins University, Baltimore, MD 21218, *Department of Zoology, Howard University, Washington, DC 20059 and TDepartment of Physiology and Biophysics. College of Medicine, Howard University, Washington, DC 20059, U.S.A. (Received 20 December
1983; revised 3 1 August 84)
Abstract-The electroretinographic spectral sensitivity of the cricket compound eyes shows the presence of two receptor types, a dominant one at 520 nm and another in the near-u.v. (,I,,,,,, 355 f 5 nm) under dark- and intense chromatic adaptation conditions (Fig. 3). The waveform of the electrical responses elicited by short-wavelength stimuli differ from those elicited by long wavelength stimuli (Fig. 1). Key Word Index: Visual spectral
sensitivity,
vision.
INTRODUCTION Neuroethologists have extensively studied the song of the cricket and its neural substrate. However, though the visual system of the cricket is well developed, possessing a pair of compound eyes, a pair of lateral ocelli and one median dorsal ocellus, comprehensive electrophysiological studies of it are lacking. On the basis of calorimetric studies, the presence of two receptor types (A,,,,, 500 nm and &,,X 565 nm) in the eye have been suggested cricket compound (Mazokhin-Porshnyakov. 1962). This communication presents electrophysiological data which suggests the presence of two receptor types, near-u.v. (J.,,, 355 + 5 nm) and green (&,,, 520 nm), differing from those described by Mazokhin-Porshnyakov. A short abstract has been published (La11 et al., 1981).
MATERIALS AND
METHODS
Adult crickets, Gryllusjrmus. were collected in the grounds of Howard University campus, Washington, DC during the months of August, September and October, and kept in the laboratory at room temperature 21-23’C) in a terrarium. The details of the optical stimulator. electronic instrumentation and experimental techniques and procedures have been described earlier (La11 et ~1.. 1982). A short summary is presented here. Intact animals were used for recording electroretinograms from either the whole eye or the ventral sector during the daylight hours (photophase). The recording electrode was a glass pipette (tip diameter 5-10pm) filled with insect Ringer solution and placed subcorneally. The reference electrode consisted of another Ringer-filled pipette placed in the antenna. An Ag-AgCI wire connected the Ringerfilled glass pipette to the input of a high impedence amplifier whose output was displayed on an oscilloscope and photographed by a camera. The optical system consisted of a two-channel
cricket
eyes
stimulator. A 150 W xenon arc provided light for the test beam and a 500 W tungsten source provided the adaptation beam. The test beam was interrupted by a grating monochromator and the adaptation beam with interference filters. Mirror optics and quartz lenses collimated each beam and a quartz beam splitter combined the two beams. The light focussed at the entrance of a fibre optic bundle (3 x 360 mm) which delivered the stimulus to the eye. Neutraldensity filters attenuated the intensity in the two beams. The intensity of the test stimuli was determined by a calibrated photodiode (United Detector Technology, Inc., Santa Monica, CA). Light flashes of 170 ms duration of selected wavelengths were administered over 4.5 log units of light intensity. Test flashes were delivered at either 1 or 2 min intervals, which permitted total recovery of the sensitivity of the eye. A “standard” flash of known wavelength and intensity was periodically given to check the overall change in the sensitivity of the eye. All experiments were conducted at room temperature (21-23 C). The amplitude of the electroretinograms provided an index of the sensitivity of the eye to quantum flux and to wavelength composition of the photic stimulus. In a few cases the electroretinograms were recorded over the entire range of available intensity gradient for 15 selected stimulus wavelengths across the spectrum (34&680 nm) and intensity-response curves were constructed. The spectral sensitivity was obtained by first determining the number of photons needed to elicit a criterion amplitude of the electroretinograms for different stimulus wavelength. A plot of l/Q as a function of wavelength gave the spectral sensitivity function. However. in most cases a simplified experimental procedure was utilized which consisted of one person observing the oscilloscope screen while another adjusted the intensity of the test flash with neutral-density filters at each wavelength until the observer signalled that a criterion response had been met (for details see Ruck. 1965). Three to 353
ABNER B
354
