486 THE SENSORIAL FUNCTIONS. 



The corapound 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 

 Melolonthay when highly magnified, is shown in 

 Fig. 422.1 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 

 vulgaris, 8820 ; in the Phalena cossus, 1 1 ,300 ; in the Libellula, 

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



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

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

 quently much less regularly ; as must necessarily happen, in 

 many parts, from the curvature of the spherical surface. 



