Sensory Discrimination: The Chemical Sense 99 



Bethe suggests that the particles of the two chemical sub- 

 stances are also differently polarized, so that one of them 

 can be followed only in one direction, the other in the oppo- 

 site direction (51). Wasmann objects to this that an ant 

 returning on its own traces would destroy them, as the 

 opposite polarizations would cancel ; and that similar con- 

 fusion would occur on a narrow and much frequented 

 road (762). He and Forel (233) both think that, granting 

 the discrimination between the outward and inward paths, 

 which is made by only a few families of ants, the direction 

 is most probably given by a perception obtained through 

 the antennae, of the " smell form" of the footsteps. 

 Since the antennae are movable organs, like the hands, 

 they may well, Forel suggests, mediate spatial perceptions 

 of the form and size of odorous patches. This hypothesis 

 would fall to the ground if Mclndoo's contention that 

 the antennas are not smell organs were sustained. 



On the whole, there is much evidence indicating that 

 smell plays an important part in determining the response 

 of ants to well-frequented roads. We may now consider 

 the case of the solitary forager. Santschi (654) believes 

 that he has seen a smell trail " intentionally " deposited by 

 an ant, dragging her abdomen along the path. Bethe, whose 

 general position that ants, and indeed all invertebrate ani- 

 mals, are reflex machines requires him to avoid any hypothe- 

 sis that would involve learning or memory on the part of 

 these animals, is of course anxious to explain the homing 

 of the solitary foraging ant as a smell reflex. He placed 

 near the entrance of a nest a large sheet of paper covered 

 with lampblack, on which the footsteps of the ants could 

 be traced. On this paper he put a supply of food. When 

 an ant had found the food, Bethe reports that in returning 

 to the nest she always followed the path by which she had 



