222 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



bring back honey or sugar, other ants are sure to try 

 the path. Hence something of the substances carried 

 over this path by the ants must remain on the path. 

 These substances must be strong enough to affect the 

 ants chemically. I can prove by the following obser- 

 vation, which must surely have been made before me 

 by many breeders of butterflies, that Bethe is justified 

 in the assumption that insects are affected by ex- 

 tremely weak chemical stimuli. I placed a female 

 butterfly of a certain species in a cigar-box, and closed 

 the box. The box was then suspended half way be- 

 tween the ceiling and floor of the room and then the 



o 



window was opened. At first no butterfly of this 

 species was visible far or near. In less than half an hour 

 a male butterfly of the same species appeared on the 

 street. When it reached the height of the window, 



O 



its flight was retarded and it came gradually toward 

 the window. It flew into the room and soon up to 

 the cigar-box, upon which it perched. During the 

 afternoon, two other males of the same species came 

 to the box. Thus we see that butterflies and certainly 

 many other insects possess a delicacy of chemical 

 irritability which, if possible, is finer than that of the 

 best blood-hound. Plateau maintains that insects are 

 attracted to the flowers by the odour rather than by the 

 colour and marking. The dioptric apparatus of insects 

 is very inferior to that of the human eye, while their 

 chemical irritability is much superior to that of our 

 olfactory epithelium. I believe that both odour and 

 colour may influence insects. 