four runs per eye were made across the spectrum both descending and ascending the wavelength scale. Dim illumination elicited low-criterion responses while bright illumination elicited high-criterion responses. For chromatic adaptation experiments the eye was adapted by a continuous high-intensity monochromatic illumination obtained by placing an appropriate interference filter in the adaptation beam. Test flashes were superimposed on the selective adaptation beam. RESULTS
Characteristics of’ the electroretinogram Electroretinogram responses elicited by two wavelengths (420 and 580 nm) are presented in Fig. 1 by dim and bright illumination were different. A response was elicited by a photic stimulus after an initial latency period of between 15-25 ms depending upon the intensity of the stimulus. The electroretinogram was dominated by an on-negative monophasic wave. When the stimulus was turned off, the response returned to the baseline. Qualitative differences existed in the waveform of the responses elicited by short (420 nm) and long (580 nm) wavelength stimuli. These waveform differences with respect to wavelength would suggest the presence of Log I = 13.95
photons
m-2
LALL PI Ul. short and long wavelength receptors in the cricket compound eyes. similar to what has been described in Limulus median eyes (Chapman and Lall, 1967), fireflies (La11 et al., 1982) and Dineutes (Bennett, 1967) compound eyes. Intensitywesponse ,function (V-log I) The relationship between the amplitude of the negative electroretinogram recorded from the compound eye and the intensity of the test stimulus for different wavelengths across the spectrum is presented in Fig. 2 for dark-adapted Gryllus jirmus. Systematic changes in the slope of the response curves for different wavelengths have been described in some arthropods (Limulus: Chapman and Lall. 1967; Dineutes: Bennett, 1967) and have been taken as evidence for the presence of different receptor types, but so far, these are exceptions. Therefore, it is a point of interest to find out whether there are any systematic changes in the intensity response function (V-logI) as a function of wavelength in G. $rmus. From the data presented (Fig. 2) there appears to be no obvious change in the slopes of V-log1 curves as a function of wavelength. The curves for different wavelengths tended to be parallel. This fact means that different receptors (as pointed out in the following section) in 15.55
I-’
photons
m-a s-l
-\ 0
-c-
-10
-15
-2.0
-2.0 420nm Fig.
580 “Ill
I. Electroretinograms recorded from a dark-adapted compound eye of an intact cricket, Gryllus firmus. elicited by light of wavelengths: 420 and 580nm. The numbers next to the response are the neutral-density attenuation of the test flash. Stimulus monitor is below the response. A 1mV and 20 ms calibration signal precedes each response. Negative deflection going upwards.
Electrophysiology
355
Spectral sensitivity curves
3.01 2.5-
of the visual system
Gryllus firmus -compound eyes dark adapt
Dark-adapted preparation. At threshold, the probability of detecting the response of a small population of receptors is greater at low criterion levels than at higher criterion levels, assuming that the threshold is the same for all receptors concerned (for details see Ruck, 1965). The spectral sensitivity curves of the dark-adapted compound eye for low criterion (Fig. 3) possessed two peaks of maximal sensitivity; a broad primary peak in the green (about 520 nm) and a secondary peak in the near-u.v. (a,,, 355 + 5 nm) region of the spectrum. The primary peak showed a close correspondence with a nomogram curve (Dartnall. 1953) for a hypothetical visual pigment with a A,,, at 520nm. Chromatic-adaptedpreparation. To test the heterogeneity of the receptor types in the compound eye; selective adaptation experiments were undertaken. Intense chromatic adaptation with orange (610 nm) and blue (460 nm) light selectively depressed the sensitivity in the longer wavelength region leaving a peak in the near-u.v. region of the spectrum. NearU.V.(390 nm) adaptation produced a curve which did not differ significantly from the dark-adapted curve. These spectral sensitivity profiles suggest the presence of at least two functionally separable receptor types in the compound eye of G. firmus; one in the green (A,,,,, 520nm), the other in the near-u.v. at about 355 * 5 nm.
560
6-i’- 82
log unit -
A
Log Intensity (Photons m-*s-l) Fig. 2. Stimulus response curves (V-logI) for one dark adapted compound eyes in G. firmus. The amplitude of the electroretinogram (mv) is plotted as a function of log intensity for different wavelengths. Curves were arbitrarily displaced along the abscissa for visual clarity. Log,, photons me2 S-I for the smallest recorded response is given at the bottom of each curve. The curves were fitted by eye to the data points.
