ARTHROPODAL VISION. 63 



Nerve fibres pass through the fenestrate membrane and terminate in a 

 delicate sheath which encloses each rlrabdome, the sheath, together with 

 the nerves that end in it constituting the retinula." 



And again on page 514, the function of the sheath cells of 

 the rhabdoine is indicated : — 



" Beneath each facet of the cornea is an ommatidium optically sep- 

 arated from its neighlxjrs by black pigment and consisting of an outer 

 segment or vitreous Ixxly and an inner segment or retinula formed of 

 sensory cells enclosing a rhabdome." 



Still another description of the onimatidia includes the 

 following : — 



"The inner portion is a group of sensory cells constituting a reti- 

 nula and enclosing a refracti\e rod, the rhabdome. The retinula is the 

 .actual percipient part of the ommatidium, its cells being comparable to 

 our own rods and cones." 



Apparently these authors do not attach much importance 

 to the question of the formation of images ; at least they 

 have ignored it and if we accept their dicl:um as to the func- 

 tion of the cells of the sheath enclosing the rhabdome we at 

 once realize the difficulty of assigning a satisfactory place for 

 an image or even a mosaic part of an image, especially where 

 we have the rhabdome itself, " a refractive rod," to deal with. 

 Turning to American text-books we quote briefly from Dr. 

 Weysse '^ on the arthropod eye : — 



11 * * * * j,^ addition there is a pair of compound eyes which 

 consist of a large number of parts, each part a structurallv complete 

 single eye." 



We might infer from this that even if an image is essen- 

 tial to distinct vision the insect would have a satisfactory pic- 

 ture of sufficiently illuminated objects within a certain range, 

 if furnished with a single ommatidium, and the reader will 



*A S^-noptic Text-Book of ZcKJlogy by Arthur Wisswald Weysse, 

 A. M., Ph. D. (Harvardj, 1904. 



