CHAPTER III. 

 THE SIX-FOOTED INSECTS. 



The Ilexapoda, or the true insects, inchule those forms 

 with three pairs of legs, and the group is further distin- 

 guished from the other tracheate forms by hxrge compound 

 eyes and for a considerable portion of the group the presence 

 of one or two pairs of wings. 



Of the various arthropods this division includes by far 

 the greater number of species and to a large extent those 

 forms which have the greatest agricultural importance. 



STRUCTURE OF INSECTS. 



There are some details in the structure of insects that 

 have a special importance in connection with the use of 

 remedies and some that from their frequent use in the 

 description of injurious species require explanation as a 

 basis for proper understanding of these principles. These 

 will be treated here as briefly as may be, bearing in mind 

 constantly this main issue in their presentation. 



The insect body is divided into three regions, head, thorax, 

 and abdomen. The first appears to be one solid segment, 

 though believed fundamentally to consist of six or seven 

 segments closely fused together. The thorax has three 

 segments usually pretty closely joined, while the abdomen 

 possesses from three to nine visible segments, and these 

 articulate so as to be free to move on each other. 



The head bears appendages, a number of definite struc- 

 tures connected with sensation or nutrition, and which are 

 of special service in separating the different groups of insects. 

 The antennse are jointed appendages usually situated on 

 the upper and front part of the head, composed of a varying 

 (38) 



