l6 ANTS AND SOME OTHER INSECTS. 



eyes. Odor, when perceived, merely draws these animals in a 

 particular direction. When the compound eyes are covered, all 

 powers of orientation in the air are lost. Many insects can adapt 

 their eyes for the day or night by a shifting of the pigment. Ants 

 see the ultra-violet with their eyes. Honey-bees and humble-bees 

 can distinguish colors, but obviously in other tones than we do, 

 since they cannot be deceived by artificial flowers of the most skil- 

 ful workmanship. This may be due to admixtures of the ultra-violet 

 rays which are invisible to our eyes. 



The ocelli (simple eyes) play a subordinate role, and probably 

 serve as organs of sight for objects situated in the immediate vicin- 

 ity and in dark cavities. 



The olfactory sense has its seat in the antennae, usually in the 

 club-shaped flagellum, or rather in the pore-plates and olfactory 

 rods of these portions of the antennae. On account of its external 

 and moveable position at the tip of the antenna, the olfactory or- 

 gan possesses two properties which are lacking in the vertebrates, 

 and particularly in man. These are : 



1. The power of perceiving the chemical nature of a body by 

 direct contact (contact-odor); 



2. The power of space-perception and of perceiving the form 

 of objects and that of the animal's own trail by means of odor, and 

 the additional property of leaving associated memories. 



The olfactory sense of insects, therefore, gives these animals 

 definite and clear-cut perceptions of space-relations, and enables 

 the animal while moving on the surface of the ground to orient it- 

 self with facility. I have designated this sense, which is thus quali- 

 tatively, i. e., in its specific energy, very different from our olfac- 

 tory sense, as the topochemical (olfactory) sense. Probably the 

 pore-plates are used for perceiving odor at a distance and the olfac- 

 tory rods for contact-odor, but this is pure conjecture. Extirpation 

 of the antennae destroys the power of distinguishing friends from 

 enemies and deprives the ant of the faculty of orienting itself on 

 the ground and of finding its way, whereas it is possible to cut off 

 three legs and an antenna without seriously impairing these powers. 

 The topochemical sense always permits the ant to distinguish be- 



