464 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



which they can examine, whereas in the simple eye the focal length 

 of the lens limits the distance at which a distinct view can be obtained. 

 Plateau thinks that the compound eyes are not complete visual organs, 

 but mainly organs of orientation, and that they do not distinguish 

 the form of objects, or, if they do, distinguish them very badly. 

 Lubbock, however, does not agree with Plateau, and claims that they 

 discern the form of bodies better than Plateau supposes. Forel 

 says that insects only distinguish the contours and forms of objects 

 in a more or less indistinct fashion, the more indistinct as the number 

 of facets is less, as the crystalline lenses are shorter, as the object is 

 farther off, or as it is smaller. Insects which have large eyes, with 

 several thousand facets, see forms fairly distinctly. By aid of their 

 compound eyes certain insects appreciate the direction and distance 

 of objects during flight; this is at least correct for near distances, 

 and Forel also thinks that they appreciate, even when at rest, the 

 distance of immobile objects. 



Some insects can distinguish colors. . Lubbock has experimentally 

 proved that bees, wasps, and ants have this power, blue being the 

 favorite color of the honeybee and violet of ants, the ants being sensi- 

 tive to ultra-violet rays. Butterflies, high in the air, will descend, 

 mistaking bits of colored paper for flowers; certain white butter- 

 flies usually preferring white flowers, and yellow butterflies showing 

 a preference for yellow flowers. 



THE SENSE OF SMELL. 



Of all the human special senses, we seem to know least about the 

 olfactory sense. This is pardonable because the sense of smell in us 

 is more or less rudimentary. Despite our degenerated olfactories, we 

 can, by special effort, cultivate this sense, and should do so by all 

 means, because odors are daily becoming more important. Alexander 

 Graham Bell, discussing the physical-chemical possibilities of odors, 

 says that an odor has already made one man famous, and wants to 

 know who is ready to evolve a new science by measuring or reflecting 

 a smell. He says : 



Find out what an odor is — whether it is an emanation and therefore subject 

 to being weighed, or a vibration and therefore capable of being reflected. Odors 

 are becoming more and more important in the worlds of scientific experiment 

 and in medicine — and the need of more knowledge will bring forth more knowl- 

 edge, as surely as the sun shines. 



The following discussion will show that in regard to odors insects 

 have already evolved a new science and are capable of classifying 

 and analyzing odors, many of which are unknown to us. They can 

 do this, of course, not as we may be able to some day, but, furthermore, 

 have evolved special organs for producing odors and highly developed 

 ones for receiving them. In fact the olfactory sense of insects, 



