ABSTRACT
Under the condition of full, normal daylight illumination a surface which reflects a fair proportion of ultra-violet, as well as the visible spectrum, is neutral or white in the vision of the bee. If the percentage of reflexion of 3600 A. is less than one-third, or about one-quarter, of the percentage of reflexion in the visible light, a degree of coloration is obtained which is sufficient to be noticed by the bees in training experiments.
When both are placed under ultra-violet absorbing filters, white paper in the vision of the bee most closely resembles a blue-green paper that possesses the highest amount of reflexion at 4900 A. Bees which have been trained to visit a blue-green surface covered by a filter glass do not do so any longer, if, by removing the filter, ultra-violet fight is added to the reflexion. The paper that possesses now two peaks of maximum reflexion, one at 4900 and the other at 3600 A., appears a light grey to the bee. On the other hand, when bees have been trained to visit an ultra-violet surface—white paper under a filter that absorbs the visible light completely—do not do so any longer if by lifting the filter the whole range of visible light is added to the reflexion of ultra-violet. The white paper at once becomes unattractive to the bee.
There remains no doubt, that among the four qualities of colour discriminated by the bee (see Fig. 1) the first and the third on one side and the second and fourth on the other are complementary colours for this insect and presumably for many others.
In the European flowers visited by bees three principal colours are now finally known: (1) that colour which is perceived if the main bulk of light reflected by the petals lies between 6500 and 4900 A. (orange-yellow for bee and man), (2) that which is perceived if the reflexion’extends from 4900 to 4000 or 3500 A. (blueviolet for bee and man), and (3) that which is perceived if the reflexion extends from 6500 A., or nearer to the red end of the visible spectrum, to 4000 A. (blue-green for the bee, but white, pink, light purple, bluish or yellowish for man).
Ultra-violet seems never to occur in European flowers in such a way as to make petals which are brilliantly white for man to appear equally white for the bee.
I. INTRODUCTION
A Method to ascertain whether a given material appears coloured for the hive bee, or appears merely neutral grey or white, has been described in a previous paper (Hertz, 1937 a). The procedure adopted may be summarized as follows. The bees were first trained to visit a feeding table placed in the open air in normal daylight. The table was covered over with a deep black paper. The material which it was desired to test was cut into the shape of a simple square or circle, about 6 cm. in diameter, and placed on the black background. The whole.surface was then covered with a sheet of glass which was known to transmit a high proportion (100-80%) of the longer ultra-violet rays, besides the visible spectrum. By placing a large drop of sugar solution on the glass over the material to be tested, attempts were made to condition the bees to visit the samples and to continue to do so when no sugar solution was present.1 The result of such experiments was that the bees reacted to samples of white of whitish paper of the ordinary kind made from wood pulp in the same way as when trained to visit blue or yellow papers; they easily learned to visit these samples exclusively. If other kinds of white or whitish paper were used, as for instance good writing paper or barytes paper, the bees continued to distribute themselves indiscriminately over the table; attempts to condition them gave no result at all. When the optical qualities of the two kinds of paper were investigated, that kind which, on a black background, proved to be completely unattractive, was found to reflect the whole of the spectrum visible for bees (6500-3000 A.). Such papers apparently are white for the vision of the bee just as they appear white for man (reflecting the red rays as well). The kind of white paper that proved to be attractive was found to reflect only the visible part of the spectrum and comparatively very little ultra-violet rays. I assume such papers to be coloured for the bee for the same reason that white light becomes coloured for man if red or any other of the visible rays are excluded from the mixed light. These white papers should appear for the bee identical in colour to a definite section of the visible spectrum, namely, that one which might represent the complementary colour to ultra-violet. Against my expectations I did not succeed, when I tried to get the bees to confuse the training colour “white devoid of ultraviolet”, with any hue of the twenty-four full-coloured papers of the Ostwald na series. Thus a final explanation regarding the complementary colours for bees could not be given at that time.
