Body movements and retinal pattern displacements while approaching a stationary object in the walking fly,Calliphora erythrocephala

When a walking fly approaches a stationary object two types of body movements are distinguishable. Type I body movements are characterized by low frequencies (0.4–1.3 Hz) and large amplitudes (28–65°). Superimposed on these movements are type II body movements which are characterized by high frequencies (7.3–10.6 Hz) and small amplitudes (5.9–8.2°) (Figs. 3–6; Table 1). Type II movements occur no matter whether the fly is fixating a pattern or orientating itself in homogeneous surroundings without any pattern. In contrast, only 72% of the flies with immobilized heads and 62% of the flies with movable heads make type I body movements. The amplitude of type I and type II body movements increases slightly after immobilization of the head. Binocular as well as monocular pattern projection occurs for the whole walking trajectory (Fig. 7–9). Monocular pattern projection seems to be more frequent in flies with immobilized heads than in those with movable heads. The degree of pattern fluctuations in the visual field of the flies increases slightly along the walking trajectory. Near the starting point in the centre of the arena it amounts to 5–7°, while at the end of the walking trajectory it amounts to 8–10° (Table 2). The following conclusions and hypothesis can be drawn from these experiments. 1. The graph BT for the direction of the fly's logitudinal axis can be approximated by the first derivative of the walking trajectory WT, that means, dWT(x)/dxBT(x) (Fig. 11). 2. The amplitudes of type II body movements are caused by the alternating movements of the legs during forward motion, while type I body movements are classified as exploring movements. During evolution of visually guided behaviour it is possible that blowflies have adapted their elementary movement detector system to type II body movements. 3. The types of pattern projection into the visual field of the fly while approaching an object can be explained by a simple neuronal network characterized by either inhibitory and/or excitatory influences of the visually activated neurones on the motor neurones generating the propulsive forces, that means the forward motion. In addition it is postulated that the large frontal and antero-lateral receptive fields of these neurones are not coupled with the motor centres on the same side of the body (Fig. 12).     

Directionality of antennal sweeps elicited by water jet stimulation of the tailfan in the crayfish Procambarus clarkii

Summary Directionality and intensity dependence of antennal sweeps elicited by water jet stimulation of the tailfan in tethered, reversibly blinded adult and juvenile crayfish (Procambarus clarkii) were analyzed.Resting crayfish keep their antennae at about 50° symmetrically to the longitudinal body axis (Figs. 2 bottom, and 3).In adults, tailfan stimulation elicits synchronous backward sweeps of both antennae, which increase for more caudal stimulus directions (Figs. 2–4 and 5A). Directions differing by 30°–60° are significantly distinguished (Fig. 4). The mean sweep of the ipsilateral antenna significantly overrides that of the contralateral antenna for rostrolateral stimulation at 40–200 mm/s stimulus velocity and lateral to caudolateral stimulation at 40 mm/s and thus lateralization of the stimulus is revealed (Figs. 2 top, 4 and 5A). Mean antennal sweeps at a given stimulus direction and distance increase with increasing stimulus velocity (40–250 mm/s, Fig. 5A).In juveniles, the directional dependence of antennal sweeps is reduced compared to that of adults, while a similar intensity dependence is found (Fig. 5B).The pronounced directionality of the antennal response in adult crayfish vanishes and response latencies increase after reversibly covering the tailfan with a small bag or the telson with waterproof paste (Figs. 6 and 7). Thus, tailfan and especially telson mechanoreceptors play an important role in the localization of water movements elicited by predators or prey behind the crayfish.     

