The Landing Responses of Insects: I. The Landing Response of the Fly, Lucilia Sericata, and Other Calliphorinae

Hollick (1941) has shown that the stimulus of an air current on the antennae causes the legs of the fly Muscina stabulans to be raised up and held in the flight position. It is possible, therefore, that when an insect slows down to land the consequent reduction of air pressure on the antennae may cause the legs to be lowered.

In these investigations the stimuli which initially attract the fly to the landing surface were not considered.

It was necessary to restrict the movements of the fly in such a way that, while the experimental conditions could be rigidly controlled, the results would, as far as possible, be applicable to the freely flying insect. In preliminary experiments flies were given varying degrees of freedom of movement ranging from suspension from a very light-weight pendulum allowing movement through 180⁰ in the vertical plane in a fore and aft direction, to tethering with a 2 ft. length of nylon thread giving freedom of movement in all planes within a sphere of radius 2 ft. For the majority of experiments, however, it was found most convenient to have a stationary insect flying in an air stream and to move the landing surface towards it through known distances at controlled speeds. The responses of rigidly mounted flies, and those more freely suspended, were filmed and compared and are described later. The conditions under which the rigidly mounted fly responds appear to resemble closely those under which the freely flying insect responds.

Specimens of Lucilia sericata were mounted after the method used by Hollick (1941). He determined the relationship of the flight system, i.e. the living fly and the surrounding air, under natural conditions in free flight and under experimental conditions with the insect held stationary. He found that when the living insect was held stationary with the body axis inclined at 10-29° to the horizontal and exposed to a stream of air whose speed ranged between 1.6 and 2.3 m./sec. it closely resembled in essentials the system in free flight. The actual flight system of the insect, therefore, was not interfered with in any significant way by mounting it in this manner, since it is likely that the conclusions reached for Muscina stabulans apply in a large measure to the closely related fly Lucilia sericata.

Flies which had emerged between 7 and 17 days previously were lightly anaesthetized and attached by the thorax to a fine piece of wire. The head was attached to the thorax by a narrow bridge of wax preventing head movements (see Fig. 3 a). The wings were then blown forward until the insect started to fly, when it was left for $12$ hr. to recover from the anaesthetic. It was then tested to ensure that recovery was complete. The criteria used were: (a) cessation of flight when the legs were brought into contact with the substratum ; initiation if they were removed, (b) Initiation of flight if the abdomen or other parts of the body were touched, (c) Retraction of the legs if the antennae were stimulated with an air current.

It was very much easier to give a controlled stimulus, and to establish the angular relationship between the fly and the landing surface when the legs were lowered, if the fly was stationary and the landing surface itself moved. An apparatus was therefore constructed to enable suitable surfaces to be projected towards the fly through a measurable distance at a measurable uniform speed. Circular black disks were normally used of diameter ranging from 1.5 to 15 cm. ; they were screwed to the end of a circular metal rod which was moved forward by the friction between it and a rotating pulley belt on to which it could be lowered. The speed of the pulley belt was controlled by a variable gear box, and the forward speed of the rod could be calculated from the speed of rotation of the pulley, since it was ensured that there was no slip between the two. The disks could be projected at any desired speed up to 150 cm./sec., the periods of acceleration and deceleration at the beginning and end of the run being very short.

The end of the rod bearing the disk projected into a white box 3 × 3 × 2 ft. (Fig. 1), provided with an observation panel. The carrier holding the fly and the jet of air in which it flew could be clamped in accurate alignment with the centre of the disk and the line of travel of the rod, the fly always being mounted in line with the centre of the disk unless otherwise stated. The disk started its travel flush with the wall of the box, the distance travelled being controlled by an adjustable rubber stop. This arrangement enabled objects of any required size to be projected at a known speed through a known distance towards the fly. It was possible to exchange the positions of the disk and fly if necessary and to fasten the fly to the rod so that it could be projected towards a stationary landing surface.

The inside of the box and all the supports were painted a matt white to present an unbroken white surface providing the maximum contrast to the black or grey disk used. In some experiments the inner surface of the box and all supports were covered with a matt black surface to contrast with white and light grey disks. Illumination was by reflected light from a 24 V., 150 W. bulb, placed behind opal glass screens to eliminate shadows cast by the moving disk. Since the flicker fusion frequency of Lucilia is well above 50 cyc./sec. (unpublished observations), the lamp was run from two 12 V. car batteries. The light intensity in the box could be varied by means of a rheostat, and measured using an S.E.I. photometer reading directly in log. ft. lamberts.