II. EXPERIMENTS WITH COLOURED PAPERS
I began a new series of experiments, when the following explanation of the results just described occurred to me: It is impossible for the eye of man to control the amount of ultra-violet, which may or may not be reflected by any given surface; and it may be that some of the colour papers used in the earlier experiments reflect ultra-violet in addition to their range of the visible spectrum. If it is true that ultra-violet and blue-green are complementary colours for the bee—as has been suggested by Kühn (1927)—a blue-green paper, reflecting ultra-violet also, must appear neutral grey or whitish for them. Whether this is actually so in the Ostwald papers can be ascertained by the method already described. In the full-coloured na series of Ostwald the blue-green papers range between No. 16 (oxide blue) and No. 22 (emerald green). Samples of each of these seven colours were put, one after the other, on the black background and attempts were made to condition the bees to visit them. As a cover to keep the papers clean a glass plate that transmits the whole of the solar spectrum was used.1 There was no difficulty in attaining regular visits to Nos. 16 and 22, representing blue and green. Papers 17 and 21 required a much longer training and the visits were less regular and reliable. In the case of the blue-green papers, Nos. 18, 19 and 20, prolonged attempts to condition the bees gave no result at all. Undoubtedly Nos. 16 and 22 are brightly coloured for the bee, Nos. 17 and 21 very much less so, and Nos. 18-20 appear for them only greyish or whitish. The most probable explanation is that the three last-mentioned blue-green papers reflect not only visible fight but also a large proportion of ultra-violet, and that Kühn was right in assuming that blue-green and ultra-violet are complementary colours for the bee. If this is actually the case, the behaviour of the bees must change completely if the ultra-violet reflexion be cut off by means of filters. I had three slightly different filters at my disposal (square glass sheets accurately fitting the paper samples).2 When any of the blue-green colour samples hitherto neglected by the bees were covered by a filter glass which absorbs ultra-violet, the behaviour changed according to expectations. There was no longer any difficulty to get the bees conditioned to pay regular visits to the coloured surfaces.3 The main and conclusive experiment was now made. When the bees had been trained to visit the fullcoloured blue-green paper na 20 covered by a filter, a sample of a whitish paper reflecting very little ultra-violet (made from wood pulp) was placed in competition with the training colour on the black background. Since the bees visited both surfaces with approximately the same eagerness, they gave ample proof that white daylight, devoid of ultra-violet or, to put it differently, the mixed light of the “visible “spectrum, is identical in colour for them with a more or less homogeneous blue-green. The same confusion took place when white papers, reflecting a high proportion of ultra-violet, were placed in competition with blue-green na 20, both samples being covered with filter glasses. It is evident, therefore, that in the vision of the bee ultra-violet and blue-green must be complementary colours.
Bees which are well trained to visit the blue-green and white papers—both devoid of ultra-violet—will not visit competing samples of yellow and blue (Ostwald na Nos. 1–4, 13–16 and 22–24).1 The light reflected by the ultra-violet absorbing glasses themselves must add a tinge of blue-green to the colour of any underlying surface. It is therefore important to know that the bees readily discriminate blue-green and visit it exclusively when the competing colour samples are also covered with filters.
In order to know the accurate range of the wave-lengths effective in the bluegreen Ostwald paper na 20 spectrographic measurements were carried out on that paper.2 The curve representing the percentage of reflexion for the different wavelengths is reproduced in Fig. 1. Kühn, when using monochromatic light in training experiments, found that the blue-green which represents for the bee a principal colour, apart from blue and green (yellow), ranges exclusively between 4800 and 5000 A. The fact that the two maxima of reflexion represented in Fig. 1 appear exactly at 4900 and 3600 A., confirms as well as possible the previous results and the conclusions so far made.1
In order to know the apparent darkness or lightness of the blue-green paper na 20—which represents a mixture of two pairs of complementary colours—the following experiment was made. For the purpose of training the bees a sample of that paper was put on a white background. Since not only colour but also relative darkness attracts their attention, the bees will learn more easily to pay regular visits to the sample on a white background the darker it is to their vision (Hertz, 1931). Apparently the paper was rather fight for the bee, because prolonged training did not give a positive result. In order to improve the training conditions by making the pattern more attractive the square paper sample was cut into strips 6 mm. in width and the strips were arranged in such a way on the white background that a pattern of cross-bars resulted. The increase in outlines had the effect that was to be expected. The bees began to visit the pattern regularly, but the reactions continued to be very slow. It is evident, therefore, that the paper na 20 is just dark enough to be distinguished by the bees from the white background; it represents a very light grey.
All these experiments prove that ultra-violet light does not affect the eye of the bee independently and apart from the rest of the spectrum, but is only one normal component in a system of four principal colours. Since this is a matter which has been misunderstood in some of the earlier investigations it is perhaps desirable to add another proof. On the usual black background a square sample of good white paper which reflects a high percentage of ultra-violet is placed. This is covered with a well-fitting sheet of black glass, known to transmit only ultra-violet light2 (3600 A. to 80%). After the bees have become well trained to visit the black (or ultra-violet) square, the white paper is removed from under the filter and placed by the side of it. A square sample of blue-green na 20, reflecting ultra-violet as shown in Fig. 1, is also put on the black background. The bees returning to their feeding table show, by their behaviour, that they completely miss the colour to which they had been conditioned. They hesitate to settle down and eventually distribute themselves indiscriminately over the table. The ultra-violet reflected from the white paper to a percentage even higher now than during training gives no longer the sensation of that colour because it is mixed with the whole range of visible light. Ostwald na 20 proves to be neither ultra-violet for the bee nor bluegreen. The black filter, devoid now of a background that reflects ultra-violet, does not attract the bees because they are conditioned to visit an ultra-violet surface and not a black one.