The responses of central octavolateralis cells to moving sources

Mechanosensory lateral line units recorded from the medulla (medial octavolateralis nucleus) and midbrain (torus semicircularis) of the bottom dwelling catfish Ancistrus sp. responded to water movements caused by an object that passed the fish laterally. In terms of peak spike rate or total number of spikes elicited responses increased with object speed and sometimes showed saturation (Figs. 7, 14). At sequentially greater distances the responses of most medullary lateral line units decayed with object distance (Fig. 11). Units tuned to a certain object speed or distance were not found. The signed directionality index of most lateral line units was between –50 and +50, i.e. these units were not or only slightly sensitive to the direction of object motion (Figs. 10, 17). However, some units were highly directionally sensitive in that the main features of the response histograms and/or peak spike rates clearly depended on the direction of object movement (e.g. Fig. 9C, D and Fig. 16). Midbrain lateral line units of Ancistrus may receive input from more than one sensory modality. All bimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. Units which receive bimodal input may show an extended speed range (e.g. Fig. 18).Abbreviations MON medial octavolateralis nucleus - MSR mean spike rate - PSR peak spike rate - p-p peak-to-peak - SDI signed directionality index     

The function of auditory neurons in cricket phonotaxis

Summary The activity of auditory receptor cells and prothoracic auditory neurons of the cricket,Gryllus bimaculatus, was recorded intracellularly while the animal walked on a sphere or while passive movement was imposed on a foreleg.During walking the responses to simulated calling song is altered since (i) the auditory sensory cells and interneurons discharged impulses in the absence of sound stimuli (Figs. 1, 3) and (ii) the number of action potentials in response to sound is reduced in interneurons (Figs. 2, 3).These two effects occurred in different phases of the leg movement during walking and therefore masked, suppressed or did not affect the responses to auditory stimuli (Figs. 3, 4). Hence there is a time window within which the calling song can be detected during walking (Fig. 5).The extra excitation of receptors and interneurons is probably produced by vibration of the tympanum because (i) the excitation occurred at the same time as the leg placement (Fig. 4), (ii) during walking on only middle and hindlegs, no extra action potentials were observed (Fig. 6), (iii) in certain phases of passive movements receptor cells and interneurons were excited as long as the ipsilateral ear was not blocked (Figs. 8, 9).Suppression of auditory responses seems to be peripheral as well as central in origin because (i) it occurred at particular phases during active and passive leg movements in receptor cells and interneurons (Figs. 1, 4, 9), (ii) it disappeared if the ear was blocked during passive leg movements (Fig. 9) and (iii) it persisted if the animal walked only on the middle and hind legs (Fig. 6).     

Responses and song pattern copying of Omega-type I-neurons in the cricket,Gryllus bimaculatus,at different prothoracic temperatures

Summary Omega-type I-neurons (ON/1) (Fig. 1A) were recorded intracellularly with the prothoracic ganglion kept at temperatures of either 8–9°, or 20–22° or 30–33 °C and the forelegs with the tympanal organs kept at ambient temperature (20–22 °C). The neurons were stimulated with synthetic calling songs (5 kHz carrier frequency) with syllable periods (SP in ms) varying between 20 and 100, presented at sound intensities between 40 and 80 dB SPL. The amplitude and duration of spikes as well as response latency decreased at higher temperatures (Figs. 1 B, 2, 6). At lower prothoracic temperatures (8–9 °C) the neuron's responses to songs with short SP (20 ms) failed to copy single syllables, or with moderate SP (40 ms) copied the syllable with low signal to noise ratio (Fig. 3). The auditory threshold of the ON/1 type neuron, when tested with the song model, was temperature-dependent. At 9° and 20 °C it was between 40 and 50 dB SPL and at 33 °C it was less than 40 dB SPL (Fig. 4). For each SP, the slope of the intensity-response function was positively correlated with temperature, however, at low prothoracic temperatures the slope was lower for songs with shorter SPs (Fig. 5). The poor copying of the syllabic structure of the songs with short SPs at low prothoracic temperatures finds a behavioral correlate because females when tested for phonotaxis on a walking compensator responded best to songs with longer SPs at a similar temperature.Abbreviations epsps excitatory postsynaptic potentials - ON/1 omega-type I-neuron - SP syllable period - SPL sound pressure level     

