i42 



AN INTRODUCTION TO ENTOMOLOGY 



investigations confirm it. The essential features of it are the follow- 

 ing: only the rays of light that pass through the cornea and the 

 crystalline-cones reach the precipient portion of the eye, the others fall 

 on tlie pigment of the eye and are absorbed by it ; in each ommatidiimi 

 the cornea transmits to the crystalline -cone light from a very limited 

 field of vision, and when this light reaches the apex of the crystalline- 

 cone it forms a point of light, not an image; hence the image formed 

 upon the combined retinulae is a mosaic of points of light, which com- 

 bined make a single image, and this image is an erect one. 



Figure 156 will serve to illustrate the mosaic theory of vision. 

 In this figure are represented the corneas (c), the crystalline-cones 



(cc), and the rhabdoms (r.) of several adja- 

 cent ommatidia. It can be seen, from this 

 diagram, that each rhabdom receives a 

 point of light which comes from a limited 

 portion of the object viewed (O) ; and that 

 the image (I) received by the percipient 

 portion of the eye is a single erect image, 

 formed by points of light, each of which 

 corresponds in density and color to the 

 corresponding part of the object viewed. 

 The distinctness of vision of a com- 

 pound eye depends in part upon the num- 

 ber and size of the ommatidia. It can be 



Fig. 156.— Diagram iUustrat- readily seen that the image formed by 

 mg the theory of mosaic ° 



vision. many small ommatidia will represent the 



details of the object better than one formed 



by a smaller number of larger ommatidia ; the smaller the portion of 



the object viewed by each ommatidimn the more detailed "will be the 



image. 



The distinctness of the vision of a compound eye depends also on 

 the degree of isolation of the light received by each ommatidiiim, 

 which is determined by the amount and distribution of the pigment. 

 Two types of compound eyes, differing in the degree of isolation of the 

 light received by each ommatidium, are recognized; to one type has 

 been applied the term day-eyes, and to the other, night-eyes. 



Day-eyes.^ — The type of eyes known as day-eyes are so-called 

 because they are fitted for use in the day-time, when there is an 

 abundance of light. In these eyes the envelope of pigment sur- 

 rounding the transparent parts of each ommatidium is so complete 

 that only the light that has traversed the cornea and crystalline -cone 



