Sensory Discrimination: The Chemical Sense 97 



the general direction of their outward course and reversing 

 it when they have found a load to be carried home. We 

 may consider very briefly the facts that have been brought 

 to the support of these various hypotheses. There are 

 really two problems involved in the homing of ants. There 

 is, first, the problem of the homing of a solitary forager, 

 who, having found food at the end often of a very long and 

 rambling course, is able to get back to the nest. Secondly, 

 there is the problem of the nature of a frequented ant road, 

 along which many ants constantly travel to and from the 

 nest. The evidence that smell functions in the homing 

 process is strongest in the case of such a frequented trail. 

 Lubbock's experiments showed that on these trails the 

 recognition of visual landmarks plays no important part. 

 For instance, he placed larvae in a dish on a table connected 

 by a bridge with an ant nest. He accustomed the ants to 

 go back and forth between the dish and the nest by a path 

 which he diversified with artificial scenery, such as rows of 

 bricks along either side, and a paper tunnel. When the 

 path was thoroughly learned, he moved the bricks and the 

 tunnel so that they led in a different direction : the ants, 

 however, were not at all disconcerted by this cataclysm of 

 nature, but followed the same track as before, evidently 

 guided by their own footprints (441, p. 259). Forel (233) 

 showed that when a piece of wood is laid across a well- 

 frequented path of certain species of ants, they are much 

 disturbed and at a loss to follow the trail, and Bethe (51) 

 reports that drawing a finger across the trail will apparently 

 break its effectiveness as a guide. That the chemical 

 deposited by the ants is volatile he concludes from the fol- 

 lowing observation. 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 



