The Effect of Colour on the Numbers of Houseflies Resting on Painted Surfaces
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THE EFFECT OF COLOUR ON THE NUMBERS OF HOUSEFLIES RESTING ON PAINTED SURFACES By D. F. WATERHOUSE* [Accepted for Publication September 22, 1947] Summary A simple technique for determining the preference of houseflies for surfdces of various colours is described. This consists of liberating adults into a Peet·Grady testing chamher fitted with movable coloured corners. The numbers of flies rest· ing on the coloured corners are recorded at intervals to provide adequate data for comparison. The ascending order of preference for the particular colours lested was white and sky hlue (equal) ,light grey, green, yellow and medium grey (equal), dusky hIue, and red. From measurements of the visual reflectances of the various colours it was found. that the order of preference could he explained. largely hy a reaction to the intensity of the light reflected hy the coloured surfaces, darker surfaces heing preferred to lighter surfaces. I. INTRODUCTION Colour perception in insects has been reviewed extensively by Weiss (1943, 1944, 1946), who points out' that most of the available information indicates that light intensity is more important than wavelength' in producing reactions. However, von Frisch and others have shown that, for bees at least, there is a definite wavelength discrimination (Wigglesworth 1939). The present tests were initiated when a proposal was made during the war that army kitchens and hospitals should be painted dark blue to repel flies. While it is well known that the common bush fly, Musca vetustissima, will not enter darkened rooms, the reaction of the housefly, M. domestica, is quite dif ferent. Thus there is a general trend in the evidence of Freeborn and Berry (1935) and Atkeson et al. (1943), in experiments carried out in dairy barns, that flies (mainly houseflies) preferred darker to lighter colours. However, because of the absence of precise experimental control and the lack of adequate physical data on the amount of light reflected by the various colours, it wal considered desirable to carry out tests paying attention to these factors. II. METHODS (a) Apparatus The tests were carried out in a standard 6 ft. by 6 ft. by 6 ft. white lacquered Peet-Grady testing chamber (Anon. 1943). Into each of the four corners of the chamber was fitted a coloured, movable, plywood corner. Each plywood corner consisted of two 5 ft. II in. by 2 ft. 3 in. pieces hinged together so that they *DiviiioD of Economic Entomology, C.Sl.R. 66 D. F. WATERHOUSE would stand up on their shorter ends at right angles to one another. During a test, these hinged pieces rested on a 2 ft. 4 in. by 2 ft. 4 in. piece of plywood placed on the floor in one corner of the chamber and were roofed by a similar piece of plywood. By this means, from one to four colours could be exposed simul taneously in the four corners of the chamber. Each false corner was separated from its neighbour by an 18 in. strip of white chamber wall. The paints used were selected from a range of full gloss, lacquer finish colours obtained from a commercial firm. They were applied with a spray gun after appropriate undercoats. The chamber was illuminated by a light bulb set in a reflector and shining in through a square glass panel in the centre of the roof. Normally a 200-watt frosted bulb was used, but in some experiments the light intensity was varied by inserting other bulbs. Although the light in this position provided equal illumination for each ()f the four coloured corners, the intensity of illumination varied from place to place on each corner. Thus the roof and upper portion of each corner was in shadow, while the remainder was directly illuminated, but situated at varying distances from the source of light. However, since each corner had an identical distribution of light from the bulb, this variation in light intensity over anyone corner should not introduce any irregularities when comparing the relative attractiveness of colours. The temperature of the chamber was thermostatically controlled, the majority of tests being carried out at 75° ± 1°F. (b) Flies Used The test insects were adult houseflies (Musca domestica 1..), bred in general accordance with the standard Peet-Grady technique (Anon. 1943). They were fed on a mixture of equal parts of milk and water and were usually tested when three to seven days old: Most batches of flies consisted of approximately equal numbers of males and females. However, in one test, females alone were used. (c) Test Procedure