Receptive field of the stemmata in the swallowtail butterflyPapilio

Summary Receptive fields of individual retinular cells in the stemmata ofPapilio xuthus L. were examined electrophysiologically, and the receptive field of the complete stemmatal system was reconstructed (Fig. 8).In stemmata I-IV, proximal retinular cells have narrow receptive fields (acceptance angles of = 1.7–5 °, Fig. 5) and small inclinations of the visual axes (inclinations of = 0.7–1.5 °, Fig. 2) with respect to the axis of the stemma, while distal ones have wide fields ( =7–13 °, Fig. 5) and large inclinations of the visual axes ( = 5–10 °, Fig. 3). In stemmata V and VI, both proximal and distal retinular cells have wide receptive fields ( = 7–26 °, Fig. 6) and have large inclinations of their visual axes ( = 9–19 °) with respect to the axis of the stemma except for one proximal cell ( = 0 °) (Fig. 4).The spatial properties of distal and proximal retinular cells, combined with the finding that distal cells are homogeneous in the spectral sensitivity while proximal ones are heterogeneous (Ichikawa and Tateda 1980), suggest that the distal cells may be concerned largely with the detection of objects and proximal cells are involved with the discrimination of the color and shape of the detected objects.     

Intracellular responses from cells of the medulla of the fly,Calliphora erythrocephala

1. Intracellular recordings were made from cells in medullae of immobilized, intact flies Calliphora erythrocephala. Stimuli were moving gratings or small spots projected onto translucent hemispheres before the fly.—2. Responses to stationary flashes included tonic and phasic slow potentials only. Sustaining and On/Off discharges were recorded from cells silent in the dark. Sustaining, dimming, On/Off, +On-Off, and-On/-Off discharges were recorded from cells spontaneous in the dark (Fig. 1, 2, and 3).—3. Some cells were relatively sensitive to 3 log unit changes in flash intensities; others were insensitive (Fig. 4).—4. Receptive fields of a few cells tested were small-field ipsilateral monocular, large-field ipsilateral monocular, or large-field binocular.—5. A number of types of nondirectional cells were found. Some gave stronger discharges to movement than to stationary flashes (Fig. 5).—6. Directionally-selective cells were generally spontaneous. Some simply fired faster in the preferred direction. Others (Fig. 6) had inhibition in the null diriction with or without hyperpolarizations.—7. Possibly-new nondirectional cells were found that were inhibited by changes of direction of movement (Fig. 7)—8. A number of cells were stained with Procion yellow, using high voltage pulses. Double stainings sometimes occurred (Fig. 8). Present Address: Psychological Laboratory, Downing Street, Cambridge CB2 3EB, U.K.     

Dynamic properties of neurons sensitive to visible movement in scarabaeid beetles (coleoptera scarabaeidae)

The responses of individual neurons of the optic lobe of beetles to moving light stimuli were studied. It was established that the reactions of neurons to the movement of a single light band in a preferred direction at a rate of 1–150 deg/sec are proportional to the logarithm of the angular velocity. The reactions to the movement of a striped pattern vary nonlinearly with the angular velocity. After an initial volley of discharges, the reaction to steady movement of the pattern drops more sharply than during a single movement of the band. When the pattern is stopped, an inhibitory pause occurs in the neuron's activity. The properties of the transitional processes can be explained by adaptation of local areas of the receptive field and by mutual inhibition between neuron systems sensitive to counter-oriented movements. The neurons which detect rotation of the optical environment have binocular receptive fields. The system for transmitting a turning command to the motor neurons has a time constant of 3–5 sec.Institute of Zoology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 325–330, May–June, 1971.     

