68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. '/'J 



Some writers classify the organ of Johnston as an auditory organ, 

 but there is httle evidence in support of the idea that it is a sound 

 receptor in any insect. From its widespread occurrence in insects, 

 however, we may infer that it is an organ of importance, and its 

 constant position suggests that it must have some specific sensory 

 function. Its structure in the Chironomidse and Culcidse, especially 

 the union of the supporting plate (fig. 31 B, a) with the base of the 

 third antennal segment (sScg), suggests that the organ in these 

 families is an apparatus for registering the movements of the flagel- 

 lum. It may, therefore, be supposed to be perceptive to slight air 

 motions, but whether to waves of sound or not is still to be 

 questioned. 



IX. THE EYES 



The word " eye '" is used in a general sense for any specific organ 

 that is sensitive to light falling upon it and capable of transmitting 

 the resulting stimulus to the central nervous system. An eye is 

 primarily a light-perceiving organ, or photoreceptor, and it is not to 

 be assumed that all eyes are capable of registering impressions of 

 form, color, or motion in external objects. The effect of the light 

 stimulus on the organism must vary with the structural development 

 of the eye and of the visual centers of the nervous system. 



The eyes of insects are usually classified as simple eyes, or ocelli, 

 and compound eyes. The compound eye constitutes a definite type of 

 organ common to insects and crustaceans, and is most probably the 

 primitive eye of these two groups. The ocelli, on the other hand, 

 are a heterogeneous group of photoreceptive organs comprising sev- 

 eral distinct kinds of eye structures, which probably either have had 

 separate origins or have followed separate lines of development 

 from a primitive type. Only a brief description of the eyes of insects 

 will be given here, because a complete treatment of the subject would 

 involve a discussion too long for the present paper. A reference to 

 the many valuable works now at hand on the structure of insect eyes 

 must also be omitted. The histology of the eyes is better known than 

 that of other insect sense organs, but still there is much that has 

 not found its way into any general review of the morphology of 

 the visual organs. 



The fundamental elements in all the varieties of insect eyes are 

 innervated photoreceptive cells of hypodermal origin. These cells 

 correspond with the sense cells of the other sense organs, and may be 

 designated the sense cells of the ocular sensillum. Associated with 

 them are other cells derived from the hypodermis, but these have 

 so little in common witb the enveloping cells of the other sense or- 



