INSECTA. 



277 



A 



The compound eyes of insects are two in number, situated on 

 the lateral aspects of the head, the form of each being more or less 

 hemispherical. When examined with a microscope, their surface 

 is seen to be divided into a multitude of hexagonal facets, between 

 which, minute hairs are generally conspicuous. The number of 

 facets or cornese, for such in fact they are, varies in different 

 genera : thus, in the ant (Formica) there are 50 ; in the common 

 house-fly (Musca domestica), 4000 ; in some dragon-flies (Libel- 

 Ma), upwards of 12,000. In butterflies (Papilio) 17,355 have 

 been counted, and some Coleoptera (Mordella) possess the as- 

 tonishing number of 25,088 distinct cornese. 



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, 

 provided with a perfect nervous apparatus, and exhibiting its pe- 

 culiar lens, iris, and pupil ; thus being completely entitled to be 

 considered a distinct instrument of vision. 



By attentively examining & 123 



the annexed figure, repre- 

 senting a section of the eye 

 of the cockchafer (Melolon- 

 tha), as displayed by Strauss 

 Durckheim, the whole struc- 

 ture of the organ will be 

 readily understood. The 

 optic nerve (Jig. 123, a), 

 derived immediately from 

 the supra-cesophageal mass 

 of nervous matter, swells 

 soon after . its origin into a 

 rounded ganglion, nearly 

 half as large as the brain it- 

 self. 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 brilliant red colour, and is placed 

 concentrically with the convex outer surface of the eye. Behind 

 this membrane, called by Strauss the common choroid, the second- 

 ary optic nerves (b) unite to form a membranous expansion of 

 nervous matter (c) which may be denominated the general retina. 



