434 THE SENSORIAL FUNCTIONS. 



humour (v), covered behind by the retina (r) which 

 is closely applied to it ; and the optic nerve (o), 

 with which the retina is continuous. 



Examples of the conglomerate eye occur in the 

 Myriapoda ; in the Scolopendra, for instance, they 

 consist of about twenty contiguous circular pellucid 

 lenses, arranged in five lines, with one larger eye 

 behind the rest, which Kirby compares to a sentinel, 

 or scout, placed at some little distance from the 

 main body. In the Julus ten-estris, or common 

 Millepede, these eyes, amounting to 28, form a 

 triangle, being 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 eyes which occupy the four lower rows are de- 

 veloped in the progress of growth ; for at an early 

 period, when the body consists of but a few seg- 

 ments, the eyes are only six in number, disposed 

 in a triangle in three rows, thus .v.'j" 



The compound eyes of insects are formed of a 

 vast number of separate cylinders or elongated 

 cones, J 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 occupies a considerable 

 space on each side of the head. The usual shape 



* Kirby and Spence's Introduction, &c., iii. 494. 



f Gervais, Ann. Sc. Nat. serie 2, vii. 56. 



X 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 Bomhyx mori, 6236; in the 

 house-fly (Mzisca 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. 