III. EXPERIMENTS WITH FLOWER PETALS
No flower seems to exist of a brilliant blue-green, but white flowers are abundant. The spectrum of the light reflected by the white petals is known for a number of American and European species, so that the question whether or not they are coloured for the bee can be answered immediately (Lutz, 1924; Lotmar, 1933). All of them should appear brightly blue-green, because the amount of reflected ultraviolet is irrelevant compared with the high percentage of reflected visible light. This is easily confirmed when samples of white petals are placed on the black background of the feeding table in competition with suitable papers. Bees that have been trained to visit white papers devoid of ultra-violet will visit indiscriminately any white flower petals picked at random. If the number and eagerness of the bees be taken as a measure, the attractiveness of the petals as a rule exceeds the effect of the paper samples. In some cases, at least, this must be due to a very high amount of reflexion of the visible fight and a nearly complete absorption of the ultra-violet rays. One instance is Convolvulus septum, the spectrum of which is given by Lotmar as: 5400 A. = 90 %, 4350 A. = 80 %, 4040 A. = 18·5 %, 3650 A. = 2·8 %, 3130 A. = 3 %.
But the number of flowers which, for bees, are apparently either identical, or more or less similar, in colour to 4900 A. are not limited to species which are white for the human eye. Bees which visit the white petals of Convolvulus, Phlox, Campanula, Althaea and Rosa will visit other flowers of the same genera which, for the human eye, are faintly but distinctly coloured pink, purple, blue or yellow, along with any other specimens of faintly coloured flowers (for instance, Knautia arvensis’). The explanation is easy to give: the pale colour effects, produced in such cases in the human eye, are made by rather slight differences in the amount of reflexion concerning the different parts of the visible spectrum. It is obvious that these slight differences do not matter very much in the case of the bee as compared with the necessarily very strong effect of a nearly complete absence of ultra-violet. The spectrum of Lavatera trimestris (Malvaceae) may serve as an example of a pink flower that is expected to be confused by bees with white ones: 6000 A. = 40 %, 5460 A. = 30%, 4350 A. = 47%, 4040 A. = 49%, 3650 A. = 6·2%, 3130 A. = 0% (Lotmar, 1933).
The only flower petals which were never visited by bees that had been fed for some time over samples of white paper lacking ultra-violet, or over white petals, were those which show a saturated orange, yellow, blue, dark purple or red.
We now select from the light, very faintly coloured ea series of Ostwald papers those which closely resemble, for the human eye, the faint flower colours just mentioned. They are placed in competition with petals of similar colour on the feeding table. It becomes at once apparent that there is no likeness between the pale flower colours and the pale paper colours; this must be due to a comparatively high amount of ultra-violet reflected by the papers.
IV. DISCUSSION
These results and arguments affect, in many instances, the interpretation of previous experiments. Lutz, in his attractive training experiments with Trígona cressoni, was the first to notice that there are two kinds of white which appear different for bees; but he is wrong when assuming that for these insects a white surface which reflects ultra-violet rays besides the visible spectrum looks anything like ultra-violet (Lutz, 1923). An important point to consider is that, in most of the experiments on colour vision in hive bees, sheets of glass of unknown qualities have been used to. cover the training table. It therefore might be useful to analyse more closely the limiting factors imposed by the qualities of such glasses. In my recently recorded experiments (Hertz, 1937 a) I found a good white paper, with the known reflexion of 60-80% of visible light and 50% of ultraviolet (3600 A.), to give no colour effect at all under a sheet of glass transmitting about 80% of 3600 A. It follows that a mixture of light, in which 60-80% of the visible rays corresponds to 32 % ultra-violet (0·802 × 0·50), is not definitely coloured in the vision of the bee. On the other hand, the bees gave positive colour reactions to another white paper which reflected 50-80% of the visible fight and about 25% 3600 A., and was placed under the same sheet of glass. This means that a mixture of light, where 50-80% reflexion of the visible light corresponds to 16% reflexion of ultra-violet (0·802 × 0·25), is no longer white but coloured for the bee. In many previous experiments glasses may have been used that did not transmit more than 50% of 3600 A. (Lotmar, 1933). By using the above-mentioned good white paper under a glass of this last kind the light ultimately effective represents 60-80% reflexion of visible light corresponding to 12·5 % reflexion of ultra-violet (0·503), a mixture that cannot be otherwise than coloured for the bee. It follows that in many instances, when investigators employed papers which they believed to be white or grey, they were actually using coloured papers, owing to the interference of the covering glass. It