HEX A POD A. 



59 



7-P 



(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 F]G 

 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. 



Three 

 ommatidia from 

 the compound eye 

 of a May-beelle. 

 (After Grenach- 

 er.) The pig- 

 ment has been 

 dissolved away 

 from two of 

 them, f, corneal 

 facet; A', crystal- 

 line cone ; /, pig- 

 ment-sheath ; /', 

 chief pigment- 

 cell ; /", pig- 

 ment-cells of the 

 second order ; R, 

 retinulas. 



