My friend Bull, an honest dog all his lifetime if ever there was one, amongst other eccentricities had the following: finding in the dust of the road the shrivelled body of a mole, flattened by the feet of pedestrians, mummified by the heat of the sun, he would slide himself over it, from the tip of his nose to the root of his tail, he would rub himself against it deliciously over and over again, shaken with nervous spasms, and roll upon it first in one direction, then in the other.
It was his sachet of musk, his flask of eau-de-Cologne. Perfumed to his liking, he would rise, shake himself, and proceed on his way, delighted with his toilet. Do not let us scold him, and above all do not let us discuss the matter. There are all kinds of tastes in a world.
Why should there not be insects with similar habits among the amateurs of corpse-like savours? We see Dermestes and Saprinidæ hastening to the arum-flower. All day long they writhe and wriggle in a swarm, although perfectly free to escape; numbers perish in the tumultuous orgy. They are not retained by the desire of food, for the arum provides them with nothing eatable; they do not come to breed, for they take care not to establish their grubs in that place of famine. What are these frenzied creatures doing? Apparently they are intoxicated with fetidity, as was Bull when he rolled on the putrid body of a mole.
This intoxication draws them from all parts of the neighbourhood, perhaps over considerable distances; how far we do not know. The Necrophori, in quest of a place where to establish their family, travel great distances to find the corpses of small animals, informed by such odours as offend our own senses at a considerable distance.
The Hydnocystis, the food of the Bolboceras, emits no such brutal emanations as these, which readily diffuse themselves through space; it is inodorous, at least to our senses. The insect which seeks it does not come from a distance; it inhabits the places wherein the cryptogam is found. Faint as are the effluvia of this subterranean fungus, the prospecting epicure, being specially equipped, perceives them with the greatest ease; but then he operates at close range, from the surface of the soil. The truffle-dog is in the same case; he searches with his nose to the ground. The true truffle, however, the essential object of his search, possesses a fairly vivid odour.
But what are we to say of the Great Peacock moth and the Oak Eggar, both of which find their captive female? They come from the confines of the horizon. What do they perceive at that distance? Is it really an odour such as we perceive and understand? I cannot bring myself to believe it.
The dog finds the truffle by smelling the earth quite close to the tuber; but he finds his master at great distances by following his footsteps, which he recognises by their scent. Yet can he find the truffle at a hundred yards? or his master, in the complete absence of a trail? No. With all his fineness of scent, the dog is incapable of such feats as are realised by the moth, which is embarrassed neither by distance nor the absence of a trail.
It is admitted that odour, such as affects our olfactory sense, consists of molecules emanating from the body whose odour is perceived. The odorous material becomes diffused through the air to which it communicates its agreeable or disagreeable aroma. Odour and taste are to a certain extent the same; in both there is contact between the material particles causing the impression and the sensitive papillæ affected by the impression.
That the Serpent Arum should elaborate a powerful essence which impregnates the atmosphere and makes it noisome is perfectly simple and comprehensible. Thus the Dermestes and Saprinidæ, those lovers of corpse-like odours, are warned by molecular diffusion. In the same way the putrid frog emits and disseminates around it atoms of putrescence which travel to a considerable distance and so attract and delight the Necrophorus, the carrion-beetle.