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English: Fig. 50.—Stages in the development of the nervous system of the water-beetle, ^cilins 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.