THE BIOLOGICAL SIGNIFICANCE OF THE OLFACTORY SENSE In the lower animals, just as in mankind and mammals, the sense of smell plays a part in the most varied activities of life. The most widespread use to which it is put is the finding of food. This is already very marked in Coelenterates. If we put a little meat juice into an aquarium containing sea-anemones, we may be sure that, in a short time, all the anemones will be most beauti fully expanded. The turbellarian worms (planarians) also have a very acute olfactory sense. If we wish to collect a number of these animals, it frequently suffices to place the body of a frog, cut up, in a running brook. All the planarians which are down stream from this place scent the bait, and creep upstream to it. Gastropods also find their food with the help of their very acute olfactory sense. If we offer a juicy fruit or a lettuce leaf enclosed in a glass to a (Roman) vineyard snail (Helix pomatia), it takes no notice of it at all. From this it had been concluded previously (by Yung) that the animals were almost blind, but it only follows that they use their eyes for a dif ferent purpose, namely for the perception of the direction whence the light comes (compare article on SIGHT), while they find food by means of their olfactory sense. As soon as we put the fruit uncovered on the table, so that it can give out its scent unhin dered, it is found very quickly.
Very characteristic examples of the searching out of food with the aid of the olfactory sense are furnished by insects. Among the Lepidoptera the female usually lays her eggs on the leaves of plants, which are, later on, devoured by the caterpillar; there is no doubt but that in this unfailing, maternal instinct, she is di rected by the olfactory sense. The sense of smell in ichneumon flies is truly wonderful. As is well known, these insects lay their eggs in the larvae of other insects, which they find out even under the most difficult conditions. (See fig. I.) On the other hand, it is just among insects that we find, also, many examples of the seeking out of the female by the male by the aid of the olfactory sense. The most noted examples again occur among Lepidoptera. In silk-moths (Bombycidae), the great devel opment of the olfactory sense is shown by the structure of the large, broad, feathery antennae. (See fig. 2.) Standfuss placed
a female, of the Emperor moth (Saturnia pavonia), which had just emerged from the cocoon, at his window, in the heart of the town, and, within 61 hours, was able to catch 127 males, which had been attracted to her. These interesting experiments have since been repeated by numerous investigators. Mell marked a number of males of the large Chinese silk-moth (Actias selene), and released them at different points on a railway to seek out the female, which had been placed in a gauze cage on the veranda of his house. 4o% found their way back from a distance of 4 kilometres, 26.6% from a distance of 11.6 kilometres. To human beings this olfactory power seems to border on the marvellous, the more so, in that we ourselves can perceive not the slightest odour from the female moth in question. The idea has been enter tained, therefore, that these insects emit, perhaps, some kind of ray, but Forel showed that the males are attracted also to the place on which, a short time previously, a female had settled. This proves definitely that it has to do with a material scent.
It is a remarkable thing that, so far, only a few instances are known of the lower animals de tecting enemies by the aid of the sense of smell, and these occur in certain molluscs. We can surely induce the scallop (Pecten), to swim away if we bring its greatest enemy, the starfish, into its vicinity.
Among social insects the sense of smell is frequently of decisive significance in the social life of the members of a community. Certain ants (Myrmica) know their comrades definitely only through their sense of smell. Forel showed that they fall upon one another with the greatest fury if their olfactory organs, the antennae, are cut off. It has long been known that bees which enter a strange hive are immediately put to death, because the scent of a foreign hive clings to them. Bees possess, in addition, a peculiar scent-organ on the end of the abdomen, which can be protruded, and is often used by them as a means of communicat ing with each other. According to von Frisch, when a worker has discovered a rich supply of food in some place, she performs a peculiar dance, which causes the other bees to follow her. They are led to the food store by means of the scent trail.