ARTHROPODAL VISION. 5g 



the corneule. And in support of Patten, J. S. Kingsley is 

 quoted as follows : "I have, however, been able to trace the 

 axial nerve through the pedicel and style and into the distal 

 portion of the crystalline cone." Now no matter where 

 images may be formed, if optic nerve elements are encoun- 

 tered by the light rays at all depths, Plateau asserts that the 

 images will be as indistinct as images in the human eye 

 formed by cones of light rays with foci behind the retina. 

 According to this investigator, then, arthropods without ej^es 

 distinguish between light and darkness through dermatoptic 

 perception, and as this form of perception is apparently 

 suited to their needs, we have said of these animals that the}- 

 " have no e\-es whatever — yet they see." 



Arthropods having only simple eyes generally have very 

 poor sight, and make much use of the organs of touch to 

 determine the forms and characters of objects, and when the 

 arthropod has both simple and compound eyes, the simple eye 

 is almost useless. Finally, the arthropod with compound eyes 

 does not see forms distincth' but in moving about among 

 motionless bodies it utilizes tactile and olfactory impressions 

 in conjunction with those produced by light. 



Plateau believes, however, with other investigators, that 

 many of the insects have a much clearer perception of objects 

 in motion. " At varying distances," he says, " these animals 

 perceive the displacement of objects of a certain ?julk infin- 

 itely better than the objects themselves." 



Prof. Exner had studied this subject from time to time 

 during man}- years before his work entitled " Die Physiologie 

 der Facettirten Augen von Krebsen und Insecten " appeared 

 in 1891. We have quoted him as believing, at an earlier 

 period, that no images could be produced by the optical 

 system of the Lampyris, but in this work he presents a photo- 

 graph of exterior objects made with the Lampyris eye, accom- 

 panying a learned and lengthy exposition of the principles 

 involved in the production of the image and the precise action 

 of the optical apparatus of this particular animal. In Lain- 



