HEXAPODA. 
59 
(Fig. 72). A study of the internal structure of this 
organ has shown that each of these hexagonal divi¬ 
sions is the outer end of a distinct eye (Fig. 73). Hence 
what at first appears to be a single eye is 
really an organ composed of hundreds of eyes; 
it is termed, therefore, a compound eye. Each 
of the small eyes of which a compound eye is 
composed is termed an ommatidium. The 
number of ommatidia of which a compound 
eye is composed varies greatly : there may 
be not more than fifty, as in certain ants, or 
there may be many thousand, as in a butter¬ 
fly or a dragon-fly. Compound eyes are not 
found in larvae, though they may possess a 
group of simple eyes on each side of the head. 
The Simple Eyes .—In addition to the com¬ 
pound eyes, many adult insects possess simple 
eyes. These are situated between the com¬ 
pound eyes. They vary in number from one 
to four; the most common number is three (see 
Fig. 71). The simple eyes are usually termed 
ocelli; sometimes, stemmata (stem'ma-ta). 
When the term ocelli is used in descriptive 
works, if there is nothing in the context to Fig n _ Three 
indicate the contrary, it is almost invariably 
of a May-bee tie. 
(After Grenach- 
er.) The pig¬ 
ment has been 
dissolved away 
from two of 
them. F, corneal 
facet: K, crystal¬ 
line cone; /, pig¬ 
ment-sheath ; F, 
chief pigment¬ 
cell ; p ", pig¬ 
ment-cells of the 
second order ; R, 
retinulas. 
applied to the simple eyes, and not to the ele¬ 
ments of the compound eyes. In the same 
way the term eye usually refers to the com¬ 
pound eyes, unless otherwise indicated by the 
context. 
The Antennce .—The antennae are a pair of 
jointed appendages inserted in the head in 
front of the eyes or between them. They 
vary in form. In some insects they are thread-like, consisting 
of a series of similar segments; in others certain segments 
are greatly modified in form. 
