The Structure and Special Physiology of Insects 25 



and the particular condition of functioning of these organs, therefore, is of 

 unique importance in the life of any particular animal. If the senses vary 

 much in their capacities among different animals, the world will have a differ- 

 ent seeming to different creatures. It will be chiefly known to any par- 

 ticular species through the dominant sense of that species. To the con- 

 genitally blind the world is an experience of touched things, of heard things, 

 and of smelled and tasted things. To the bloodhound it is known chiefly 

 by the scent of things. It is a world of odors; the scent of anything deter- 

 mines its dangerousness, its desirableness, its interestingness. As insects 

 know it, then, the world depends largely upon the particular character and 

 capacity of their sense-organs, and we realize on even the most superficial 

 examination of the structure of these organs, and casual observation of the 



FIG. 50. Stages in the development of the nervous system of the water-beetle, jEcilius 

 sulcatus; i showing the ventral nerve-cord in the earliest larval stage, and 7 the 

 system in the adult. (After Brandt; much enlarged.) 



responses of insects to those stimuli, like sound-waves, light- waves, dis- 

 solved and vaporized substances, which affect the sense-organs, that the 

 insects have some remarkable special sense-conditions. But the difficul- 

 ties in the way of understanding the psychology of any of the lower animals 

 are obvious when it is recalled that our only knowledge of the character 

 of sense-perceptions has to depend solely on our experience of our own per- 

 ceptions, and on the basis of comparison with this. We do not know if 

 hearing is the same phenomenon or experience with insects as with us. 

 But a comparison of the morphology of the insect sense-organs with that 

 of ours, and a course of experimentation with the sight, hearing, smelling, 

 etc., of insects, based on similar experimentation with our own senses, leads 

 us to what we believe is some real knowledge of the special sense-condi- 

 tions of insects. 



