22 FUNCTION OF VISION. 



have received the names of ocelli and stemmata, and are generally three in minder, and 

 rtmnged In the form of a triangle ; hut there may be bul two, or only one. Their structure 

 qualities them for viewing such objects only as are elose at hand. 



The compound eye presents a reticulated appearance under the microscope : the surface 

 is convex or globular; but the organ is immovable, except by the motion of the head. 

 The reticulated appearance is produced by the lines that mark the boundary of each eye 

 or lens : these are hexagonal, and their number is almost incredibly great in some species. 

 They van in this respect from 50 in the ant, to 25,000 in a species of Mordclla : the 

 butterfly has 17,000, and the dragonfly 12,511. Each eye is furnished with an apparatus 

 sufficiently perfect for the exercise of vision in its sphere : it has its lens for refraction, 

 its choroid for the correction of aberration, and its retina for the reception of the images 

 of external objects. Each single e}e, however, must embrace an extremely limited held of 

 vision, and there is no doubt that it requires the use of many of these eyes to see a single 

 object ; for only those rays of light that fall perpendicularly upon the eye can reach the 

 optic nerve. 



The eyesof predaceous insects, such as the dragonfly, are large, prominent and globular: 

 hence they enjoy, altogether, a large field of vision. In those insects, on the other hand, 

 which are confined in their range, or are parasitic, the field of view is diminished by a 

 reverse of circumstances. 



The nerve of each eye terminates in a common nerve : this must be regarded as the 

 sensorium commune, the nervous plane upon which the image of an object is spread. Some- 

 times the eye is pedunculated, or placed upon a footstalk : sometimes it is semicircular, 

 in consequence of the implantation of the antenna 1 , and indeed this implantation may be 

 9uch as to give the semblance of four eyes. Tn other instances the size of the eye is a sexual 

 mark. 



We are too much in the habit of looking vaguely upon the insect tribes. While we 

 recognize the movements of the vertebrated class as resulting from distinct acts of the will, 

 and as controlled by internal feelings, we are little disposed to entertain the view that the 

 apparatus of a fiy or a beetle indicates similar internal motives for action ; or, in other 

 words, we do not possess so lively a sense of the perfection of the being of the insect, as 

 of the being of the higher order. We see, however, that insects have eyes to see, cars to 

 hear, and organs of smell ; a highly developed nervous apparatus, and an active circula- 

 tion : in fine, the insect moves in a world of its own, which takes no part in the sphere 

 belonging to themolluscaor vertclmita. Its senses and organs of animal life, however, give 

 it a wide sphere of activity, and have prepared it for fulfilling important functions, and 

 furnished it with a capability to affect very materially the interests of man. Being widely 

 diffused, and their life overflowing with activity, always moving as if impelled forward by- 

 important business or engaged in errands of the most momentous character, they seem to 



