COMPOUND EYES OF A h'TIfh'Ol'ODS. a21 



bcr of oimiKitidiii ; thus in 7'< //ijiff/onin ou\\ imc oininaliiliuiii 

 exists oil eiicli siilo of tlio lie;ui ; Orcluaclla lias six on uaiii side 

 of the head ; 7'omoceru8, IsojjoTna, liave seven ; Uajeeria, Lepi- 

 docyrius, SmyntAurus and Pajnrivs, eight. In tlie ants we 

 observe a siinihir gradation in the number of oininatidia. 



What reasons can we assign for this enormous multiplication 

 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 tliat can 

 account for the enormous multip'lication of the similarly formed 

 pits in the skin. The subject has been so fullj' discussed by 

 Lubliock that I need not enter into details here. "A(rcording to 

 his (Johannes Miiller's) view, those rays of light only wliicli 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 difl'erent 

 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 moi'e numerous are the facets, the more distinct will the 

 vision be. In fact, the picture perceived by the insects will be 

 mosaic, in wluch the number of points will correspond with the 

 number of facets."' Tlie 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 Lim.ulus m\<r\\i 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 Serolis or in some Trilobites ; a slight change again 

 might produce a protuberant ocular area mounted on an ophthal- 

 amic 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 difl'erent directions so as to command a still wider 

 field of vision. In this connection I may refer to a series of 

 diagrams (Figs. 74-81, PI. XXXV). The black heavy layer repre- 



' Lublx)ck: Senses, Instinct and Intelligence, p. 103. 