Four hundred to five hundred flies were liberated in the chamber in which corners of the desired colours had been placed. The operator sat on a stool in the centre of the chamber and disturbed all flies from each of the four corners in rotation and then from the glass panel under the light. After an interval of three minutes to allow the flies to settle, the numbers of flies on each coloured corner were counted as rapidly as possible, starting with the corner which had been disturbed first. Following this count, the flies were again disturbed and counted after settling, on this occasion the corner first disturbed being'the one adjacent and clockwise to the one first disturbed previously. When each of the four corners had been the first to be disturbed, a cycle of four counts was com plete. At least four repetitions of each cycle of four counts were usually carried out with each batch of flies to provide sufficient data for statistical analysis. THE EFFECT OF COLOUR ON HOUSEFLIES 67 The ahove cyclical procedure was necess.aryto compensate for the fact that the corner last to he disturhed for anyone count always had fewer Hies resting on it than would have heen expected from the counts when it was not disturhed last. The validity of the cyclical method of counting is discussed later. In one series of experiments, a cycle of counts taken inside the chamher in the fashion just mentioned was alternated with a cycle taken hyan operator through glass panels in opposite sides of the chamher. During the outside cycle of counts the Hies were not dlsturhed hut, as hefore, counts were taken at three minute intervals. This series of experiments was de~igned to detect the presence of any "self-preservation" reaction which might cause the Hies to react differently to coloured panels when disturhed than when allowed free choice to settle and move without disturbance. (d) Colours Tested Table 1 contains information* on the colours tested. Plywood painted with each colour and illuminated by illuminant A (light from a gas-filled tungsten lamp operating at a colour temperature of 2,840oK.) was matched to the neares.! shade in the Munsell Book of Colour. From the Munsell notation.the ISCC-NBS name was derived (Judd and Kelly 1939), and the I.C.I. trichomatic coefficients for illuminant A (gas-filled light bulb) obtained (Kelly.et al. 1943). The visual reHectances were measured with illuminant A, incident normally on the sample , and viewed at 45 ° to the normal. This source of light is the standard equivalent of the 200-watt bulb used to illuminate the chamber. TABLE 1 DESCRIPTION OF THE COLOURS TESTED I.C.I. Trichromatic Visual Name Used Munsell ISCC-NBS Name Coefficien'ts Reflectance Notation x y z (=y) White 10Y8/2 Pale yellow-green 0.460 0.422 0.118 0.68 Sky hlue 5BG7/2 Pale hlue·green 0.421 0.420 0.159 0.50 Dusky hlue 5B3/4 Dusky hlue 0.333 0.390 0.277 0.04 Green 10GY6/6 Strong yellowish-green 0.419 0.482 0.099 0.31 Yellow 10YR7/10 Moderate yellowish-orange 0.545 0.431 ' 0.024 0.37 Red 5R4/14 Vivid red 0.641 0.331 ·0.028 0.16 Light grey N7/ Light grey 0.446 0,408 . 0.146 0.45 Medium grey NS/ Medium grey 0.446 0.407 0.147 0.23 *Supplied hy the Division of Physics, C.S.I.R. & I 68 D. F. WATERHOUSE Light grey was prepared by mixing one part of black paint with eleven parts of white, while medium grey was a mixture of one part of black and three I parts of white. I, i i (e) Analysis of Results The relative attractiveness of colours was compared on the basis of the ratio of the numbers of flies on the panels in question, these numbers being summated over the four phases of a cycle. Logarithms of ratios for replicate cycles were used for tests of significance. When a number of cycles was involved the mean value for a colour ratio was calculated as the antilog of the mean log of the ratios of each cycle. III. RESULTS A. STANDARDIZATION EXPERIMENTS (a) Symmetry of Method of Counting In order to test for any possible asymmetry in the test chamber or in the method of taking counts, one pair of diagonally opposite corners was set up with white plywood and the other with dusky blue plywood. One hundred and six cycles, each of four counts, were then made, as described earlier, ten different batches of flies being used. Between tests with each batch ·of flies both the painted corners and the chamber were wiped clean with a moist cloth. The results obtained are shown in Table 2. TABLE 2 COUNTS OF FLIES RESTING ON WHITE AND DUSKY BLUE CORNERS Corner 1 2 3 4 Colour Blue White Blue White No. of flies 24,389 3,797 24,715 3,741 There was no significant difference in the ratios.