VISION. 343 



ing disposed in seven rows, the number in each regularly 

 diminishing from the base to the apex; an arrangement 

 which is shown in Fig. 421.* 



The compound eyes of insects arc formed of a vast num- 

 ber of separate cylinders or elongated cones,t closely packed 

 together on the surface of a central bulb, which may be con- 

 sidered as a part of the optic nerve; while their united bases 

 or outer extremities constitute the surface of a hemispherical 

 convexity, which often occupies 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 uni- 

 form 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 sur- 

 face 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.% The internal structure of these eyes will be 

 best understood from the section of that of the Lihelhda 

 vulgata, or gray Dragon-fly, shown in Fig. 424, aided by 

 the highly magnified views of smaller portions given in the 

 succeeding figures, in all of which the same letters of refe- 

 rence are used to indicate the same objects.^ The whole 

 outer layer (c c) of the compound eye may be considered 



• Kirby and Spence's Introduction, Sec, ill. 494. 



f The number of these cones or cylinders which compose the entire org^n 

 differs much in different species. In the ant, there are only 50; in a Scara- 

 baeus, 3180; in the Bomhyxmori^ 6236; in the house-fly (Musca domestica,) 

 8000; in the Melolontha vulgaris, 8820; in the Phalena cossus, 11,300; in the 

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



^ In the Fhaknae, and other tribes, they are arranged in squares (as shown 

 in Fig-. 423,) instead of hexagons, and frequently much less regularly; as 

 must necessarily happen, in many parts, from the curvature of the spherical 

 surface. 



§ These figures, as well as the account of the anatomy of the eye of the 

 Libellula, are taken from the memoir of Duges, in the Annales des Science* 

 Naturelles, xx. 341. 



