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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; m, nucleus; nv, nerve 
fibrille; pc, pseudocone; pg, 
g*, cells containing iris pig- 
ment; pg*, cell containing ret- 
inal pigment; 7, one of the six 
ENTOMOLOGY 
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; ¢, trachea; tv, 
tracheal vesicle-—After HrcKson. . 
