The olfactory organ of insects is situated at the bottom of little crypts in the antennæ and in the palpi of the mouth apparatus, more particularly in the antennæ. And those insects, like bees, wasps, butterflies and moths, that frequent flowers, are attracted to them by their perfumes as well as by their colours. It has been found, for example, that covering up flowers from view does not put a stop to the visits of insects. Some naturalists go so far, indeed, as to say that odour is their principal guide. At all events, the sarcophagic and stercophagic insects are attracted to their food chiefly, if not entirely, by odour. Fabre has recorded how such insects are lured to their death by certain insectivorous plants which exhale a smell like that of putrid beef.
In this connection I may interpolate here an experience which shows that this class of insect may be attracted solely by odour. Incidentally, it also manifests how the olfactory sense of insects can be utilised in the matter of hygiene.
A clever plumber of my acquaintance was once called to a large drapery establishment in the West End of London, because the dressmakers at work in one of the rooms were making complaints of an evil smell that haunted the place. So much had they been troubled, indeed, that several of them had been made ill by it. On examining the workroom my friend found everything apparently faultless. It was a large, well-lighted and airy apartment, and he himself was unable to detect anything amiss in the atmosphere. Plans were consulted, but no evidence could be found of any possible source of unpleasant odour. His opinion therefore was, that the ladies were—ladies, that is to say, fanciful, and the matter was dropped. But the ladies were not consenting parties to this opinion, and the complaints continued. More of the assistants fell ill as a consequence, they said, of the smell, so that he was again sent for. On this occasion, it being the height of summer, he called, on his way to the draper’s emporium, at a butcher’s shop, and much to that man’s surprise, asked permission to capture a few of his bluebottle flies. These he took with him to the draper’s, and, the suspected room having been emptied of furniture and occupants, he closed all the windows and doors and released his flies. After waiting patiently for some time, he observed that these amateur detectives of his had all made for one part of the room, where they were settling on the wall. Here he had an opening made, and found hidden behind the plaster an open drain-pipe, old and foul, which had formerly been connected with a lavatory, and had been enclosed and forgotten during some alterations made on the building several years before.
The olfactory sense of insects has been credited with perhaps even more wonderful powers than those we have just been writing about. For instance, both Lubbock and Forel have shown that the extraordinary aptitude ants possess for finding their way back to their nest after their peregrinations in the mazy labyrinth of their world depends upon the sense of smell. On their return to the nest they follow the scent left by their own footsteps.
This “homing” instinct, or “orientation,” which is found in many species of insects and animals, has long been a matter of interest to scientific naturalists. The subject is, however, much too large for us to enter fully into on the present occasion.
Winged insects like bees and wasps manifest also the homing instinct. In their case the return to the nest or hive is effected probably altogether under the guidance of vision. This is what we should expect, as elevation in the air secures for these creatures a wide and unimpeded view of their world. Circumstances are obviously different in the case of ants and other creeping things, whose immediate outlook, like that of four-footed mammals, is circumscribed to an area of but a few inches or feet at the most.
Investigating the orientation of ants, Forel found, first of all, that while the covering of their eyes with an opaque varnish “embarrassed” them to some extent, they went hopelessly astray when their antennæ were removed.
He also repeated Lubbock’s well-known experiments of supplying the ants with bridges over obstacles in the neighbourhood of their nests, noting their behaviour when the bridges were changed, removed, or reversed, with the result that he came to credit the olfactory system of ants with much greater powers than the more cautious Lubbock would have believed.
These insects, says Forel, exploring with their mobile antennæ the fields of odour they encounter, form in their memory a kind of “chemical topography.”
Thus when an ant sets out from her nest she distinguishes the various odours and varying strengths of odours she comes upon, noting and memorising them as in two main fields, one on her left side, the other on her right. In order to find her way back again all she has to do is to unwind, so to speak, the roll in her memory, transposing right and left, and this successfully accomplished will bring her back to the point she started from.