Sense of Smell in Flies

Abstract

By far the most efficient of fly-destroyers with which I am acquainted is a dilute solution of formaldehyde. If two teaspoonfuls of formalin (40 per cent, formaldehyde) be added to a soup-plate filled with water, flies go to it, one after the other, to drink, especially in the early afternoon. Some die in the water; many fall in the immediate neighbourhood of the plate; others succumb on window-sill or floor. As the result of leaving a single plateful of the solution on the kitchen table (I am writing in the south of France) hundreds of dead flies are each day swept up from the floor. Formalin water is free from the gruesome associations of fly-papers and other traps which hold their struggling victims. It may even be turned to ornamental uses. A wire cage placed in the centre of the dish may be crowned with flowers, which flourish equally as well, with some slight but interesting changes in tint, in dilute formalin as in pure water. The solution neither attracts nor repels flies. Two similar dishes placed side by side, the one containing pure water and the other formalin, are visited, so far as one can judge, with equal frequency. It is somewhat strange that so small a dose proves fatal when taken into the fly's alimentary canal. I find that, to free a room from flies by vaporising formalin, the air must be rendered quite irrespirable by a human being. The room needs to be amply ventilated before one ventures into it.