A second apparatus was designed to stimulate the fly with a change of light intensity unaccompanied by movement in the visual field. A controlled increase or decrease of illumination was produced over a chosen area of the fly’s visual field at a measurable speed, the change of intensity here being uniform over all the receptors involved. The fly was mounted in a matt black box facing a wall containing a circular opal glass disk, illuminated from outside by a lamp powered by batteries as before. The illumination of the screen was reduced by covering the lamp gradually with a light-tight metal canister and increased by withdrawing the canister from the lamp. The canister ran on wheels on two horizontal rails, and was screwed to one end of a rod moving on a pulley belt as described in the previous apparatus (Fig. 2A, B). To ensure that the light intensity of the screen would change uniformly over its surface, without the canister movement being visible, the lamp was mounted 1 ft. 10 in. away at a slight angle, and a second sheet of opal glass interposed near the lamp. The apparatus was then calibrated so that the position of the canister corresponding to a particular intensity on the opal disk was indicated by a pointer moving over a horizontal scale. Thus by appropriate placing of stops any desired decrease or increase in light intensity of the disk could be produced. A lever was pressed which lowered the rod on to the pulley belt and the canister ran forward at a known speed through a known distance resulting in a uniform decrease of intensity over the disk. For an increase of intensity the direction in which the pulley belt moved was reversed. The rate of change in intensity was proportional to the speed of the canister. The fly was mounted as in the previously described apparatus, the centre of its head being in line with the centre of the circular opal glass screen. The area of the visual field stimulated by the change in light intensity could be controlled by changing the radius of the disk or the distance away of the fly.

The response of rigidly mounted flies, stimulated by the approach of black disks of varying sizes, was filmed with a 16 mm. ciné camera (film speed 64 frames/sec.) so that leg movements might be studied in detail.

In the normal flight position the femur and tibia of the first and second pair of legs were folded up together at the side of the body with the tarsi extending forward. The third pair of legs streamed behind in the air current set up by the wings (Fig. 3 a). When the disk was 3 or 4 in. away from the fly the tarsi of the first and second pair of legs began to be lowered (Fig. 3 b). As the disk continued to approach the tibia and femur of the first two pairs of legs were extended forward ; the second were unfolded and extended downward and slightly forward (Fig. 3,c). When the disk was very near, the first and second pair of legs were completely unfolded and extended forward and the third pair were bent forward (Fig. 3d). As soon as the first pair of legs made contact with the disk the wings stopped beating and the body was bent round so that the first, the second and then the third pair of legs came in contact with the surface (Fig. 3 e). Whenever the stimulus was adequate to elicit a response the movement of the legs followed this pattern exactly.

The camera speed was not fast enough to allow a detailed analysis of the wing movements. However, it was possible to detect, though not to measure accurately, a reduction in the rate of the wing beat as the insect began to lower its legs. This was accompanied by a change in the inclination of the stroke plane from the habitual 30-50° to the horizontal axis of the body to almost 90°, possibly a reaction which, if it took place in free flight, would tend to prevent the fly from stalling as it slowed down. This change in the rate and plane of the wing beat did not occur until the legs began to be lowered. The reactions of the legs and wings were exactly the same when rigidly mounted flies were moved towards stationary surfaces, and when flies tethered to 2 ft. of nylon thread were allowed to land on a stationary surface, or, while remaining fairly still themselves, had a landing surface moved towards them. Although it was not possible to film freely flying insects landing, the behaviour of free, hovering specimens of Syrphus balteatus on the slow approach of a suitable landing surface can be observed with the naked eye. The legs are lowered in a similar manner to Lucilia and, if observed in a strong beam of sunlight, the stroke plane can be seen to alter also. The uniformity of response of both legs and wings under these varied conditions suggest that this is the normal landing response of these insects, and that it is not modified in any way by moving the landing surface towards a stationary fly. However, the insect is normally free to vary its speed of approach to the landing surface, while under the experimental conditions the landing surface approaches at a constant speed. In all the filmed records the rate of wing beat appeared to remain fairly constant until the legs had begun to be lowered, when there was a marked slowing in the rate in spite of the fact that the stream of air in which the insect was flying remained constant. It seems likely therefore that the fly approaches surfaces at a fairly constant rate until it is quite near the landing surface; it then lowers the legs and slows down almost simultaneously. The timing of these two reactions suggests that the lowering of the legs is not the result of a sudden reduction in pressure on the antennae as the insect slows down or of marked changes in the apparent rate of approach of the insect to the object. It seems very probable that the behaviour of the insect under the experimental conditions closely resembles that of the freely flying insect.

