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 p 

 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 

 indicate the contrary, it is almost invariably 

 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 Antenna. — 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. 



Fig. 73. — Three 

 ommatidia from 

 the compound eye 

 of a May-beetle. 

 (After Grenach- 

 er. ) The pig- 

 ment has been 

 dissolved away 

 from two of 

 them. F, corneal 

 facet ; K, crystal- 

 line cone ; /, pig- 

 ment-sheath ; P, 

 chief pigment- 

 cell ; p" , pig- 

 ment-cells of the 

 second order ; R, 

 retinulae. 



