526 THE SENSES AND SENSORY ORGANS. 
5. THE COMPOUND EYE OF THE BLOW-FLY. 
a. The Structure of the Dioptron. 
The dioptron consists of the cornea, the basilar membrane, 
and the scleral ring, which form its case, and of the ommatea 
or radially arranged eyelets. Each ommateum corresponds to 
a corneal facet, and there are between 4,000 and 5,000 facets 
in each eye. 
The Cornea.—I have already shown elsewhere [283] that 
there are several very distinct forms of cornea in the Arthro- 
poda. These I named the continuous cornea, the faceted 
cornea, the kistoid cornea, and the lenticular cornea. 
The cornea of the Blow-fly is a kistoid cornea. That is, it 
consists of a thin cuticular lamina with concavo-convex facets, 
each facet strengthened and supported by a hexagonal frame 
which separates the adjacent lenses from each other. 
Thus the cuticular lamina of the cornea exhibits very 
shallow hexagonal alveoli with concave ends when viewed 
from its inner surface. These alveoli are filled up by bi-convex 
lenses. 
The substance of the lenses stains readily in mature pupz, 
is highly refractive during life and shortly after death, but soon 
loses its refringent power. In dried specimens, each lens is 
seen split into four segments, and the same result is brought 
about by treating a fresh cornea with ether. These segments 
drop out in sections and leave only the cuticular alveoli. 
In well-preserved specimens the lenses remain i situ and 
exhibit no apparent structure. In specimens preserved with 
osmium peroxide the lens is well seen, it is stained a pale 
brown, and is strongly bi-convex, without, so far as I can see, a 
trace of apparent structure. In pupe about eight days old the 
lens is represented by four nucleated cells. 
In the Cockroach (Blatta) and the Earwig (Fovficula) the 
cornea is kistoid, and I have frequently separated the lenses ; 
this I have been unable to accomplish in the Blow-fly. The 
lenses so separated may be ruptured by pressure, and then 