Identified nonspiking interneurons in leg reflexes and during walking in the stick insect

In the stick insect Carausius morosus identified nonspiking interneurons (type E4) were investigated in the mesothoracic ganglion during intraand intersegmental reflexes and during searching and walking.In the standing and in the actively moving animal interneurons of type E4 drive the excitatory extensor tibiae motoneurons, up to four excitatory protractor coxae motoneurons, and the common inhibitor 1 motoneuron (Figs. 1–4).In the standing animal a depolarization of this type of interneuron is induced by tactile stimuli to the tarsi of the ipsilateral front, middle and hind legs (Fig. 5). This response precedes and accompanies the observed activation of the affected middle leg motoneurons. The same is true when compensatory leg placement reflexes are elicited by tactile stimuli given to the tarsi of the legs (Fig. 6).During forward walking the membrane potential of interneurons of type E4 is strongly modulated in the step-cycle (Figs.8–10). The peak depolarization occurs at the transition from stance to swing. The oscillations in membrane potential are correlated with the activity profile of the extensor motoneurons and the common inhibitor 1 (Fig. 9).The described properties of interneuron type E4 in the actively behaving animal show that these interneurons are involved in the organization and coordination of the motor output of the proximal leg joints during reflex movements and during walking.Abbreviations CLP reflex, compensatory leg placement reflex - CI1 common inhibitor I motoneuron - fCO femoral chordotonal organ - FETi fast extensor tibiae motoneuron - FT femur-tibia - SETi slow extensor tibiae motoneuron     

Structure and kinematics of the prosternal organs and their influence on head position in the blowfly Calliphora erythrocephala Meig.

Summary The blowfly Calliphora has a mobile head and various, presumably proprioceptive, sense organs in the neck region. The prosternal organs are a pair of mechanosensory hair fields, each comprising ca. 110 sensilla. We studied their structure (Figs. 2–4), kinematics (Figs. 5, 6) and, after surgery, their influence on head posture (Figs. 7–11) in order to reveal their specific function.The hair sensilla are structurally polarized, all in roughly the same direction, and are stimulated by dorsoventral bending of the hairs (Figs. 3, 4). This occurs indirectly by flap-movements of two contact sclerites (Figs. 3, 6); they move in the same direction during pitch turns of the head, in opposite directions during roll turns, and barely at all during yaw turns of the head (Fig. 5).Bending and arresting all hairs of one field elicits a head roll bias to the non-operated side (Fig. 7) during tethered flight in visually featureless surroundings. In contrast, shaving all hairs of one field elicits a head roll to the operated side (Figs. 8–10). The surgically induced bias of head posture is not compensated within three days (Fig. 10). Our results show that the prosternal organs of Calliphora sense pitch and roll turns of the fly's head, and control at least its roll position.Abbreviations HP° TP° angular positions of the sagittal planes of the fly's head and thorax, respectively, relative to an external reference - HR° = HP — TP head roll angle of the fly's head relative to its thorax, HR>0° for clockwise head roll, looking in flight direction - N number of flies - n number of measurements - PO prosternal organ - SD standard deviation - SEM standard error of the mean     

