PHYSIOLOGY AND BEHAVIOR OF THE ARTHROPODA 



337 



More recent work shows that in addition to receiving the Hght from 

 directly in front, each ommatidium transmits Hght less and less effec- 

 tively as the incident light arrives more and more obliquely. Eyes with 

 few ommatidia gather light from wide angles. All of the light trans- 

 mitted through the cornea and cone is brought to a point at the in- 

 ternal tip of the cone, where it enters the ends of the seven to fifteen 

 rhabdomes (Fig. 17.12) which form a single retinula. \\^hether each 

 retinula records a single light impression or whether its constituent 

 rhabdomes respond to different properties of the light, such as its ex- 

 ternal direction or its color, is at present an unsolved problem. 



' The curtains of pigment separating adjacent ommatidia vary from 

 arthropod to arthropod and in many species from day to night. Diurnal 

 species usually have a complete curtain formed by two sets of pigment 

 cells (Fig. 17.12) so that each retinula can receive light only from its 

 own lens system. In nocturnal species, however, the pigment is re- 

 stricted to the outer layers and the retinulae are separated some dis- 

 tance from the inner ends of the cones. In such eyes light from a distant 

 point can pass through several adjacent lenses to be superimposed on a 

 single underlying retinula. 



In both kinds of eyes the pigment may migrate according to the 

 light intensity. The pigment of the nocturnal eye (Fig. 17.12) spreads 

 inward under bright light, reducing the nimrber of facets that can 

 superimpose an image. In this way the total light reaching the light- 

 sensitive regions is reduced to avoid glare. 



The visual acuity of arthropods has been studied extensively. A 



-Corne-al l<znS 

 Cone lens 



Pigment cells 



DarK condition."^ 



Bright condition 



Rhabd-onze 



RctinuLa cells- 



"Nerve f^- 



Figure 17.12. A, Insect ommatidia, showing a diurnal type (left) and a nocturnal 

 type (center). In the nocturnal type, the pigment is shown in two positions, adapted for 

 very dark conditions on the left side, and for relatively bright conditions on the right. B, 

 Nocturnal type of eye adapted for dark conditions, showing how light can be concentrated 

 upon one rhabdome from several lenses. If the pigment moved downward, light from 

 peripheral lenses would be screened out. 



