Sensory Discrimination: the Chemical Sense 93 



bridge led down to the artificial ant nest. Ants and pupae 

 were placed on the stage. After the ants had through 

 random movements learned the way down the incline 

 a second incline was placed so as to lead from the opposite 

 side of the stage to the nest. No ants went down this way. 

 The inclines were then exchanged so that the one bearing the 

 scent of the ants' footprints was on the opposite side and the 

 unscented incline in the old place ; the ants continued to go 

 down in the old place. It is unsafe to criticise an experiment 

 without having actually seen it, but Turner's account does not 

 exclude the possibility that the ants were guided in setting 

 out on their homeward course by the scent of their footprints 

 on the cardboard stage, which seems to have remained un- 

 changed. He confirms Bethe's observation that the path- 

 ways to and from the nest are different, but does not find 

 that even a single ant follows her own footsteps in both di- 

 rections. The direction of the light, not smell, is the ruling 

 factor in pathfinding, according to Turner, who offers the 

 following experimental evidence. When the stage with the 

 first incline was arranged as before and a 16 c.p. lamp 

 placed near the side to which the incline was attached, the ants 

 learned to go down the pathway to the nest. When the in- 

 cline on the opposite side was added, and, after waiting a time 

 to make sure that no ants went down that way, the lamp 

 was moved to the other side, marked disturbance was shown 

 by the ants. "In most cases, some would finally go down 

 the new incline; in a few cases after the lapse of several 

 minutes all went down the new incline." Altering the in- 

 tensity of the light had no effect ; the disturbance was caused 

 by any decided change in its direction (408). 



In all probability, different species of ants vary in the 

 degree to which they make use of smell as a guide. Pieron, 

 for example, finds that Formica cinerea depends more upon 