The effect of forewing depressor activity on wing movement during locust flight

Locusts are passively yawed in the laminar air current of a wind tunnel (Fig. 1). In order to study the influence of depressor muscles of the forewing on its movement, electromyography is combined with true 3-dimensional inductive forewing movement recording. In quick response to the yaw stimulus, many kinematic parameters (e.g. shape of the wing tip path, amplitudes of wingstroke, ratios of downstroke to upstroke duration, time interval between beginning of downstroke and time of maximum pronation etc.) vary differently in both forewings (Figs. 3–5). Pronation changes in correlation to yawing reciprocally on both forewings with comparable differences of pronation angles (Fig. 5a). Maximum pronation is decreased on that side, to which the animal is-passively-yawed, whereas the slope of the wing tip paths remains almost constant. Therefore, decreasing pronation most probably indicates increasing thrust. The animal appears to perform a disturbance avoidance behaviour. Although the burst length of muscle firing is almost constant here, the onset of 8 depressor muscles (1 st basalar and subalar muscles of all 4 wings) varies in correlation to the stimulus (Figs. 6–8). The changing time intervals between the 1 st basalar muscle M97 and subalar muscle M99 are responsible for the alterations of forewing downstroke. Quantitative analysis of combined motor and movement pattern (Fig. 9) shows the following: (i) the maximum pronation and time interval between the onset of 1 st basalar muscle M97 as well as subalar muscle M99 and the beginning of downstroke are positively correlated (Figs. 10 and 12a and b). (ii) Maximum pronation is greatest, when muscles M97 and M99 act simultaneously (Fig. 12c). Thus, both muscles work synergistically, concerning pronation. Muscle M99 is of less importance than muscle M97. On failing activity of the depressor muscle M97, downstroke is greatly reduced. Some depressor as well as elevator muscles are switched on and off separately on each side (Fig. 11).     

The eye-movements of the mantis shrimp Odontodactylus scyllarus (Crustacea: Stomatopoda)

Summary Odontodactylus scyllarus makes discrete spontaneous eye-movements at a maximum rate of 3/s. These movements are unpredictable in direction and timing, and there is no detectable co-ordination between the two eyes. The eye-movements were measured with a computer-aided video method, and from 208 of these the following picture of a typical movement emerges. It has roughly equal horizontal and vertical components of 7–8°, taking the eye-stalk axis about 12° around a great circle, and also a rotational component of about 8°. The 3 components can occur independently of each other and are thus separately driven by the brain (Fig. 6). The average duration is 300 ms, and average velocity is 40° s (Fig. 5). Most movements are made in a direction approximately at right angles to the orientation of the specialised central band. It is shown that the slow speed of the eye-movements is compatible with scanning, that is, the uptake of visual information during the movement rather than its exclusion as in conventional saccades.Mantis shrimps also make target-acquiring and tracking eye-movements which tend to be somewhat larger and faster than other spontaneous movements. Rotating a striped drum around the animal induces a typical optokinetic nystagmus whose slow phases are smooth, unlike target tracking which is jerky (Fig. 7). Eye-movements may therefore be conveniently grouped into 3 classes: targetting/tracking, scanning, and optokinetic.     

Characterization of spatial and temporal properties of monkey LGN Y-cells

LGN Y-cells in 3 anaesthetized (N₂O/O₂) and paralyzed rhesus monkeys were investigated with stimuli, intensity modulated by gaussian white noise, and with moving and counterphase modulated spatial sine wave gratings. The results support the model, postulated on the base of electrophysiological recordings in the retina of cat and mudpuppy, which consists of a linear centre and surround mechanism whose responses are modified in a frequency-selective multiplicative way by a nonlinear mechanism in the receptive field. This nonlinear mechanism is also held responsible for the second-order harmonic responses, which are the defining characteristic of Y-cells. The temporal and spatial characteristics of these mechanisms were determined. The responses obtained with the GWN stimulation and with modulated spatial sine wave gratings both indicate that the optimal temporal frequency of the linear mechanisms is near 7 Hz at 70 td and near 5 Hz for the nonlinear mechanism. The optimal spatial frequency for the linear mechanism is between 0.5–2 cycles/deg and between 6–12 cycles/deg for the nonlinear mechanism.     

Visual cortical detector neurons in the Siberian chipmunkEutamias sibiricus

Three functional classes of neurons are described in the visual cortex of the Siberian chipmunk: neurons not selective for direction of movement and orientation, neurons selective for movement in a particular direction, and neurons selective for orientation. Unselective and directionally-selective neurons were activated maximally at speeds of movement of 100–500 deg/sec or more, most orientation-selective neurons at speeds of 10–50 deg/sec. For all three classes of neurons clear correlation was observed between selectivity for velocity of movement and character of responses to presentation of stimuli stationary in the receptive field. With reference to this sign the neurons were divided into two groups: phasic (fast) and tonic (slow). Phasic (fast) neurons predominate in the visual cortex ofEutamias sibiricus.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 807–814, November–December, 1984.     