DISCUSSION
this compound eye have the same slope to their V-log1 curves even though the threshold and relative sensitivities of the eye for the different light stimuli are different.
t
The presence of two spectral receptor types in the cricket compound eye is suggested by the experimental results: (a) differences in the waveform of the
Gryllus flrmus -compound eye //
Wavelength
- nm
Fig. 3. A comparison of the spectral sensitivity of the compound eye in G. jirmus under dark (0) for 250 PV response and under chromatic adaptation of u.v.: 390 nm, (0) blue: 460 nm (A), and orange: 610 nm (v) for 50 PV response. Bars when present represent + 1SE of the mean. Dotted line is a nomogram curve (Dartnall, 1953) for a hypothetical visual pigment with a maximum at 520nm.
356
ABNER B. LALL et ul.
electroretinograms at different wavelengths, (Fig. 1). (b) the presence of two peaks in the dark-adapted spectral sensitivity curves, (Fig. 3) and (c) differential effects of intense, long and shortwave monochromatic lights on the spectral sensitivity curves (Fig. 3). These criteria have been used by several workers (La11 et al., 1982; Menzel, 1979; Walther. 1958) to characterize receptor mechanisms in insect vision. The evidence for the presence of a green receptor in the cricket eye is: (a) the presence of a dominant peak in the dark-adapted spectral sensitivity curves at 520 nm (Fig. 3). (b) the partial isolation of a green peak by intense near-u.v. (390 nm) and blue (460 nm) adaptation (Fig. 3). The green spectral sensitivity is presumably mediated by a visual pigment at P520 since close correspondence exists between spectral sensitivity curves and the Dartnall’s nomogram (Fig. 3). Among insects, a green receptor system is almost of universal occurrence (reviews: Goldsmith and Bernard. 1974; Menzel. 1981) and is presumably selected for providing high sensitivity, low threshold scotopic vision in nocturnal insects, since the ambient photic environment in forest or grassland is maximal in the green (Seliger et ul., 1982). The presence of a short wavelength receptor (j.,,,,, 355 k 5 nm) is strongly suggested due to: (a) the presence of a shoulder in the dark-adapted curve and (b) high sensitivity in the near-u.v. region with selective adaptation intense orange (610 nm) and blue (460nm) lights (Fig 3). The presence of a near-u.v. receptor system is also quite universal among insects (reviews: Goldsmith and Bernard, 1974; Menzel, 1979 and Stark and Tan. 1982). and is attributed to the presence of a near-u.v. photopigment (Schwemer et rd.. 1971) and a sensitizing antenna pigment (Hardie and Kirschfeld, 1983). The antenna pigment presumably catches the available quanta and transfers the energy of the absorbed quanta to the green photopigment. The functional significance of the near-u.v. system has been difficult to fathom. Recently, based on the analysis for optimization ol signal to noise ratio. it has been proposed that a near-u.v. pigment (350 nm) is selected for the detection of polarized light (Seliger et rd.. 1983). The utilization of near-u.v. polarized light as an orientation cue has been extensively studied in the honeybee (von Frisch. 1965) and the desert ant (Duelli and Wehner. 1973). The utilization of the sun compass for homing is suggested for the wood cricket (Beugnon. 1983). Since it has been argued that the detection of polarized light in the sky is optimal at 350 nm (Seliger et cd.. 1983), it is likely that in the cricket it is the u.v. polarization which is used a celestial cue for homing. analogous to what occurs in the honeybee. Hence the presence of near-u.v. receptors (i,,,.,, 355 _t 5 nm) in the cricket would facilitate the detection of polarized light as a celestial cue for homing.
