343 



as to form a hard and resistant surface, while in other places it 

 is flexible enough to permit a free movement of the body as a 

 whole or the functioning of its various parts. 



Increase in size, or growth, is provided for by shedding from 

 time to time, during the young or developmental period, this outer 

 covering. This process is called "molting" and these changes 

 of skin mark the early stages of development in insects. 



MOUTH PARTS. 



The anatomy of the head is of interest, for by the structure of 

 the mouth-parts one can determine the nature of the injury 

 wrought by plant-feeding species. The mouth-parts of some 

 insects of which those of the beetles and the grasshoppers ofifer 

 examples, are formed for biting off or gnawing into and masti- 

 cating the portions of the plant upon which the insect feeds. 

 That is, insects with biting mouth-parts actually chew, masticate 

 and swallow the portions of the plant upon which they feed. The 

 jaws of this type of insects move from side to side like swinging 

 doors, instead of up and down as is the case in higher animals. 



In such insects as leaf-hoppers, scale-insects and plant-lice, the 

 mouth-parts are formed for sucking. The food of plant-feeding 

 species, having a mouth after this fashion, is not portions of the 

 plant itself, but the sap or juice thereof. The portions of the 

 plant fed upon are left intact, but the result of the myriad of small 

 pumps sucking out the very life of the plant can be imagined. 



DIGESTION. 



It might be well to refer in this very general manner to the 

 organs of digestion of insects. Tliey consist of the alimentary 

 canal and its appendages. The alimentary canal is a tube run- 

 ning the length of the body in almost a direct line. This tube is 

 separated into definite parts and supplied with various structures. 

 The shape and size of the parts and the presence of certain sup- 

 plementary structures depends on the food of the insect, that is, 

 whether it is a chewing insect and takes into its system portions 

 of the plant to be digested or is a sucking insect and feeds simply 

 upon the sap or juice. A poison placed upon the foliage of a 

 plant is carried with the portion eaten by a chewing insect di- 

 rectly into the system. On the other hand, a sucking insect would 

 not be injured by placing a poison on the surface of its food since 



