Sensory Discrimination : the Chemical Sense 91 



of Bethe's reflex theory, looks with suspicion on this tendency 

 gradually to straighten the path, and thinks an animal re- 

 flexly drawn along by a chemical stimulus on the ground 

 would make no improvements in the route (426). The evi- 

 dence that it is the sense of smell, if not a smell reflex, that 

 guides the ants remains, however, strong. Bethe further 

 points out that the chemical stimulus deposited by the feet 

 of the ants is volatile. If a strip of paper be placed across 

 an ant path, the ants on coming to it stop, quest about, and 

 are delayed until one accidentally runs across the strip and 

 others follow. (Why, asks Wasmann, if they are being re- 

 flexly drawn along, do they not merely stop short when the 

 stimulus fails, instead of hunting for it ?) The piece of paper 

 is thus gradually adopted into the ant road; if it is subse- 

 quently removed, the ants stop and are bewildered at the 

 place where it was, showing that the earlier traces of their 

 footsteps, under the paper, have evaporated. Again, Bethe 

 thinks he has evidence that the chemical stimulus left by the 

 feet of ants going from the nest is different from that deposited 

 by those going to the nest, and that ants on the way. home 

 will not follow a track made by the feet of other ants on the 

 outward journey, and vice versa (30). That they will follow 

 their own individual track in either direction is shown by 

 the smoked paper experiment just described, and also by an 

 experiment of Fielde's, where ants finding their way through 

 a labyrinth from a heap of pupae to the nest and back 

 again followed each one its own trail without regard to the 

 others (218). Bethe found that when the usual road to an 

 ant nest had been interrupted by the removal of a heap of 

 sand, and the road across the breach had been established 

 solely by incoming ants, the outgoing ants refused to follow 

 it, and made a new road for themselves (30). Wasmann 

 thinks this may have been done merely on account of the 



