THE INTERNAL ANATOMY OF INSECTS 141 



Three types of compound eyes are recognized: first, the eucone 

 eyes, in these each ommatidium contains a true crystalline-cone, as 

 described above, and the nuclei of the cone-cells are in front of the 

 cone; second, the pseudocone eyes, in these the four cone-cells are 

 filled with a transparent fluid medium, and the nuclei of these cells are 

 behind the refracting body; and third, the acone eyes, in which 

 although the four cone -cells are present they do not form a cone, either 

 solid or liquid. 



d. THE PHYSIOLOGY OF COMPOUND EYES 



The compound eyes of insects and of Crustacea are the most com- 

 plicated organs of vision known to us. It is not strange therefore, that 

 the manner in which they function has been the subject of much dis- 

 cussion. It is now, however, comparatively well-understood; 

 although much remains to be determined. 



In studying the physiology of compound eyes, three sets of struc- 

 tures, found in each ommatidium, are to be considered: first, the 

 dioptric apparatus, consisting of the cornea and the crystalline-cone; 

 second, the percipient portion, the retinula, and especially the rhab- 

 dom; and third, the envelope of pigment, which is found in three 

 sets of cells, the iris-pigment-cells, the retinular cells, and the accessory 

 or secondary pigment -cells. 



The dioptrics of compound eyes is an exceedingly complicated 

 subject ; a discussion of it would require too much space to be intro- 

 duced here. It has been quite fully treated by Exner ('91). to whose 

 work those especially interested in this subject are referred. The 

 important point for our present discussion is that by means of the 

 cornea and the crystalline -cone, light entering the cornea from within 

 the limits of a certain angle passes through the cornea and the crystal- 

 line-cone to the rhabdom, which is formed of the combined rhab- 

 domeres of the nerve-end-cells, constituting the retinula, the precipient 

 portion of the ommatidium. 



The theory of mosaic vision. The first two questions suggested by 

 a study of physiology of compound eyes have reference to the nature 

 of the vision of such an eye. What kind of an image is thrown upon 

 the retinula of each ommatidium? And how are these images com- 

 bined to form the image perceived by the insect? Does an insect 

 with a thousand ommatidia perceive a thousand images of the object 

 viewed or only one? 



The theory of mosaic vision gives the answers to these questions. 

 This theory was proposed by J. Miiller in 1826; and the most recent 