By using Porshynakov
calorimetric techniques. Mazokhin(1962) concluded that adult cricket, Gryllus domesticus. compound eyes contain two kinds of receptors, blue-green with peak sensitivity at about 500 nm and yellow-green at about 565 nm. Our techniques shows the presence of a green (i,,,,, 520nm)
and a near-u.v. (E.,,, 355 nm) receptor. Futhermore, all wavelengths less than about 490 nm were stated by Maxokhin-Porshynakov to produce identical effects on the electroretinogram provided that stimulus intensities were adjusted appropriately. The latter statement is not true for G. jknus as indicated in Fig. 1. Mazokhin-Porshnyakov’s calorimetric method involves the eye being exposed for a time to a stimulus of monochromatic wavelength (e.g. 520 nm) and then abruptly this wavelength is replaced by a mixture of two wavelengths (e.g. 430 nm plus 640 nm). Absence of change in the electroretinogram resulting from such a substitution signifies a calorimetric match. A characteristic feature of this method is simultaneous excitation of two or more classes of photoreceptors when such are present. It is entirely conceivable that the response of one class can suppress or inhibit the response of another class (Zettler and Autrum, 1975). or that screening pigments can absorb differentially across the spectrum (Stowe. 1980; Leggett, 1979). We suggest that our intense chromatic bleach experimental design lacks some of the calorimetric method and conclude that in the visible region of the spectrum, the cricket compound eyes possess only a green receptor and not two receptors, blue-green and yellow. as suggested by Mazokhin-Porshnyakov. Further confirmation of our findings is being attempted by single cell recordings. .ilc~no~ledRemmrs-This investigation was supported in part by NIGMS Training Grant IT-02 G05010-01 MARC tto Howard University): NSF Grant HES 75-09824 (to COT). DOE contract EY03277 and NSF Grants BNS-SO12215 and BNS-83-11127 (to H. H. Seliger) and grants from Mr Robert L. Conway and The Kettering Family Foundation (to ABL). Some of the data presented here are part of a doctoral dissertation presented to the Graduate School. Howard University, by Egbert T. Lord. The authors also thank the two anonymous reviewers for their suggestions on the manuscrtpt. This is contribution No. 1256 of the McCollum-Pratt Institute and Department of Btology. The Johns Hopkins University.
REFERENCES
Bennett R. R. (1967) Spectral sensitivity studies in the whirligig beetle, Dineures ciliutus. J. Insect. Physiol. 13, 621-633. Beugnon G. (1983) Terrestrial and celestial cues in visual orientation of the wood-cricket Nemohris svirestris (Bose). Biol. Beh. 8, 159-169. Chapman R. M. and Lall A. B. (1967) Electroretinogram characteristics and the spectral mechanisms of the median ocellus and the lateral eye in Lirnulus po/vphemzrs J. Gen. Physiol. 56, 2267-2287. Dartnall H. J. A. (1953) The interpretation of spectral sensitivity curves. Br. Med. Bull. 9. 34-30. Duelli P. and Wehner R. ( 1975) The spectral sensitivity of polarized light orientation in Cutu~l~phis bicolor (Formicidae: Hymenoptera). .I. Camp. Physiol. 102, 43-45. von Frisch K. (1967) The Dunce Lunguuge and Orientation o/‘Bees. p. 556. Harvard University. Cambridge. Goldsmith T. H. and Bernard G. D. (1974) The visual system of insects. In Physiology qf I&ec/o, ‘Vol. II. 2nd edn (Ed. by Rocksein M.), pp. 1655272. Academic Press. New York. Hardie R. C. and Kirschfeld K. (1983) Ultraviolet sensitivity ot Ry photoreceptors ” -.. - R, and R,: evidence for a sensitizing tunctton. tlzoph~s. Sfruct. Mech. Y. 171-180
Electrophysiology Lall A. B. and Lord E. T. (1981) Electrophysioiogy of the cricket visual system. In ARVO. 1981: Invest Ophth. and Visual Sci. Suppl., 20, I8 I. La11 A. B.. Lord E. T. and Trouth C. 0. (1982) Vision in the firefly Photuris lucicrescens (Coleoptera: Lampyridae): Spectral sensitivity and selective adaptation in the compound eye. J. Camp. Physiol. 147, 195-200. Leggett L. M. W. (1967) A retinal substrate for colour d&crimination in crabs. J. Comp. Physiol. 133, 159-166. Lord, E. T. (1980) A comparison of spectral characteristics of scotopic vision in two insect species-Photuris lucicrescens (Coleoptera: Lampyridae) and Grvllus firmus (Orthoptera: Gryliidae). Unpublished doctoral thesis. Howard University, Washington, DC. Mazokhin-Porshnyakov G. A. (1962) Receptor interaction in cricket eve. Biofizika 7, 442449. Menzel R. (1979). Spectral sensitivity and color vision in invertebrates. In Comparative Physiology and Evolution of Vision in Invertebrates A: Invertebrate Photoreceptors. (Ed. by Autrum H.), pp. 504-580. Springer, Berlin. Mote M. 1. and Goldsmith T. H. (1970) Spectral sensitivities of color receptors in the compound eye of cockroach Periplanera. J. exp. Zool. 173, 137-146. Ruck P. (1965) The components of the visual system of a dragonfly. J. Gen. Physiol. 49, 289-307.