Stimuli which were inadequate to produce the full response were sometimes able to produce a partial unfolding of the legs, which would afterwards be returned to the normal flight position. Such partial unfoldings ranged from a very slight movement of the tibia of the first and second pairs of legs to a nearly complete unfolding of the legs, according to the strength of the stimulus. It was easy to see with the naked eye what stage in the unfolding of the legs was reached and so to assess the effectiveness of the stimuli. The unfolding of the legs was arbitrarily divided into three stages, corresponding to the three stages illustrated in Fig. 3 b-d, to provide a convenient way of recording the degree of response.

The films also revealed that as the stimulus strength was reduced the fly’s response was delayed until nearer the end of the disk’s run. Finally, a little before the stimulus strength was so reduced that incomplete responses began to appear, the flies were responding just before the disk stopped moving. The experiments described in this paper are mainly concerned with finding the threshold stimulus for a full response. The experimental conditions are such, therefore, that if a fly responds it is always responding just before the disk stops moving, i.e. during the last 1-2 mm. of its travel. It can therefore be assumed, without appreciable error, that the distance between the fly and the disk when it has stopped moving is the same as the distance between the disk and the fly when it responded, a distance which it would otherwise be difficult to measure.

It was possible that the fly’s legs were lowered in response to a visual stimulus mediated by the compound eyes or the ocelli. Alternatively, as the insect slowed, a reduction in the pressure of the air current impinging upon the antennae might have caused the legs to drop. There also remained the possibility that under the experimental conditions air currents set up by the moving disk might stimulate the tactile hairs on the head and body of the fly.

The response of intact flies was compared with that of flies whose compound eyes or ocelli or both had been covered with an opaque black varnish and with flies whose antennae had been removed or whose heads had been enclosed in transparent celluloid capsules. In each case twenty flies were in turn stimulated ten times with a black disk 10 cm. in diameter approaching against a white background at 50 cm./sec., with and without a glass screen between the disk and the fly. The positions of the disk and the fly were then reversed and the fly was projected towards the disk. The flies were also stimulated by rapidly increasing or decreasing the pressure of the jet of air in which they were flying. The results for all flies are summarized in Table 1.

It is clear that the response is not affected by air currents set up by the approaching disk, since it is given in exactly the same manner when the fly is shielded from such currents by a glass plate. The legs appear to be lowered in response to a visual stimulus mediated by the compound eyes alone. The presence of the intact antennae is not necessary for the response to be given in a normal manner. Increasing the pressure on the antennae as the disk approaches (by opening the tap controlling the compressed air supply), to compensate for any reduction in pressure due to alteration in the wing beat rate, does not prevent the response from being given. If the reduction in pressure was great enough some flies would lower their legs in response to this stimulus alone. However, the reduction in pressure necessary was much greater than would normally be caused by the insect slowing in flight. The flies rapidly became adapted to this stimulus and after one or two full responses, followed by some incomplete ones, the legs were not lowered.

Tests were made to determine which visual stimuli would elicit the landing response. Twenty flies were each stimulated in turn with objects moved towards and away from them, objects appearing to change in size without moving, and changes in the light intensity of the surroundings. In addition they were themselves moved towards and away from objects. In order to relate, as far as possible, the behaviour of the rigidly fixed specimens to that of the freely flying insects, the tests were repeated wherever possible with flies tethered to 2 ft. of nylon thread giving them considerable freedom of movement. Under these conditions the insects either fly around within the radius permitted by the thread or else, although flying, remain more or less in the same position.