is perhaps well worth while to consider the fundamental experiments of von Frisch (1914) from this point of view. His bees, after having been trained to visit blue or yellow papers, never confused these colours with many different shades of grey. But confusion between the training paper and the grey papers took place when they had been fed on blue-green (Hering papers). He drew the conclusion that the blue-green papers were not coloured for the bee, but appeared grey. He certainly was correct if in his experiments he used a sheet of glass which transmitted a high percentage of ultra-violet light. If instead he used a sheet of glass transmitting comparatively little ultra-violet the confusion must have been due to the fact that both the blue-green and the grey papers appeared definitely and equally coloured for the bee. The principal result of von Frisch’s experiments and his conclusion that the bees discriminate blue and yellow colours by the wave-lengths they represent, not by their shade or darkness, is, of course, not affected by this doubt. The reasoning remains unaltered whether a certain colour is competing with many different shades of grey or of any other colour. I am myself aware of having once used a sheet of glass transforming, for bees, grey into colour (Hertz, 1932). As others had done before me, I tried to condition bees to visit exclusively a definite shade of grey, but noticed that after prolonged training the bees became attracted far more to the reverse side of any paper where the whitish wood pulp material was not covered with paint than to the actual training paper. I suspected a hidden colour effect, but I could not give the right explanation at that time. Now it is evident that the grey training paper appeared more or less coloured to the bee. After being conditioned they gave preference to that paper in which the training colour appeared most brilliant. It would be wrong to suppose, on the other hand, that all grey surfaces must appear coloured under glass plates of this kind. It might be that some surfaces, which are without definite colour to the human eye, reflect a surplus of ultra-violet and that a glass plate by absorbing part of the ultra-violet rays serves to destroy the ultraviolet colour effect without causing the complementary colour to appear.
Kühn has been the only investigator to use monochromatic light in training bees and the first to ascertain that the ultra-violet and blue-green rays are discriminated by their wave-lengths (Kühn, 1927). One of his experiments is very suggestive. He trained bees to visit a strip of blue-green light (4900 A.), thrown on to a table; afterwards he crossed that strip with a strip of mixed white light. The bees showed themselves well conditioned by visiting the blue-green light exclusively. In repeating this experiment one ought to place a filter, absorbing ultra-violet rays, into the beam of the mixed light. If I am correct, the bees would cease immediately to discriminate between 4900 A. and the white strip of light.
The system of colours as perceived by man is usually given in the form of a triangle, the angles of which represent red, green and blue. Lines’ connecting the loci of complementary colours meet in a point in the centre that represents white. We may suppose that the system of colours as perceived by the bee can be represented in a similar way. The colour triangle shown in Fig. 2 represents the facts as far as they are known to-day. How far the diagram represents the truth can be ascertained only in experiments with mixtures of monochromatic light. One most interesting question to solve is whether there is any definite likeness between the longest and the shortest rays in the colour system of the bee.
If future experiments would disprove a similarity between 6400 and 3200 A., and further, if it would be impossible to get the full effect of 4900 A. by mixing homogeneous yellow-green and greenish-blue rays, a simple triangle could no longer serve to represent the colour system of the bee.
ACKNOWLEDGEMENTS
I am very much indebted to Dr A. D. Imms whose help made these investigations possible.
REFERENCES
It is impossible to repeat here all the precautions usually taken in experiments with bees to avoid erroneous conclusions, they are to be found in earlier papers. A description of the technique and the methods of the present experiments is given in Hertz (1937b)
“Uviolglas” supplied by Schott & Gen, Jena.
Supplied by Schott & Gen, Jena.
It is necessary in experiments of this kind to cover carefully the edges of the little filter glasses, It is necessary in experiments of this kind to cover carefully the edges of the little filter glasses, arrangement of filters and papers is kept clean by means of an Uviolglas.
The percentages of reflexion in the visible light is known for some of these colours (Hertz & Imms, 1937).
By Messrs Adam Hilger Ltd., London.
If the examination of paper na 20 had revealed that the blue-green colour of this paper was If the examination of paper na 20 had revealed that the blue-green colour of this paper was that the bees respond alike under equal conditions; daylight devoid of ultra-violet and daylight devoid of ultra-violet and red are identical for bees.
Supplied by Schott & Gen, Jena.