of the visual system
351
Schwemer J., Gogala M. and Hamdorf K. (1971) Der-UVSehrfarbstoff der insekton: Photochemie in vitro and in viva. Z. vergl. Ph.ysiol. 75, 174188. Seliger H. H., La11 A. B., Lloyd J. E.. Biggley W. H. (1982) The colors of firefly bioluminescence. Experimental evidence for the optimization model. Photochem. Photo&l. 35, 381-388. . Seliger H. H., Lall A. B. and Biggley W. H. (1983) Blue and ultra-violet spectral sensitivities: prediction of an optimization model. Photochem. Photobioi. 37 Supplement. p. 585 Abstract, 1983. Stark W. S. and Tan K. W. P. (1982) Ultraviolet hght: photosensitivity and other effects on the visual system. Photochem. Phorobiol. 36, 371-380. Stowe S. (1980) Spectral sensitivity and retinal pigment movement in the crab Leprograpsus variegatus (Farbricus). J. exp. Biol. 87, 73-98. Walther J. B. (1958) Changes induced in spectral sensitivity and form of retinal action potential of the cockroach eye by selective adaptation. J. Insect Physioi. 2. 142-151. Zettler F. and Autrum H. (1975) Chromatic properties of lateral inhibition in the eye of a fly. J. Camp. Phr~iol. 97, 181-188.
Vol. 31. No. 5. pp. 353-357. Britain. All rights reserved
1985 Copyright
0022-191Oi85 $3.00 + 0.00 I(‘ 1985 Pergamon Press Ltd
ELECTROPHYSIOLOGY OF THE VISUAL SYSTEM IN THE CRICKET GRYLLUS FIRMUS (ORTHOPTERA: GRYLLIDAE): SPECTRAL SENSITIVITY OF THE COMPOUND EYES ABNER B.
LALL,
ECBERT
T.
LORD*
and C. OVID TROUTHt
McCollum-Pratt Institute and Department of Biology, The Johns Hopkins University, Baltimore, MD 21218, *Department of Zoology, Howard University, Washington, DC 20059 and TDepartment of Physiology and Biophysics. College of Medicine, Howard University, Washington, DC 20059, U.S.A. (Received 20 December
1983; revised 3 1 August 84)
Abstract-The electroretinographic spectral sensitivity of the cricket compound eyes shows the presence of two receptor types, a dominant one at 520 nm and another in the near-u.v. (,I,,,,,, 355 f 5 nm) under dark- and intense chromatic adaptation conditions (Fig. 3). The waveform of the electrical responses elicited by short-wavelength stimuli differ from those elicited by long wavelength stimuli (Fig. 1). Key Word Index: Visual spectral
sensitivity,
vision.
INTRODUCTION Neuroethologists have extensively studied the song of the cricket and its neural substrate. However, though the visual system of the cricket is well developed, possessing a pair of compound eyes, a pair of lateral ocelli and one median dorsal ocellus, comprehensive electrophysiological studies of it are lacking. On the basis of calorimetric studies, the presence of two receptor types (A,,,,, 500 nm and &,,X 565 nm) in the eye have been suggested cricket compound (Mazokhin-Porshnyakov. 1962). This communication presents electrophysiological data which suggests the presence of two receptor types, near-u.v. (J.,,, 355 + 5 nm) and green (&,,, 520 nm), differing from those described by Mazokhin-Porshnyakov. A short abstract has been published (La11 et al., 1981).