Disks 9 cm. in diameter were projected a distance of 25 cm. towards the flies at a speed of 40 cm./sec. finishing their run 2.5 cm. away from the flies. The same speed and distances were used when the flies were projected towards the disk. In order to produce the effect of a change in size of the object, without a change in its distance from the fly, a diaphragm 6 cm. in diameter, inserted into a sheet of cardboard, was placed 4 cm. away from the flies and opened and closed in 1 sec.

The light intensity was varied by 1·0 log ft. lamberts in 1 sec. Each fly was given ten tests and its response noted. The results are summarized in Table 2.

The tests showed that it was not essential for the fly to be stimulated by a moving object, or by an object apparently increasing in size in order to elicit the landing response. Under certain circumstances, for example, a white disk retreating against a black background or a black diaphragm closing, the fly will respond to an object appearing to decrease in size. In addition a general decrease in the light intensity of the surroundings, involving no movement in the visual field, proved a very effective visual stimulus. The lowering of the legs does not, therefore, depend upon there being a landing surface at a given distance from the fly.

Much the most effective visual stimulus was one which produced a decrease of intensity in the visual field, for example, a black disk approaching against a white background, a white disk retreating against a black background, a white diaphragm opening and revealing a black surface, a black diaphragm closing over a white surface and a general decrease in the light intensity of the surroundings. A white disk approaching against a black background and a black diaphragm opening to reveal a white surface occasionally evoked a response, but the flies never responded to the withdrawal of a black disk against a white background, or to a general increase in the light intensity of the surroundings. Stimuli causing an increase of intensity over part of the visual field appear to be effective only when accompanied by an increase in the size of the stimulating object. It appears that this latter factor may, on occasion, play a part in eliciting the response though it is certainly not essential.

Controlled stimulation of insects attached to nylon thread was difficult. Some insects flew around within the limits imposed by the tethering thread and avoided the moving disk. Others, while flying, remained in more or less the same position and gave the landing response when the disk had approached to within 1-3 cm. of them. Other insects supported in this way were moved towards stationary disks and these too gave the landing response. According to Hertz (1934) bees are able to discriminate between moving objects and the apparent movement of objects due to their own movement. Lucilia also appears to be able to do this in some way, the moving fly distinguishing between a comparatively large moving object which it avoids and the same object stationary on which it prepares to land.

Flies do not discriminate, at least in their behaviour, between approaching a stationary disk and, while stationary, being approached by a disk. The landing response given in both cases is identical in all respects. It seems valid, therefore, to assume that the behaviour of the stationary fly stimulated by an approaching disk resembles closely that of the freely flying insect approaching a stationary surface.

Since in these preliminary tests the most effective stimuli were the approach of a dark disk against a light background and a general decrease in the light intensity of the surroundings, these stimuli and the responses given to them were analysed in greater detail.

As the disk approaches it will appear to increase in size and certain ommatidia will be stimulated by a change of light intensity, the number depending (amongst other factors) upon the size of the disk, its distance from the fly and the distance it travels. The rate at which these ommatidia are stimulated depends upon the speed of approach of the disk. Another possible stimulating factor is the rate of change of angular size of the disk which varies continuously as it approaches the fly. In addition the contrast between the disk and its background may affect the response given. The effect on the landing response of varying each of these factors in turn was examined.

The simplest estimate of the area of the fly’s visual field traversed by the edge of a disk as it approached was obtained by calculating the increase in the solid angle subtended at the eye by the disk as it moved forward. Owing to the rather complex geometry of the fly’s eye it would be difficult to calculate accurately the exact number of ommatidia stimulated by the disk, even if the variation in ommatidial angle were known in detail over the whole eye. The ommatidial angle was measured by cutting sections in the horizontal and vertical planes of the eye. It remained fairly constant over the central part of the eye at 2° in both the horizontal and vertical planes, increasing to 5° at the outer edge of the eye. As the basis of an approximate calculation it has been assumed that the eye is a hemisphere and the ommatidial angle constant. In such an ideal case the number of ommatidia stimulated would be proportional to the change in solid angle (cos θ₁ cos θ₂) where 2θ₁, and 2θ₂ are the angles subtended by the disk at the beginning and end of its run. Angle θ, the change in the angle subtended by the disk when it moves forward is thus (2θ₂ - 2θ₁). In the range of angles used in these experiments (20-100°) θ is approximately proportional to the change in solid angle, at least to within the limits of experimental error. It has therefore been used for convenience as an approximate estimate of the number of ommatidia stimulated. Accurate proportionality would not be expected for large values of θ where the ommatidial angle is not constant, or for small values of θ where the proportionality to change in solid angle breaks down. In addition, Burtt & Catton (1954) have shown that although the inclination of the ommatidial axes in the locust eye was 2.4° in the horizontal and 1° in the vertical meridian of the eye, the actual visual field of one ommatidium was approximately 20°. They suggest that this overlapping of ommatidial fields is general amongst insect eyes and if this is so in the case of Lucilia the number of ommatidia stimulated for a given value of θ will be greater than that expected. The effect of an increase in the field of vision of the ommatidia would be to restrict slightly the upper limit of the range over which the calculation applies. Over the greater part of the experimental range, however, θ gives a fair measure of the number of ommatidia stimulated by the disk.

