BRANCH ARTHROPODA; CLASS INSECT A : THE INSECTS 1^3 



The body-wall is usually firm and rigid, with thinner 

 flexible places between the segments and body-parts for 

 the sake of motion. The body-wall is composed of a 

 cellular skin or hypoderm, and an outer non-cellular 

 cuticle in which is deposited a horny substance called 

 chitin. This chitinous cuticle or exoskeleton serves as 

 an armor or protective covering for the soft body within, 

 and also as a point of attachment for the many muscles 

 of the body. 



Insects vary a great deal in regard to shape and ap- 

 pearance of the body, and certain of the external organs 

 are greatly modified in different insects to adapt them to 

 the varied conditions under which they live. Especially 

 interesting and important are the variations in the char- 

 acter of the mouth-parts and wings, the organs of food- 

 getting and locomotion. In our consideration later of 

 some of the more important groups of insects the modifica- 

 tion of these parts will be specially referred to. Despite 

 the great number ol insects, however, and their varied 

 habits of life, a strong uniformity of body-structure is 

 noticeable, all of them holding pretty closely to the 

 typical body-plan. 



The most interesting feature of the internal anatomy of 

 the insect body is the respiratory system. Insects breathe 

 through tiny paiied openings, called spiracles, in the sides 

 of the abdominal (and sometimes the thoracic) segments 

 (the number and disposition of the pairs of spiracles varying 

 much in different insects). These spiracles are the external 

 openings of an elaborate system of air-tubes or tracheae 

 (fig. 47) which ramify throughout the whole body and carry 

 air to all the organs and tissues. The blood has apparently 



(nothing to do with respiration as it has in the' vertebrate 

 animals, where it carries oxygen to all the body tissues. 



The other systems of organs are well developed and in 

 many respects more complex and elaborate than those of 



