5i6 THE SENSES AND SENSORY ORGANS. 



The Number of Corneal Facets is therefore equal to that of the 

 ommatea, or radial segments of the eye. It varies greatly in 

 different insects. 



In the Dragon-flies there are from 25,000 to 30.000 facets, 

 while in Eciton there is only i, in Ponera from i to 5, and 

 in Scolenopsis fugax from 6 to 9. Myrmecina Latreilli has 15 ; 

 Pheidole pallidula, 30 ; Atta barbara from 80 to 90 in the 

 small workers, and 230 in the large ; Camponotus ligniperdus, 

 500; and Formica from 600 to 1,200. Most of these numbers 

 are quoted from Forel [194]. In Typhlopone eyes are absent. 



In many insects with few facets and in the Myriopoda the 

 separate ommatea are distinct from each other. It was 

 formerly believed that the compound eye took its origin by the 

 multiplication and union of separate simple eyes : but the 

 view held by Grenacher appears to be more correct. He says : 

 ' The stemma, to use a simile, is not the mother of the com- 

 pound eye, but the sister ' ; and he further says : ' The multi- 

 plication of the individual elements of the primitive eye has 

 led to the formation of the stemma ; the multiplication in the 

 number of simple eyes, their nearer aggregation, and the 

 development of fewer elements in each, has led to the forma- 

 tion of the compound eye'- a view which appears probable. 



The Ommateum. Each ommateum consists of a corneal facet, 

 a cone or pseudo-cone, and a great rod. 



The Cone when present is a highly-refractive body, usually 

 termed the sclero-cone, or crystalline cone, with its base 

 towards the corneal facet, and its apex towards the great rod. 

 In the Diptera this structure is wanting, and its place is taken 

 by a conical cavity, filled with a semi-solid body, resembling a 

 vitreous, which has been termed a pseudo-cone. 



The Great Rods. Each great rod consists of an axial struc- 



DKM KiniON OK I'lAll. XXX V. 



L. i. A frontal section of the compmiml eye of the Blow-fly seen with a \ 

 objective, dJ, the dioptron ; g, the optic ganglion : on, the optic nerve ; rt, the 

 retina; /;/;-, trachea! vessels in section. 

 KM.. 2. Two of the ocelli drawn to the same si-ale. 