The rate of change of angular size of the approaching disk depends upon its radius, its speed of approach and its distance from the fly and can be calculated as follows.

Let p be the radius of the disk in cm. ; v be the disk velocity in cm./sec. ; x be the distance of the disk from the fly and a be the angle subtended by α radius of the disk (in radians).

Since the disk diameter, the distance x and the disk velocity are all known, the rate of change of angular size at the moment when the fly responds can be calculated.

The fly was mounted facing a disk, 9 cm. in diameter, which travelled forward 25 cm. and stopped 10 cm. away from the fly. The speed of the disk was varied until, by trial and error, a point was found at which the fly gave a full response every time it was stimulated. The fly was then stimulated ten times at ½ min. intervals at this speed and the degree of unfolding of the legs, if any, resulting from each stimulus was recorded. The speed was then reduced by a known amount, and a further ten stimuli given. This was repeated until the fly ceased to respond. The speed was then increased by the same steps and the tests repeated. After an interval of $14$ hr. the two sets of tests were repeated and after a second interval the fly was stimulated for the third time. The average percentage of full responses given at each speed was then determined and plotted against the rate of approach of the disk, measured in cm./sec. Similar graphs could also be plotted for each of the other stages in the unfolding of the legs if desired.

This method is only valid if the variation in the percentage response with the strength of the stimulus remains constant so that reproducible results are obtained. In order to test this, and to determine whether the fly became adapted to or fatigued by the stimulus, an experiment similar to that described above was performed, except that only two series of ten tests were given. The stimulus required to obtain 50% response was read off from the graph and the fly was stimulated 100 times at this stimulus strength (in this case 31 cm./sec.) at $12$ min. intervals, the type of response given being recorded as usual. This was repeated for a further two groups of 100 stimuli. Finally, the initial experiment in which the stimulus was varied was repeated. The two graphs plotted from the two sets of results obtained from one fly at an interval of three hours were identical (see Fig. 4). The percentage of full responses given in each of the groups of 100 tests was compared with the percentage response read off from the two graphs at the stimulus value used. The five sets of results obtained, 50, 52,47, 51, and 50%, agree to within 5 %. The results of this experiment justified the use of the method previously described of estimating the average percentage response given to a particular stimulus from a series of ten tests, repeated several times, and showed that the results were reproducible over a period of several hours. In a further test flies were stimulated at a level just above the threshold for full responses at 5 sec. intervals to determine how long they would respond. As long as they continued to fly, which they did for periods of from 1-5 hr. and in one case 8 hr., they gave the full response to each stimulus. If fed with glucose solution upon cessation of flight they would very often begin to fly again and would continue to give the full response. Flies thus showed no signs of becoming adapted to or fatigued by this stimulus.

Ten flies were each stimulated by eleven black disks whose diameters ranged from 2 to 12 cm. The disks travelled 30 cm. towards the flies against a white background at a speed of 30 cm./sec., stopping 10 cm. away from them. Fig. 5 shows that the average percentage of full responses given by the ten flies was directly proportional to the disk diameter.

Ten flies were stimulated in turn by a disk 9 cm. in diameter starting 35 cm. away from them and approaching at 40 cm./sec. The distance travelled by the disk was varied between 15 and 25 cm. The average percentage of full responses given by the ten flies was directly proportional to the distance travelled by the disk, a greater response being obtained the nearer the disk approached (Fig. 6).

