THE SPECIAL SENSES 



377 



The sense of smell varies a great deal in its degree of 

 development in various animals. With the strictly aquatic 

 animals and these include most of the lower invertebrates, 

 as the polyps, the star-fishes, sea-urchins, and most of the 

 worms and molluaks the sense of smell is probably but 

 little developed. There is little opportunity for a gas or 

 vapor to come to these animals, and only as a gas or vapor 

 can a substance be smelled. With these animals the sense 

 of taste must take the place of the olfactory sense. But 

 among the insects, mostly terrestrial animals, there is an 

 extraordinary development of the sense of smell. It is in- 

 deed probably their principal special sense. Insects must 

 depend on smell far more than on sight or hearing for 

 the discovery of food, for becoming 

 aware of the presence of their enemies 

 and of the proximity of their mates 

 and companions. The organs of 

 smell of insects are situated princi- 

 pally on the antennae or feelers, a 

 single pair of which is borne on the 

 head of every insect (Fig. 229). That 

 many insects have an extraordinarily 

 keen sense of smell has been shown 

 by numerous experiments, and is con- 

 stantly proved by well-known habits. 

 If a small bit of decaying flesh be in- 

 closed in a box so that it is wholly 

 concealed, it will nevertheless soon 

 be found by the flies and carrion 

 beetles that either feed on carrion 

 or must always lay their eggs in de- 

 caying matter so that their carrion-eating larvae may be 

 provided with food. It is believed that ants find their 

 way back to their nests by the sense of smell, and that 

 they can recognize by scent among hundreds of individ- 

 uals taken from various communities the members of their 



FIG. 229. Antenna of a leaf- 

 eating beetle, showing 

 smelling-pits on the ex- 

 panded terminal segments. 



