THE SENSES OF INSECTS McINDOO. 463 



The compound eyes are not complex or specialized derivations of 

 the simple ones, but are of independent origin and of obviously dis- 

 tinct structural character. In structure they are wonderfully com- 

 plex and most delicately organized, being far more so than those of 

 the vertebrates or mollusca. Externally each compound eye presents 

 a number (from 7 to 27,000) of facets or microscopic polygonal cu- 

 ticular windows. These are the cornea of the eye. Behind each 

 facet or corneal lens there is a distinct and independent subcylin- 

 drical eye element which is composed of (1) a crystalline lens (want- 

 ing in many insects), (2) of pigment matter, and (3) of a special 

 nerve ending. According to Mtiller's mosaic theory of insect vision, 

 each of these eye elements perceives that bit of the external object 

 which is directly in front of it — that is, from which light is reflected 

 perpendicularly to its corneal lens. All of these microscopic images, 

 each of a small part of the external object, form a mosaic of the 

 whose object, and thus give the familiar name " mosaic vision" to the 

 particular kind of seeing accomplished by the compound eye. 



The character or degree of excellence of sight by the two kinds of 

 eyes obviously varies much. The fixed focus of the simple eyes is 

 extremery short, and probably their range of vision is restricted to 

 an inch or two in front of the insect's head. It is generally agreed 

 among entomologists that they avail little beyond distinguishing 

 between light and darkness. Relative to the compound ej'es the focus 

 is also fixed, but is longer, and the range of vision probably extends 

 to 2 or 3 yards. According to Midler's theory, which is supported 

 by several reliable observers and which is generally accepted by ento- 

 mologists, the larger and more convex the eyes the larger will be 

 the visual field, while the smaller and more abundant the facets 

 the sharper and more distinct will be the image. Although no change 

 in focus can be effected, as in the human eye, nevertheless Exner 

 claims that certain accommodation or flexibility of the seeing func- 

 tion is obtained by the movement of the pigment, which tends to 

 regulate the amount of light admitted into the eye, and by differences 

 in size and pigmental character of the eye element, which tends to 

 make part of the eye especially adapted for seeing objects in motion 

 or in poor light, and another part for seeing in bright light and for 

 making a sharper image. 



Both Plateau and Exner claim that while the compound eye is 

 inferior to the vertebrate eye for making out the forms of objects, 

 it is superior to the latter in distinguishing the smallest movements 

 of objects in the total field of vision. They believe that it is used 

 mainly for perceiving moving bodies. Mallock agrees with Plateau 

 in that insects do not see well ; at any rate, as regards their power of 

 defining distant objects and relative to insects having compound eyes 

 there is hardly any practical limit to the nearness of the objects 