It was possible to replot the results of the two previous experiments in terms of the increase in the angle subtended at the eye by the disk (θ) and the rate of change of angular size of the disk at the point at which the flies responded (see Figs. 7 a, b and 8 a, b). The response given by the flies is seen to be directly proportional to the increase in the angle subtended at the eye by the disk. It is not, however, directly related to the rate of change of angular size of the object.

Eighteen flies were each tested with thirteen disks having diameters ranging from 3 to 15 cm. and travelling 30 cm. at 50 cm./sec. The distance between the fly and each disk at the end of its run (hereafter known as distance x) was varied and the percentage response given was plotted against the value of distance x. The value of distance x necessary for 100% response with each disk used could then be read off from the graphs. The stimulus necessary for 100% response in each of the tests was tabulated in terms of the disk diameter and distance x required, the value of θ and of the rate of change of angular size which this represents (see Table 3).

The results show that the flies do not always respond at a fixed distance from the approaching object, but at such a distance that the increase in the angle subtended at the eye is approximately the same whatever the size of the approaching object. Since θ is approximately proportional to the number of ommatidia stimulated it appears that to obtain a given response a given number of ommatidia must be stimulated by a change in light intensity as the disk passes across their visual field.

Ten flies were stimulated in turn with a black disk, 9 cm. in diameter, travelling 25 cm. at speeds varying between 18.5 and 40 cm./sec. The percentage of full responses given by each fly was plotted against the rate of approach of the disk (cm./sec.). In each case the percentage response given was directly proportional to the rate of approach of the disk, as previously shown in Fig. 4.

The visual differentiation of an object depends upon the contrast presented when the object and its background differ in brightness or in colour or in both these characteristics. In order to present a simple stimulus situation the contrast in these experiments was confined to differences in physical brightness only. Grey disks of a wide range of shades were used. They were presented first against a white background and then against a black background. The contrast between the disk and the background was measured in terms of the relative brightness difference (I₁ — where I₂ is the brightness of the background and I₂ is the brightness of the disk measured in log ft. lamberts, since brightness contrast depends upon relative rather than absolute brightness differences (Weston, 1945). The brightness of a surface is a function of the illumination it receives and of its reflexion factor. At the start of the disk’s run it is flat against the wall of the box and at this stage the disk and the background receive practically identical illumination. The brightness contrast is, therefore, a measure of the difference between their reflexion factors. As the disk moves towards the fly it moves nearer the light source and its illumination increases according to the law of inverse squares. However, the forward movement is small compared with the distance from the light source and measurements of the light intensity of the disk at the beginning and end of its run showed that there was very little error involved if the average of these two intensities was taken as the brightness of the disk.

Ten flies were stimulated in turn with fifteen disks, 9 cm. in diameter and of varying shades of grey, projected against a white background. The disks approached at a speed of 20 cm./sec. and the value of θ was 6o°. The average percentage of full responses given by the flies was directly proportional to the relative brightness difference measured directly in log ft lamberts (see Fig. 9). The experiment was repeated with θ equal to 100, 80, 40 and 30°, although for the latter two values it was necessary to increase the rate of approach to 45 cm./sec. to obtain 100% response. A similar result was obtained in each case. The walls of the box were then covered with a matt black paper and the disks projected against a black background. The response obtained was not so consistent as when a white background was used, some flies responding very infrequently, while in no case was the response directly related to the relative brightness difference.

Since the percentage response given by the flies was directly proportional to the increase in the angle subtended at the eye by the disk (θ), the rate at which it approaches (V) and its contrast with the background it seemed likely that there might be some relationship between these three factors.

The relationship between θ and V was first examined, keeping I₁— I₂)/I₁ constant, the speed necessary to achieve 100% response being determined for different values of θ. Ten flies were stimulated by a black disk, 9 cm. in diameter, with a background intensity of 1.5 log ft. lamberts. θ was first set at 20° and the rate at which the disk approached was varied, the percentage response given by each fly being plotted against the rate of approach of the disk. θ was increased by intervals of 10° and the experiment was repeated. The rate of approach of the disk necessary to produce 100% response for each value of θ was read off from the graphs. Fig. 10 shows that the log of these rates of approach plotted against the log of the values of θ yields a straight line of slope — 1. The rate at which the disk must approach the fly in order to produce 100% response is therefore inversely proportional to the increase in the angle subtended at the eye by the disk. Results obtained from other experiments have been included in Fig. 10 for comparative purposes and show close agreement.

