516 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 1, in Ponera from 1 to 5, and 
in Scolenopsis fugax from 6 to 9. Myrmecina Latreilli has 15; 
Pheidole pallidula, 30; Atta barbara from 80 to go 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- 
DESCRIPTION OF PLATE XXXV. 
Fic. 1,—A frontal section of the compound eye of the Blow-fly seen with a 4 inch 
objective. dd, the dioptron ; g, the optic ganglion ; 0, the optic nerve; r/, the 
retina; ¢ ¢v, tracheal vessels in section. 
Fic. 2,—Two of the ocelli drawn to the same scale. 