Hydrodynamic orientation of crayfish (Procambarus clarkii) to swimming fish prey

Reversibly blindfolded crayfish (Procambarus clarkii) react to small swimming fish (Astyanax fasciatus mexicanus) approaching or passing nearby with antennal and cheliped movements and body turns (Fig. 3). We studied the accuracy and dynamics of crayfish orientation responses to the previously analyzed hydrodynamic disturbances caused by the fish, mostly produced by tail flicks.Antennal and cheliped movements started slightly before the onset of turning responses (Fig. 4). Antennal sweeps were performed most rapidly. 50% of the appendage sweeps resulted in contacts with the fish (Fig. 5).Most turns were directed toward the stimulus (Fig. 6). Response amplitudes increased with increasing stimulus angle. Turns were accurate for small stimulus angles, but smaller than expected for larger ones. Sweeps of ipsilateral antennae and chelipeds were generally directed backwards, while those of contralateral appendages were smaller and directed forwards. The amplitudes of appendage sweeps first increased with increasing stimulus angle and then decreased again for more caudal stimulus directions. Lateral stimuli (60°–120°) from opposite sides were usually significantly distinguished. The amplitudes of the different elements of orientation behaviour were highly correlated with each other, indicating that they were directed by the same sensory input.     

Frequency and temporal pattern-dependent phonotaxis of crickets (Teleogryllus oceanicus) during tethered flight and compensated walking

Summary Phonotactic responses ofTeleogryllus oceanicus were studied with two methods. Tethered crickets were stimulated with sound while they performed stationary flight, and steering responses were indicated by abdominal movements. Walking crickets tracked a sound source while their translational movements were compensated by a spherical treadmill, and their walking direction and velocity were recorded.During both flight and walking, crickets attempted to locomote towards the sound source when a song model with 5 kHz carrier frequency was broadcast (positive phonotactic response) and away from the source when a song model with 33 kHz carrier frequency was used (negative phonotactic response) (Figs. 2, 4).One-eared crickets attempted, while flying, to steer towards the side of the remaining ear when stimulated with the 5 kHz model, and away from that side in response to the 33 kHz model (Fig. 3). While walking, one-eared crickets circled towards and away from the intact side in response to the 5 kHz and 33 kHz models, respectively (Fig. 6).Positive and negative responses differed in their temporal pattern requirements. Phonotactic responses were not elicited when a non-calling song pattern (2 pulses/s) was played with a carrier frequency appropriate for positive phonotactic responses (5 kHz), but this pattern did elicit negative responses with 33 kHz carrier frequency (Figs. 7–10). When an intermediate carrier frequency, 15 kHz, was used, the response type (positive or negative) depended on the stimulus temporal pattern; the calling song pattern elicited primarily positive responses, while the non-calling song pattern elicited negative responses (Figs. 11, 12, 14, 15). A curious phenomenon was often observed in the flight steering responses; while most responses to 15 kHz song pattern were primarily positive, they often had an initial negative component which was supplanted by the positive component of the response after approximately 2–5 s (Figs. 11, 12).In recent experiments onGryllus campestris, Thorson et al. (1982) described frequency-dependent errors in phonotactic direction (anomalous phonotaxis) and showed how such errors might arise from the frequency-dependent directional properties of the cricket's auditory apparatus. Our findings, particularly the dependence of response type on temporal pattern when 15 kHz carrier frequency was used, argue that frequency-dependent directional properties alone cannot account for positive and negative phonotaxis inT. oceanicus. Rather, these represent qualitatively different attempts to locomote towards and away from the sound source, respectively.We discuss the possibility that central integration of these opposing tendencies might contribute to anomalous phonotaxis.     

