Previous communications in this series have reported that surface warmth, darkness, and moisture are attractive factors for Aedes mosquitos, the demonstrations having been made on inanimate objects. The attractiveness of warmth independent of moisture was demonstrated on plastic balls for Aedes aegypti (L.) in the laboratory (Peterson & Brown, 1951) and on clothed robots for Canadian species of Aedes in the open air (Brown, 1951). The superior attractiveness of dark-hued or black surfaces was similarly demonstrated in the laboratory (Brown, Sarkaria & Thompson, 1951) and with robots in the field; it was later shown that the attractiveness of a surface was inversely proportional to its reflectivity in the range of wavelength between 475 and 625 millimicrons (Brown, 1954). The attractiveness of moisture was demonstrated with moist air emitted from an olfactometer in the laboratory, and with moistened clothing on robots in the field. The purpose of the experiments to be reported in this paper is to investigate the effect of these three factors on the attractiveness of the skin of the human hand.
Material and Methods.The species employed was the yellow fever mosquito, Aedes aegypti, which was reared according to the methods described by Brown, Sarkaria & Thompson (1951). Tests were conducted using the adult females in a large cage of 360 cu. ft. capacity. Two ports were cut in the side of the cage, 16 in. apart and 4-5 ft. from the floor, and to them were attached reversible sleeves of white broadcloth, 18 in. long, each with an elastic band to fit round the wrist.The human hand was the test object, and the hands of two different individuals were compared for their attractiveness. They were simultaneously thrust into the cage and held there, fingers outstretched and appressed, for 15 sec. while an observer counted the number of mosquito landings on each. They were removed, the hands interchanged between the sleeves, and again introduced for 15 sec; a total of 18 such counts constituted a test. The results of each test were statistically analysed by applying Student's t test to the variance of the counts.Skin temperatures were measured by pressing copper-constantan thermocouples into the back of the hand; these were connected through a switch to a pointer-type galvanometer (G.-M. Laboratories) sensitive to 0-1 mieroamp. per mm. Skin hues were selected visually and confirmed by photographing from 10 ft. under a floodlight with Ortho-X sheet film and printing on to Kodak F-4 paper. Skin humidity was measured as the mg. of moisture transpired in 30 min. when the hand of a sitting subject was enclosed in a 4-g. polythene bag (Canadian