112 



ENTOMOLOGY 



FIG. 142. 



pc- 



tv 



nv 



Structure of an ommatid- 

 ium of Calliphora vomitoria. 



A, radial section (chiefly) ; 



B, transverse section through 

 middle region; C, transverse 

 section through basal region; 

 bm, basement membrane; c, 

 cornea; n, nucleus; nv, nerve 

 fibrillae; pc, pseudocone; pg 1 , 

 pg 2 , cells containing iris pig- 

 ment; pg 3 , cell containing ret- 

 inal pigment; r, one of the six 



Each ommatidium is adapted to trans- 

 mit light along its axis only (Fig. 

 143), as oblique rays are lost by ab- 

 sorption in the black pigment which 

 surrounds the crystalline cone and the 

 axial rhabdom. Along the rhabdom, 

 then, light can reach and affect the 

 terminations of the optic nerve. . Each 

 ommatidium does not itself form a 

 picture ; it simply preserves the inten- 

 sity and color of the light from one 

 particular portion of the field of 

 vision ; and when this is done by hun- 

 dreds or thousands of contiguous om- 

 matidia, an image results. All that 

 the painter does, who copies an object, 

 is to put together patches of light in 

 the same relations of quality and posi- 

 tion that he finds in the object itself 

 and this is essentially what the com- 

 pound eye does, so far as can be in- 

 ferred from its structure. 



Exner, removing the cones with the 

 corneal cuticula (in Lampyris) , looked 

 through them from behind with the 

 aid of a microscope and found that the 

 images made by the separate omma- 

 tidia were either very close together 

 or else overlapped one another, and 

 that in the latter case the details corre- 

 sponded; in other words, as many 

 as twenty or thirty ommatidia may co- 

 operate to form an image of the same 

 portion of the field of vision; this 



retinal cells which compose the retinula; rh, rhab- 

 dom, composed of six rhabdomeres; t, trachea; tv, 

 tracheal vesicle. After HICKSON. 



