HEXAPODA 



29 



pages 128 to 130. These pages should be carefully studied before 

 attempting to use the table that follows them. 



The segments of the body in a fully developed insect are grouped into 

 three regions: head, thorax, and abdomen (Fig. 58). In the larval state 

 this grouping of the segments is not well shown. 



The Head and its Appendages 



The head is the first of the three regions of the body formed of several 

 body-segments grown together. 



The head bears the compound eyes, the simple eyes or ocelli, the an- 

 tennae, and the mouth-parts. 



The compound eyes. — On each side of the head of an adult insect is 



an organ, which is 

 recognized at once as 

 an eye. But when 

 one of these eyes is 

 examined with a mi- 

 croscope it is found 

 to present an appear- 

 ance very different 

 from that of the eye 

 of higher animals; its surface is divided 

 into a large number of six-sided divisions 

 (Fig. 59). A study of the internal struc- 

 ture of this organ has shown that each of 

 these hexagonal divisions is the outer 

 ^^npvC- end of a distinct portion or element of 



-| / II \ ^ the eye (Fig. 60). Hence what at first 



"X^vJLy^" a PP ears to De a single eye is really an 

 1§Nf€* organ cc 



*/. 



Fig. 59. — Part of surface 

 of compound eye, greatly 

 enlarged. 





r-jpv 



omposed of hundreds of distinct 

 structures; it is termed, therefore, a com- 

 pound eye. Each of the small elements 

 of which a compound eye is composed is 

 termed an ommatidium (plural ommati- 

 dia). 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 butterfly or a 

 dragonfly. 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 

 compound eyes, many adult insects pos- 

 sess simple eyes. These are situated be- 

 tween the compound eyes. They vary in 

 number from two to three; the most 



common number is three (see Fig. 58). The simple eyes are termed ocelli. 

 Nymphs also possess ocelli but in the case of most larvae there are 



simple eyes which are different in origin from the ocelli of adult insects. 

 The antennae. — The antennas are a pair of jointed appendages artic- 



FlG. 60. — An ommatidium of Machilis 

 c, cornea; ky, corneal hypodermis; cc, crystal- 

 line-cone-cells; :', iris-pigment cells; r, retin- 

 ula cells; rh, rhabdom; b, basement mem- 

 brane; ap, accessory pigment cells. 