MATERIALS AND
METHODS
Adult crickets, Gryllusjrmus. were collected in the grounds of Howard University campus, Washington, DC during the months of August, September and October, and kept in the laboratory at room temperature 21-23’C) in a terrarium. The details of the optical stimulator. electronic instrumentation and experimental techniques and procedures have been described earlier (La11 et ~1.. 1982). A short summary is presented here. Intact animals were used for recording electroretinograms from either the whole eye or the ventral sector during the daylight hours (photophase). The recording electrode was a glass pipette (tip diameter 5-10pm) filled with insect Ringer solution and placed subcorneally. The reference electrode consisted of another Ringer-filled pipette placed in the antenna. An Ag-AgCI wire connected the Ringerfilled glass pipette to the input of a high impedence amplifier whose output was displayed on an oscilloscope and photographed by a camera. The optical system consisted of a two-channel
cricket
eyes
stimulator. A 150 W xenon arc provided light for the test beam and a 500 W tungsten source provided the adaptation beam. The test beam was interrupted by a grating monochromator and the adaptation beam with interference filters. Mirror optics and quartz lenses collimated each beam and a quartz beam splitter combined the two beams. The light focussed at the entrance of a fibre optic bundle (3 x 360 mm) which delivered the stimulus to the eye. Neutraldensity filters attenuated the intensity in the two beams. The intensity of the test stimuli was determined by a calibrated photodiode (United Detector Technology, Inc., Santa Monica, CA). Light flashes of 170 ms duration of selected wavelengths were administered over 4.5 log units of light intensity. Test flashes were delivered at either 1 or 2 min intervals, which permitted total recovery of the sensitivity of the eye. A “standard” flash of known wavelength and intensity was periodically given to check the overall change in the sensitivity of the eye. All experiments were conducted at room temperature (21-23 C). The amplitude of the electroretinograms provided an index of the sensitivity of the eye to quantum flux and to wavelength composition of the photic stimulus. In a few cases the electroretinograms were recorded over the entire range of available intensity gradient for 15 selected stimulus wavelengths across the spectrum (34&680 nm) and intensity-response curves were constructed. The spectral sensitivity was obtained by first determining the number of photons needed to elicit a criterion amplitude of the electroretinograms for different stimulus wavelength. A plot of l/Q as a function of wavelength gave the spectral sensitivity function. However. in most cases a simplified experimental procedure was utilized which consisted of one person observing the oscilloscope screen while another adjusted the intensity of the test flash with neutral-density filters at each wavelength until the observer signalled that a criterion response had been met (for details see Ruck. 1965). Three to 353
ABNER B
354
four runs per eye were made across the spectrum both descending and ascending the wavelength scale. Dim illumination elicited low-criterion responses while bright illumination elicited high-criterion responses. For chromatic adaptation experiments the eye was adapted by a continuous high-intensity monochromatic illumination obtained by placing an appropriate interference filter in the adaptation beam. Test flashes were superimposed on the selective adaptation beam. RESULTS
Characteristics of’ the electroretinogram Electroretinogram responses elicited by two wavelengths (420 and 580 nm) are presented in Fig. 1 by dim and bright illumination were different. A response was elicited by a photic stimulus after an initial latency period of between 15-25 ms depending upon the intensity of the stimulus. The electroretinogram was dominated by an on-negative monophasic wave. When the stimulus was turned off, the response returned to the baseline. Qualitative differences existed in the waveform of the responses elicited by short (420 nm) and long (580 nm) wavelength stimuli. These waveform differences with respect to wavelength would suggest the presence of Log I = 13.95
photons
m-2
LALL PI Ul. short and long wavelength receptors in the cricket compound eyes. similar to what has been described in Limulus median eyes (Chapman and Lall, 1967), fireflies (La11 et al., 1982) and Dineutes (Bennett, 1967) compound eyes. Intensitywesponse ,function (V-log I) The relationship between the amplitude of the negative electroretinogram recorded from the compound eye and the intensity of the test stimulus for different wavelengths across the spectrum is presented in Fig. 2 for dark-adapted Gryllus jirmus. Systematic changes in the slope of the response curves for different wavelengths have been described in some arthropods (Limulus: Chapman and Lall. 1967; Dineutes: Bennett, 1967) and have been taken as evidence for the presence of different receptor types, but so far, these are exceptions. Therefore, it is a point of interest to find out whether there are any systematic changes in the intensity response function (V-logI) as a function of wavelength in G. $rmus. From the data presented (Fig. 2) there appears to be no obvious change in the slopes of V-log1 curves as a function of wavelength. The curves for different wavelengths tended to be parallel. This fact means that different receptors (as pointed out in the following section) in 15.55
I-’
photons
m-a s-l
-\ 0
-c-
-10
-15
-2.0
-2.0 420nm Fig.
