I 12 



ENTOMOLOGY 



C 



pc 



tv 



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 

 fibrillse; pc, pseudocone; pg 1 , 

 pg z , cells containing iris pig- 

 ment; pg z , 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. 