The relationship of θ, V, and (I₁— I₂)/I₁ was examined using the results of previous experiments. The rate of approach necessary to obtain 100% response when each of the grey disks was used was known for a wide range of values of θ. Fig. 11 shows the relationship between the product θ V and(I₁— I₂)/I₁ each case. θ V is inversely proportion al to (I₁— I₂)/I₁ i.e is a constant for a constant response. The response given by the fly is therefore directly proportional to the product of ⁠. These results suggest that the response given by the flies is directly proportional to the total decrease of intensity (the number of ommatidia stimulated × the relative brightness decrease) × the rate of decrease.

If the effective stimulus given by an approaching object is the decrease of intensity it produces, it should be possible to elicit the response by stimulating the flies with a decrease of light intensity which does not involve a moving object. The apparatus used for these experiments has been described above. The fly faced an opal glass disk the intensity of which could be decreased by a known amount at a known rate. An estimation of the total visual field of the two eyes affected was obtained by calculating the angle subtended at the eye by the opal glass disk in one plane. This angle is hereafter known as angle θ₂. In the range of values used it is approximately proportional to the number of ommatidia stimulated by the change in light intensity, assuming as before a hemispherical eye and a constant ommatidial angle.

The experiments performed with a moving disk were repeated with an increase and then a decrease of light intensity as the stimulus, using the same technique as before. θ₂, the relative brightness change (I₃ — I₄)/I₃ measured in log ft. lamberts (where I₃ is the intensity at the beginning and I₄ the intensity at the end of the change), and the rate of the change (V₂) were varied in turn. No fly ever responded when the intensity of the opal disk was increased. A normal landing response was given when the intensity was decreased and figs. 12, 13 and 14 show that the average percentage of full responses given by the ten flies tested was directly proportional to θ₂, to the rate of the decrease measured in terms of the speed at which the canister moved over the light source, and to the relative brightness decrease (I₃ — I₄)/I₃.

The relationship between θ₂, V₂ and (I₃-I₄)/I₃ was also investigated. The rate of decrease of intensity necessary for 100% response was determined for values of θ₂ ranging from 30 to 100° for a given value of (I₃ - I₄)/I₃. A slope of approximately — i was obtained when the log of the rate of decrease was plotted against the log of θ₂ (see Fig. 15), indicating that the rate of the decrease of intensity required for 100% response was inversely proportional to θ₂. Similarly the rate of decrease necessary for 100% response was found for various values of (I₃ — I₄)/I₃ when θ₂ was constant. The rate of decrease of intensity required for 100% response was inversely proportional to (I₃ — I₄)/I₃ (see Fig. 16). The percentage of full responses given by the flies is therefore directly proportional to the product of θ₂ (and hence to the number of ommatidia stimulated), the relative decrease of intensity and the rate of the decrease. A decrease of light intensity which in no way involves movement in the visual field produced a response identical with, and dependent upon, the product of the same three factors as the response given to a dark object approaching against a light background.

Nine flies were each stimulated by a black disk approaching against a white background. (I₁—I₂)/I₁ was kept constant at 0.86 throughout the experiment. The disk approached at 50 cm./sec. when stimulating the first three flies. The speed was reduced to 40 cm./sec. for the next three flies and to 30 cm./sec. for the last three flies used. θ was varied in each case and the percentage response obtained plotted against the value of θ. One compound eye and the ocelli of each fly were then completely covered with a quick-drying black varnish and the experiment was repeated. With one eye covered the visual field will be restricted, though not halved, because the visual fields of the two eyes overlap ; but the actual number of ommatidia stimulated will, however, be reduced by half.

All the flies responded normally when one compound eye was covered; binocular vision is therefore not essential for the response. Table 4 compares the values of 9 necessary for 100 % response when the eyes are intact and when only one compound eye is functioning. The value of θrequired is approximately doubled, confirming the implication of the previous results that the response is proportional to the product of θV at least over the range of values of θ, V and (I₁ — I₂)/I₁ used in these experiments, values which are within the limits of those normally encountered by the fly.