580 “Ill
I. Electroretinograms recorded from a dark-adapted compound eye of an intact cricket, Gryllus firmus. elicited by light of wavelengths: 420 and 580nm. The numbers next to the response are the neutral-density attenuation of the test flash. Stimulus monitor is below the response. A 1mV and 20 ms calibration signal precedes each response. Negative deflection going upwards.
Electrophysiology
355
Spectral sensitivity curves
3.01 2.5-
of the visual system
Gryllus firmus -compound eyes dark adapt
Dark-adapted preparation. At threshold, the probability of detecting the response of a small population of receptors is greater at low criterion levels than at higher criterion levels, assuming that the threshold is the same for all receptors concerned (for details see Ruck, 1965). The spectral sensitivity curves of the dark-adapted compound eye for low criterion (Fig. 3) possessed two peaks of maximal sensitivity; a broad primary peak in the green (about 520 nm) and a secondary peak in the near-u.v. (a,,, 355 + 5 nm) region of the spectrum. The primary peak showed a close correspondence with a nomogram curve (Dartnall. 1953) for a hypothetical visual pigment with a A,,, at 520nm. Chromatic-adaptedpreparation. To test the heterogeneity of the receptor types in the compound eye; selective adaptation experiments were undertaken. Intense chromatic adaptation with orange (610 nm) and blue (460 nm) light selectively depressed the sensitivity in the longer wavelength region leaving a peak in the near-u.v. region of the spectrum. NearU.V.(390 nm) adaptation produced a curve which did not differ significantly from the dark-adapted curve. These spectral sensitivity profiles suggest the presence of at least two functionally separable receptor types in the compound eye of G. firmus; one in the green (A,,,,, 520nm), the other in the near-u.v. at about 355 * 5 nm.
560
6-i’- 82
log unit -
A
Log Intensity (Photons m-*s-l) Fig. 2. Stimulus response curves (V-logI) for one dark adapted compound eyes in G. firmus. The amplitude of the electroretinogram (mv) is plotted as a function of log intensity for different wavelengths. Curves were arbitrarily displaced along the abscissa for visual clarity. Log,, photons me2 S-I for the smallest recorded response is given at the bottom of each curve. The curves were fitted by eye to the data points.
DISCUSSION
this compound eye have the same slope to their V-log1 curves even though the threshold and relative sensitivities of the eye for the different light stimuli are different.
t
The presence of two spectral receptor types in the cricket compound eye is suggested by the experimental results: (a) differences in the waveform of the
Gryllus flrmus -compound eye //
Wavelength
- nm
Fig. 3. A comparison of the spectral sensitivity of the compound eye in G. jirmus under dark (0) for 250 PV response and under chromatic adaptation of u.v.: 390 nm, (0) blue: 460 nm (A), and orange: 610 nm (v) for 50 PV response. Bars when present represent + 1SE of the mean. Dotted line is a nomogram curve (Dartnall, 1953) for a hypothetical visual pigment with a maximum at 520nm.
356
ABNER B. LALL et ul.
