STRUCTURE OF COMPOUND EYES. 



333 



Fig. 166. 



such, in fact, they are) varies in different genera : thus, in the Ant 

 (Formica) there are 50 ; in the common House-fly (Musca domesticd), 

 4000 ; in some Dragon-flies (Libellula), upwards of 12,000. In Butter- 

 flies (Papilio) 17,355 have been counted ; and some Coleoptera (Mor- 

 della) possess the astonishing number of 25,088 distinct corneae. 



(872.) But in order to appreciate the wonderful organization of these 

 remarkable organs of sight, it is necessary to examine their internal 

 structure : every cornea is then found to belong to a distinct eye, pro- 

 vided with a perfect nervous apparatus, and exhibiting its peculiar lens, 

 iris, and pupil ; thus being completely entitled to be considered a distinct 

 instrument of vision. 



(873.) By attentively examining the annexed figure, representing a 

 section of the eye of the Cockchafer (Melolonfhd), as displayed by Straus- 

 Durckheim, the whole structure of the organ will be readily understood. 

 The optic nerve (fig. 166, a}, derived immediately from the supra- 

 cesophageal mass of nervous 

 matter, swells soon after its 

 origin into a rounded gan- 

 glion nearly half as large as 

 the brain itself. From the 

 periphery of the ganglion so 

 formed arise a considerable 

 number of secondary nerves 

 (6), which are very short, and 

 soon come in contact with a 

 layer of pigment (d), that in 

 the Cockchafer is of a bril- 

 liant red colour, and is placed 

 concentrically with the con- 

 vex outer surface of the eye. 

 Behind this membrane (called 

 by Straus - Durckheim the 

 common choroid), the second- 

 ary optic nerves (b) unite to 

 form a membranous expan- 

 sion of nervous matter (c), 

 which may be denominated 

 the general retina. From the 

 nervous expansion so formed 

 arise the proper optic nerves (e), appropriated to the individual eyes, or 

 ocelli, as we shall term them. These nervous filaments are as numerous 

 as the facets of the cornea, and traverse the common choroid to radiate 

 towards the individual eyes whereunto they are respectively destined, and 

 the structure of which we must now proceed to examine. In fig. 166, B, 

 a portion of the circumference of the compound eye is represented upon 



II I 



Structure of the eye of a Cockchafer. A, sectional 

 view : a, optic ganglion ; 6, secondary nerves ; c, ge- 

 neral retina, in front of which is a layer of pigment, d ; 

 e, proper optic nerves, supplying the individual facets 

 of the compound eye. B, a group of ocelli, much 

 magnified : /, bulb of optic nerve ; g, layer of pigment ; 

 h, vitreous humour ; i, cornea. 



