INSECT STRUCTURES 9 



body wall is not only the covering of the insect but also 

 the skeleton, there being no hardened framework inside 

 the body as is the case with the higher animals. An ex- 

 ternal skeleton like this is an exoskeleton. From this exo- 

 skeleton there are inward projections which serve as at- 

 tachments for the muscles and other internal organs. The 

 possession of the exoskeleton enables the insects to keep 

 their form after death and renders preservatives unneces- 

 sary for any but the softest bodied species. 



10. Internal Organs. Internally insects differ widely 

 from the higher animals. Yet they have organs and sys- 

 tems of organs constructed for the performance of similar 

 functions. The names applied to these organs are, as a 

 rule, the same as for the organs in the higher animals 

 having similar uses. 



The systems of internal organs in an insect as well as 

 in other animals, are grouped according to function. The 

 more important of these groups are: Digestive, Circula- 

 tory, Respiratory, Nervous, Reproductive and Muscular. 

 The Excretory system is distinguished by some writers. 

 In addition, there are the fat-bodies, certain connective 

 tissues and special organs. 



11. The Digestive Tract. Young insects, and many 

 adults, have the interior of the body almost completely 

 taken up by the digestive tract. This is, primarily, a tube 

 running the length of the body. As the tube is often 

 longer than the body it is found to be more or less con- 

 voluted. Different parts of this tube are modified and 

 are given names according to their uses. The anterior 

 end forms the mouth and the back part of the mouth cav- 

 ity is the pharynx. Mouth and pharynx are for reception 

 of food and serve the same purposes as in higher animals. 

 Salivary glands are found in insects. These are usually 