Directionality of acoustic orientation in flying crickets

Summary Abdominal flexions associated with flight steering were measured in tethered flyingTeleogryllus oceanicus stimulated with a model of conspecific calling song presented at various intensities and from many directions.Flexions increased in size with stimulus intensity until a plateau level was reached. Flexion amplitude was then approximately constant over a range of 20–30 dB, and decreased at still higher intensities (Figs. 2, 3). The shape of this intensity function results from binaural processing; in unilaterally deafened crickets flexion amplitude increased monotonically with stimulus intensity (Fig. 4).Abdominal flexions were graded with respect to sound location; they were larger for laterally placed sound sources and smaller for sound sources near the midline (Figs. 5, 6).A model for the specification of flight steering movements is presented which accounts for our findings (Fig. 7).     

Elementary detectors for vertical movement in the visual system of Drosophila

Optomotor thrust responses of the fruitfly Drosophila melanogaster to moving gratings have been analysed in order to determine the arrangement of elementary movement detectors in the hexagonal array of the compound eye. These detectors enable the fly to perceive vertical movement. The results indicate that, under photopic stimulation of a lateral equatorial eye region, the movement specific response originates predominantly from two types of elementary movement detectors which connect neighbouring visual elements in the compound eye. One of the detectors is oriented vertically, the other detector deviates 60° towards the anterior-superior direction (Fig. 5b). The maximum of the thrust differences to antagonistic movement is obtained if the pattern is moving vertically or along a superior/anterior — inferior/posterior direction 30° displaced from the vertical (Fig. 3d,e, Fig. 6). Only one of the detectors coincides with one of the two detectors responsible for horizontal movement detection. This indicates that a third movement specific interaction in the compound eye of Drosophila has to be postulated. — The contrast dependence of the thrust response (Fig. 2) yields the acceptance angle of the receptors mediating the response. The result coincides with the acceptance angle found by analysis of the turning response of Drosophila (Heisenberg and Buchner, 1977). This value corresponds to the acceptance angle expected, on the basis of optical considerations, for the receptor system R 1–6. — The movement-specific neuronal network responsible for thrust control is not homogeneous throughout the visual field of Drosophila. Magnitude and preferred direction of the thrust response in the upper frontal part of the visual field seem to vary considerably in different flies (Fig. 6).     

The role of retinula cell types in fixation behaviour of walkingDrosophila melanogaster

Summary Fixation behaviour of free walking wild typeDrosophila and various retinal mutants was tested in a circular arena. Optomotor response was also measured as a test of the function of R1-6.ora andsev,ora do not fixate a narrow stripe (10° or 20°, Fig. 1) but are able to orient towards broad stripes (110° or 180°, Fig. 1). The behaviour ofsev is not different from wild type. Fixation behaviour ofw rdgB is similar toora (Figs. 5, 6). The mutantora has a maximum optomotor response at low contrast frequencies (Fig. 2), but the threshold for this response is at least one log unit higher than in wild type orsev (Fig. 8). The light intensity threshold at 550 nm of fixation to a broad stripe (110°) is 1–2 log units higher inora than in wildtype, and 4 log units higher insev,ora and the structural brain mutantVam (Fig. 7).The conclusions are that retinula cells R1-6 mediate fixation to a narrow stripe at high and low ambient light intensities, and to a broad stripe at low ambient light levels. R8, possibly in conjunction with R1-6, contributes to orientation towards broad stripes at high light intensities. This hypothesis is supported by evidence that blue-adapted white-eyed flies are able to orient towards a broad stripe at high blue light intensities (Figs. 9 and 12). Blue adaptation totally eliminates the optomotor response (Figs. 10, 11) and so the optomotor response observed inora at low contrast frequencies (Figs. 2 and 8) is most likely due to the small remnants of the rhabdomeres of R1-6 that remain.Abbreviations PDA prolonged depolarising afterpotential - ERG electroretinogram