Ill STRUCTURE 89 



Maximum. Minimum. 

 Head ... 7 3 



Thorax ... 6 3 



Abdomen ... 11 5 



Total .24 11 



Although it is not probable that ultimately so great a difference 

 as these figures indicate will be found to prevail, it is certainly 

 at present premature to say that all Insects are made up of the 

 same number of primary segments. 



A brief account of the structure of the integument will be 

 found in the chapter dealing with the post-embryonic develop- 

 ment. 



The three great regions of the Insect body are functionally as 

 well as anatomically distinct. The head bears the most important 

 of the sense organs, viz. the antennae and ocular organs ; it includes 

 the greater of the nerve-centres, and carries the mouth as well 

 as the appendages, the trophi, connected therewith. The thorax 

 is chiefly devoted to the organs of locomotion, bearing externally 

 the wings and legs, and including considerable masses of muscles, 

 as well as the nerve centres by which they are innervated ; through 

 the thorax there pass, however, in the longitudinal direction, 

 those structures by which the unity of the organisation is com- 

 pleted, viz. the alimentary canal, the dorsal vessel or " heart " for 

 distributing the nutritive fluid, and also the nerve cords. The 

 abdomen includes the greater part of the organs for carrying on 

 the life of the individual and of the species ; it also frequently 

 bears externally, at or near its termination, appendages that are 

 doubtless usually organs of sense of a tactile nature. 



In the lower forms of Insect life there is little or no actual in- 

 ternal triple division of the body ; but in the higher forms such 

 separation becomes wonderfully complete, so that the head may 

 communicate with the thorax only by a narrow isthmus, and the 

 thorax with the abdomen only by a very slender link. This 

 arrangement is carried to its greatest extreme in the Hymenoptera 

 Aculeata. It may be looked on as possibly a means for separating 

 the nutrition of the parts included in the three great body 

 divisions. 



Along each side of the body extends a series of orifices for the 

 admission of air, the stigmata or spiracles ; there are none of 

 these on the head, but on each side of most of the other segments 



