206 INTRODUCTION TO ZOOLOGY chap. 



and in having no power of flight. A very large majority of 

 insects, when adult, possess wings and can fly. Fig. 137 

 shows diagrammatically the general structure of the body of 

 an adult insect. 



In all the higher insects, the division of the body 

 the^Bodv" ^"^^^ three regions — head, thorax, and abdomen — is 

 very distinct. 



The liead is relatively small, but is very firm and compact. 

 It bears (1) a single pair of jointed antennae which are the 

 special tactile sense organs ; (2) three pairs of modified 

 appendages round the moutli, the so-called " mouth-parts " ; 

 (3) a pair of large compound eyes (see p. 208). Assuming 

 that each pair of appendages corresponds to one segment of 

 the primitive form, the head is formed of at least four fused 

 segments, and many zoologists think that more than these 

 have gone to its construction; however, ho other signs of 

 segmentation in it are visible externally. It is separated 

 from the thorax by a narrow neck, which is largely mem- 

 branous, and can be extended, or folded so that the back of 

 the head is overlapped by the first thoracic segment. The 

 head has, therefore, great freedom of motion. 



The ihcfrax consists typically of three segments, the divisions 

 between which can often be easily seen. It bears ventrally 

 three pairs of jointed appendages, the legs, and dorsally, 

 attached to its two hinder segments, are usually two pairs 

 of membranous wings, though one pair may be absent, as in 

 flies, or the first pair may be modified into hard sheaths, as 

 in beetles. To this thoracic region, therefore, are attached 

 all the organs of locomotion, and it must necessarily be 

 broad and strong, to allow for the presence and attachment 

 of the strong muscles which move the wings and limbs. 



The abdomen is more distinctly segmented than the thorax, 

 and is not so compact and hard. It bears no appendages 

 except at the tip, where occasionally a pair of jointed feeler- 

 like processes is present (see Fig. 232). In the case of some 

 female insects, there may be at the end of the abdomen a 

 sharply pointed tube, the "ovipositor," which is used in 

 depositing the eggs in the place where they are to be hatched, 

 often within the tissues of a plant. Sometimes, again, there 

 may be paired stinging or piercing organs present just within 

 the end of the body. The number of segments in the 



