Olfactory cells are held in place by ordinary epithelial cells—the sustentacular cells—which contain pigment. Olfactory cells are found in animals as low in the scale as the sea-anemone. They occur in the integument of the animal, and their structure is the same as in man, the only difference evolution has brought about being that in the higher animals they are protected by lodgment in a cul-de-sac. Their function in the sea-anemone is probably limited to the sensing of food, but we do not yet know much about this particular organism.
It is otherwise with the olfaction of insects. Here the work of painstaking observers like Lubbock, Fabre, and Forel, has supplied us with a mass of information of the utmost interest, which we shall now proceed to discuss in some detail, commencing with the work of that remarkable French naturalist, Fabre, whose interest in the subject was aroused by an accident—the accident of which the genius of observation knows so well how to take advantage.
Having by chance a living female Great Peacock moth captive in his house, Fabre was surprised one night by the advent of some forty others of the same species—males in search of a mate. At once the question arose in his mind: How was it that they had been attracted?
Sight could not have guided them, because, apart from the comparative rarity of this moth in that particular district, the night of their arrival was dark and stormy, his house was screened by trees and shrubs, and the female was ensconced under a gauze cover. He observed, besides, that the males did not make straight for their objective, as is characteristic of movement when directed by sight. They blundered and went astray, some of them wandering into rooms other than that in which the female was lying. They behaved, that is to say, as we ourselves do when we are trying to locate the source of a sound or a smell. But sound was ruled out by the fact that they must have been summoned from distances of a mile or a mile and a half.
Olfaction remains, and with this in his mind Fabre undertook several experiments, some of which, as it happens, support, while others oppose, the theory of an olfactory cause.
When the female was sequestered under the gauze cover, and in drawers or in boxes with loosely-fitting lids, the males always succeeded in discovering her. But when she was placed under a glass cover, or in a sealed receptacle, no male at all appeared. Further, Fabre found that cotton-wool stuffed into the openings and cracks of her receptacle was also sufficient to prevent the summons reaching the males. This last observation should be borne in mind in view of further discussion later on regarding the nature of the lure.
Similar observations and experiments were made on the Lesser Peacock, with very much the same kind of result. But in dealing with this moth Fabre made an observation which, if it was accurate, tells against the theory of olfaction, or at least against such olfaction as we ourselves experience. At the time when he was carrying out his experiments the mistral was blowing hard from the north, and as nevertheless males arrived, they must all have come with the wind; no moth ever hatched could beat up against the mistral. But then, if the guide is an odour, the wind, blowing it to the south, would have prevented it ever reaching the males! Here, then, we have a circumstance which leaves us groping for an explanation.
In watching the behaviour of the third moth on his list, the Banded Monk, on the other hand, Fabre discerned a circumstance very strongly suggestive of the operation of an odorous lure. He found that, if the female was left for a time in contact with some absorbent material and was afterwards shifted, the males were attracted, not to her new situation, but to the place where she had originally been lying. Subsequent experiment showed that a period of about half an hour was necessary to lead to the impregnation of the neighbourhood with the effluvium she elaborated.
The obvious test was employed of trying to drown the supposed odour of the female by filling the room she was in with powerful aromas, like naphthaline, paraffin, the alkaline sulphides, and the like. But in spite of the presence of these stenches, in our experience overwhelming to fainter exhalations, the males still continued to arrive in droves. This result led Fabre to doubt whether it could really have been an odour that attracted them. But surely this negative conclusion ignores the possibility of the moths being anosmic to these gross scents while highly specialised for one particular olfactory stimulus to which, as a matter of fact, we ourselves are wholly insensitive.
Apart from this particular problem, however, to which we return below, biologists agree that insects undoubtedly possess an olfactory sense capable of appreciating the same kind of odours as ours does. Lubbock, for example, demonstrated that ants give signs of perceiving the presence of musk and other perfumes. There is no doubt, indeed, that the olfactory sense plays a great, it may be a preponderating part in their life-activity.