NOTES ON A COLOUR TROPISM OF ASTEROCHITON VAPORARIORUM, WESTV,'. 359 



held 21 and the south side of the trap 470. The distant plant held 122 flies. The 

 trap was now removed, and ten daj^s later the two plants which were near it and were 

 kept in the same position held 80 and 133 flies respectively, no emergence having 

 occurred upon them and the source of invading flies remaining approximately the 

 same as when the trap was exposed. 



Experiment 16. 



January 8th ; the main source of flies was a nettle plant growing in a pot and 

 massively infested. A small uninfested tomato plant one foot in height was placed 

 at a distance of two feet and the tube lined with yellow paper, as described in 

 experiment 13 above, was stuck into the soil of the pot holding the tomato. After 

 four-and-a-half hours of moderate sunlight a count of the flies was made. The 

 tomato plant with a total foliage area (both surfaces of leaves) of 120 sq. in. held 

 106 flies, about one to the square inch, and the trap with an area of 14 sq. in. held 

 160 flies, or about 11 to the square inch. 



Conclusions. 



Both sexes of the insect were drawn to the yellow traps, so the habit is probably 

 not associated with mating. The bodies of the mature insects are yellow and the 

 scale has a large internal organ of a yellow colour, the mycetoma, which is con- 

 spicuous in the younger scales. The insect is distinctly gregarious, quite apart from 

 the fact that it seeks 3'ounger foliage, and sometimes the majority of the flies on a 

 plant are found gathered on one or two leaves. It appeared possible that this gre- 

 garious habit was at the root of the attraction, and a number of adults were enclosed 

 in a glass tube. This and an empty tube were smeared with tanglefoot and placed 

 in the cage. The tube of fly caught 28 and the empty tube 25 in eight hours' 

 exposure. There was clearly no definite attraction here, and it is probable that the 

 yellow body of the insect is not sufficiently conspicuous to be perceived. 



Several manifestations of the phenomenon were observed apart from the experi- 

 ments recounted. The attraction to clothing, and especially to the khaki garments 

 which were commonly worn by the tomato hands at this time, has been referred to. 

 The flies may often be seen on yellow flowers. It was noticed particularly that 

 the yellow flowers of narcissus were more favoured than the foliage of the plant, 

 and many eggs were laid on them, though these inevitably perished. In the autumn, 

 when heavily infested tomato houses were being cleared out in the neighbourhood, 

 and the air in places was full of the disturbed insects, great numbers were seen hovering 

 over yellow privet, but the insect does not care for leathery foliage and has not been 

 known to breed on this shrub. 



The most favoured foods of the insect out of doors in England are the following 

 plants : — hollyhock, calceolaria, tobacco, sunflower, French and runner beans, and 

 vegetable marrow. All these have a distinctly yellowish-green foliage. The colour 

 attraction is believed to be associated with choice of food and to draw the migrating 

 insects to light- foliaged plants as opposed to those which have dark leaves ; but for 

 the reason given, the elaborate experiments which would be necessary to prove 

 this could not be undertaken. It is possible also that a solution of the problem could 

 only be found by studying the insect in its native haunt, which is said to be Brazil. 



It might be possible to devise some method of partial check to the pest by means 

 of these traps. Literally enormous numbers can be caught on a yellow screen by 

 shaking infested plants and holding the screen near them. The flies fly up when 

 disturbed in this way and pass to the screen as though drawn by a magnetic force. 

 Any such method of check would be clumsy and is not worth elaboration, since 

 perfect control of the pest can be obtained in the greenhouses by fumigations with 

 hydrocyanic acid, and out of doors, in England at any rate, the insect is not a pest 

 of major importance, and can only rarely be considered a pest at all. 



