562 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1928 



exists to a certain degree. Recognition of a known location is prob- 

 ably the function of the contact-odor sense, while the perception of 

 direction is effected wholly by visual memory. 



Many observers have strongly held that the sense of smell explains 

 how ants and termites retrace or follow their tracks, and these social 

 insects have actually been observed to lay down odorous trails, but 

 not until recently has anyone conclusively demonstrated the part 

 played by smell. 



Henning became greatly interested in this problem, and decided 

 to help clarify it, if possible. The many authorities on this subject 

 are agreed that the three senses of sight, touch, and smell are involved, 

 but the question remains as to which one of these plays the most 

 important part. Some of the writers think that smell is the most 

 important, but do not explain just how this sense operates, finding a 

 difficulty in the fact that the various trails run from the nest in 

 every direction, and apparently the same odor clings to all of them. 



Henning first attacked the problem by observing the behavior of 

 ants upon artificial paths instead of upon those made by themselves. 

 He caused the ants under observation to crawl over paper covered 

 with lampblack, and observed that after having traveled for a dis- 

 tance of 1 millimeter an ant made three strokes upon the black paper 

 with the end of her abdomen. After they had crawled over the paper 

 for a time or two the characteristic odor of formic acid was plainly 

 perceptible, indicating that they had imparted this particular odor 

 to the trail. It was later determined that they excreted this acid 

 whenever they were running over the paper, thus giving a cue for 

 experimental tests in which only the olfactory sense is involved. He 

 next painted a pathway up a tree, from the ground to the highest 

 point he could reach, with a weak solution of formic acid. The result 

 obtained was most interesting. Even before he had made a connec- 

 tion by means of the acid from the foot of the tree to one of the 

 natural runways passing from the nest, 10 to 20 ants made their way 

 up the artificial path upon the tree, though this path had not been 

 visited by them before. These pioneers were quickly followed by 

 others, until nearly the whole personnel of the nest had thus been 

 led astray by the artifice. The conclusion was drawn that the insects 

 reacted strongly to the odor of the formic acid, which was more con- 

 centrated than that in their own bodies. The stimulus was so strong 

 that even while the path was still moist the ants began to travel over 

 it, though they particularly dislike to get their feet wet and always 

 avoid damp places. Curiously enough, the same effect was pro- 

 duced not only by formic acid but by other chemicals having a similar 

 odor, such, for example, as formaldehyde. This experiment also 

 showed that it is not the smell of food which exerts the attraction but 

 the smell peculiar to the insects themselves. 