The leg movements during landing of many Diptera and representatives of other insect orders have been observed. The results are summarized in Table 5. Those insects which fold their legs beneath the body in some way during flight all lower at least the first and second pair of legs and extend them forward when approached by a suitable landing surface and most give a similar response to a decrease of light intensity. The response is very marked in those members of the Odonata which have been tested, all legs being extended forward to form a ‘basket’ with its open end directed forwards. It is possible that this reaction to an approaching object is utilized by the insect when catching prey on the wing.

The fly, Lucilia sericata, lowers its legs from their retracted flight position before landing in response to a visual stimulus to the compound eyes. Although by virtue of its overlapping frontal fields of vision the fly is presumably capable of some degree of distance estimation by a binocular method, the lowering of the legs is not based upon an estimation of the distance between the fly and the landing surface. The approach to a landing surface, at least as far as lowering the legs is concerned, and probably also with reduction in speed, is measured in terms of the product of: the number of ommatidia stimulated, the log of the relative decrease of intensity by which they are stimulated and the rate at which they are stimulated. This type of stimulus may well be more effective for an insect such as L. sericata than one based on distance judgement with the legs lowered at a fixed distance from the object, since that would require a well-defined landing surface. As it is, the insect is equally readily stimulated to slow down and extend its legs ready to make contact when approaching well-defined landing surfaces, more or less uniformly shaded walls or ill-defined shadowed areas into which it is likely to be attracted by chemical stimuli. Waterhouse (1948) has shown that in general houseflies land upon the darkest surface available provided that they are not additionally stimulated in other ways. Observations upon L. sericata and Calliphora erythrocephala in the laboratory indicate that they also show a tendency to land in a shaded corner or upon the darkest wall of the cage. Nevertheless, Lucilia is fully capable of landing upon a light, or even a plain white wall. It is extremely likely, however, that even on approaching very light surfaces there are sufficient irregularities of illumination to provide the necessary stimulus. Observations on blind flies have shown that if the legs are not lowered as the flies approach a landing surface, as soon as the wing tips make contact with the surface in front of them as they sweep forward, the legs are extended and moved about until the surface is grasped. The movement is not so smooth as the normal landing reaction but it is effective and it is possible that flies encountering a completely evenly illuminated surface, which must be very rare, land on it in this way.

The fact that the perception of approach to a landing surface appears to be based on the multiplication of : the contrast presented to successive ommatidia, the number of ommatidia so stimulated and the rate of successive stimulation, would seem to support Hassenstein’s claim (1951) that perception of movement in an insect’s visual field is based upon the evaluation of the contrasts presented, by a process of multiplication in the central nervous system. The after-effect resulting from stimulation of one ommatidium is multiplied by the response of the subsequently stimulated ommatidium as the object moves across the visual field. The rate of stimulation is important in that the sooner the second ommatidium is stimulated the less will the after-effect of the first ommatidium have faded. When Lucilia is stimulated by a change of light intensity alone the response is still related to the product of θ₂V₂ (I₃ —I₄)/I₃. It may be that in perceiving changes of light intensity the central nervous system is comparing the previous intensity and the new intensity continuously by a process of multiplication.

1. The leg movements and changes in the wing beat pattern which occur when the fly, Lucilia sericata, is in thé proximity of a landing surface have been filmed and the sequence of events described in detail.

2. The legs are lowered from their retracted flight position as the result of a visual stimulus mediated by the compound eyes alone.

3. The moment at which the legs are lowered is not based upon an estimation by the fly of the distance between itself and the surface it is approaching. A normal landing response can be evoked merely by decreasing the light intensity of the surroundings without any movement occurring in the visual field.

4. The effective stimulus is based upon a multiplication of : the change of intensity at successive ommatidia when a fly approaches a landing surface, the number of ommatidia so stimulated and the rate of their successive stimulation. A given value of this product is required to evoke the landing response.

5. The landing responses of other Díptera and representatives of other insect orders have been described.

I wish to express my gratitude to Prof. R. J. Pumphrey for his encouragement and advice during all stages of this work and for his criticism of the manuscript. I am also indebted to Dr F. S. J. Hollick, who first introduced me to this problem, and to Dr J. W. Jones, Dr D. J. Harrington and Dr J. D. Carthy for their help and advice. My thanks are due also to the Mistress and Fellows of Girton College Cambridge for the award of an Ethel Sargent Research Studentship.