230 ANIMAL LIFE 
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 mollusks—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 antenne or feelers, a 
single pair of which is borne on the 
head of every insect (Fig. 143). 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 
Fro. 143.—-Antenna of aleat. Concealed, it will nevertheless soon 
ane Botley arte be found by the flies and carrion 
panded terminaleegments, beetles that either feed on carrion 
or must always lay their eggs in de- 
caying matter so that their carrion-eating larva 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 tuken from various communities the members of their 
