486 THE^SENSOUIAL FUNCTIONS. . 



The compound eyes of insects are formed of a 

 vast number of separate cylinders or elongated 

 cones,* closely packed together on the surface 

 of a central bulb, which may be considered as 

 a part of the optic nerve; while their united 

 bases or outer extremities constitute the surface 

 of a hemispherical convexity, which often occu- 

 pies a considerable space on each side of the 

 head. The usual shape of each of these bases is 

 that of a hexagon, a form which admits of their 

 uniform arrangement with the greatest economy 

 of space, like the cells of a honey-comb; and 

 the hexagonal divisions of the surface are very 

 plainly discernible on viewing the surface of 

 these eyes with a microscope, especially as there 

 is a thin layer of black pigment intervening 

 between each, like mortar between the layers of 

 brick. The appearance they present in the 

 Melolontha, when highly magnified, is shown in 

 Fig. 422. t The internal structure of these eyes 

 will be best understood from the section of that 



* The number of these cones or cylinders which compose the 

 entire organ differs much in different species. In the ant, there 

 are only 50 ; in a Scarabceus, 3180 ; in the Bombyx mori, 6236 ; 

 in the house-fly (Musca domestica), 8000 ; in the Melolontha 

 vulgai'is, 8820 ; in the Phalena cossus, 11,300 ; in the Libellula, 

 12,544; in ihePapilio, 17,325; and in the Mordella, 25,088. 



+ In the PhalencB, and other tribes, they are arranged in 

 squares (as shown in Fig. 423), instead of hexagons, and fre- 

 qu^itly much less regularly; as must necessarily happen, in 

 many parts, from the curvature of the spherical surface. 



