110 
ENTOMOLOGY 
somewhat the same plan as the human eye, its capacity for 
forming images must be extremely limited; for since the form 
of the lens 1s fixed 
Fic. 140. 
An ocellar retinula of 
the honey bee, composed 
of two retinal cells. A, 
longitudinal section; B, 
transverse section; 7, 1, 
nerves; p, pigment; 7, 
rhabdom. — After  ReEptr- 
KORZEW. 
elongate elements, 
nally in a facet. 
and also the distance between the lens and 
the retina, there is no power of accommo- 
dation, and most external objects are out 
of focus; to make an image, then, the 
object must be at one definite distance 
from the lens, and as the lens is usually 
strongly convex, this distance must be 
small; in other words, insects, like spiders, 
are very near-sighted, so far as the ocelli 
the 
number of retinal rods implies an image of 
are concerned; furthermore, small 
only the coarsest kind. 
If the compound eyes of a grasshopper 
are covered with an opaque varnish and 
the insect is placed in a box with only a 
single opening, it readily finds its way out 
by means of its ocelli; if all three ocelli are 
also covered, however, it no longer does 
so, except by accident, though it can make 
its escape when only one of the ocelli is 
The ocelli, then, can dis- 
and they 
are probably more serviceable to the in- 
left uncovered. 
tinguish light from darkness 
sect in this way than in forming images. 
Compound Eyes.—<As regards deli- 
cacy and intricacy of structure, the com- 
pound eye of an insect is scarcely if at all 
inferior to the eye of a vertebrate. In 
radial section (Fig. 141), a compound eye 
appears as an aggregation of similar 
or ommatidia, each of which ends exter- 
The following structures compose, or are 
concerned with, each ommatidium: (1) cornea, (2) crystal- 
line lens, or cone, (3) rhabdom and retinula, (4) pigment (aris 
