MORPHOLOGY OF COMPOUND EYES OF ARTHROPODS. 153 



have seven; Degeeria, Lepidocyrtus, Smynthurus^ and 

 PapiriuSj eight. In the ants we observe a similar gradation 

 in the number of ommatidia. 



What reasons can we assign for this enormous multiplica- 

 tion of similarly constructed parts? What advantage follows 

 from this arrangement? If the view of the nature of the 

 compound eye which is put forward in the preceding pages be 

 a true one^ Miiller's celebrated theory of mosaic vision is the 

 only one that can account for the enormous multiplication of 

 the similarly formed pits in the skin. The subject has been so 

 fully discussed by Lubbock that I need not enter into details 

 here. " According to his (Johannes Miiller's) view, those rays 

 of light only which pass directly through the crystalline cones, 

 or are reflected from their sides, reach the corresponding 

 nerve-fibre. The others fall on and are absorbed by the 

 pigment which separates the different facets. Hence each 

 cone receives light only from a very small portion of the field 

 of vision, and the rays so received are collected into one spot 

 of light. The larger and more convex, therefore, is 

 the eye, the wider will be its field of vision; while 

 the smaller and more numerous are the facets, the 

 more distinct will the vision be. In fact, the picture 

 perceived by the insects will be mosaic, in which the number 

 of points will correspond with the number of facets." ^ The 

 whole explanation of the problem seems to me to be contained 

 in the passage above cited ; and no further comment will be 

 necessary more than a statement that the increase in the 

 number of ommatidia is a decided advantage to their possessor. 

 An eye like that of Limulus might by a slight change be 

 converted into one of a more protuberant nature so as to 

 command a wider field of vision, as we see in some species of 

 Scrolls or in some Trilobites; a slight change again might 

 produce a protuberant ocular area mounted on an ophthalamic 

 stalk, and accompanied by the accessory apparatus of vision, 

 such as the socket for protection or the set of muscles to 

 move the eye-stalk in diff"erent directions so as to command a 

 Lubbock, ' Senses, Instinct, and Intelligence,' p. 163. 



VOL. XXXI, PART II. NEW SER. L 