electroretinograms at different wavelengths, (Fig. 1). (b) the presence of two peaks in the dark-adapted spectral sensitivity curves, (Fig. 3) and (c) differential effects of intense, long and shortwave monochromatic lights on the spectral sensitivity curves (Fig. 3). These criteria have been used by several workers (La11 et al., 1982; Menzel, 1979; Walther. 1958) to characterize receptor mechanisms in insect vision. The evidence for the presence of a green receptor in the cricket eye is: (a) the presence of a dominant peak in the dark-adapted spectral sensitivity curves at 520 nm (Fig. 3). (b) the partial isolation of a green peak by intense near-u.v. (390 nm) and blue (460 nm) adaptation (Fig. 3). The green spectral sensitivity is presumably mediated by a visual pigment at P520 since close correspondence exists between spectral sensitivity curves and the Dartnall’s nomogram (Fig. 3). Among insects, a green receptor system is almost of universal occurrence (reviews: Goldsmith and Bernard. 1974; Menzel. 1981) and is presumably selected for providing high sensitivity, low threshold scotopic vision in nocturnal insects, since the ambient photic environment in forest or grassland is maximal in the green (Seliger et ul., 1982). The presence of a short wavelength receptor (j.,,,,, 355 k 5 nm) is strongly suggested due to: (a) the presence of a shoulder in the dark-adapted curve and (b) high sensitivity in the near-u.v. region with selective adaptation intense orange (610 nm) and blue (460nm) lights (Fig 3). The presence of a near-u.v. receptor system is also quite universal among insects (reviews: Goldsmith and Bernard, 1974; Menzel, 1979 and Stark and Tan. 1982). and is attributed to the presence of a near-u.v. photopigment (Schwemer et rd.. 1971) and a sensitizing antenna pigment (Hardie and Kirschfeld, 1983). The antenna pigment presumably catches the available quanta and transfers the energy of the absorbed quanta to the green photopigment. The functional significance of the near-u.v. system has been difficult to fathom. Recently, based on the analysis for optimization ol signal to noise ratio. it has been proposed that a near-u.v. pigment (350 nm) is selected for the detection of polarized light (Seliger et rd.. 1983). The utilization of near-u.v. polarized light as an orientation cue has been extensively studied in the honeybee (von Frisch. 1965) and the desert ant (Duelli and Wehner. 1973). The utilization of the sun compass for homing is suggested for the wood cricket (Beugnon. 1983). Since it has been argued that the detection of polarized light in the sky is optimal at 350 nm (Seliger et cd.. 1983), it is likely that in the cricket it is the u.v. polarization which is used a celestial cue for homing. analogous to what occurs in the honeybee. Hence the presence of near-u.v. receptors (i,,,.,, 355 _t 5 nm) in the cricket would facilitate the detection of polarized light as a celestial cue for homing.
By using Porshynakov
calorimetric techniques. Mazokhin(1962) concluded that adult cricket, Gryllus domesticus. compound eyes contain two kinds of receptors, blue-green with peak sensitivity at about 500 nm and yellow-green at about 565 nm. Our techniques shows the presence of a green (i,,,,, 520nm)
and a near-u.v. (E.,,, 355 nm) receptor. Futhermore, all wavelengths less than about 490 nm were stated by Maxokhin-Porshynakov to produce identical effects on the electroretinogram provided that stimulus intensities were adjusted appropriately. The latter statement is not true for G. jknus as indicated in Fig. 1. Mazokhin-Porshnyakov’s calorimetric method involves the eye being exposed for a time to a stimulus of monochromatic wavelength (e.g. 520 nm) and then abruptly this wavelength is replaced by a mixture of two wavelengths (e.g. 430 nm plus 640 nm). Absence of change in the electroretinogram resulting from such a substitution signifies a calorimetric match. A characteristic feature of this method is simultaneous excitation of two or more classes of photoreceptors when such are present. It is entirely conceivable that the response of one class can suppress or inhibit the response of another class (Zettler and Autrum, 1975). or that screening pigments can absorb differentially across the spectrum (Stowe. 1980; Leggett, 1979). We suggest that our intense chromatic bleach experimental design lacks some of the calorimetric method and conclude that in the visible region of the spectrum, the cricket compound eyes possess only a green receptor and not two receptors, blue-green and yellow. as suggested by Mazokhin-Porshnyakov. Further confirmation of our findings is being attempted by single cell recordings. .ilc~no~ledRemmrs-This investigation was supported in part by NIGMS Training Grant IT-02 G05010-01 MARC tto Howard University): NSF Grant HES 75-09824 (to COT). DOE contract EY03277 and NSF Grants BNS-SO12215 and BNS-83-11127 (to H. H. Seliger) and grants from Mr Robert L. Conway and The Kettering Family Foundation (to ABL). Some of the data presented here are part of a doctoral dissertation presented to the Graduate School. Howard University, by Egbert T. Lord. The authors also thank the two anonymous reviewers for their suggestions on the manuscrtpt. This is contribution No. 1256 of the McCollum-Pratt Institute and Department of Btology. The Johns Hopkins University.
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