The Structure and Special Physiology of Insects 25 
and the particular condit'on 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, Mcilius 
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. 
