CHAPTER IV. 



THE EXTERNAL ANATOMY OF A BEETLE. 

 (Pterostichits calif ornicus. ) 



With the knowledge of the external anatomy of the 

 locust as a basis we may attempt to examine comparatively 

 some of the various conditions of the body exhibited among 

 different orders of insects. With the varied habits of in- 

 sects there are necessarily correlated various modifications 

 of structure, internal and external. The modifications of 

 the external structure are those taken special cognizance of 

 and used in the present analytical tables and keys for insect 

 classification, and must be studied to some degree before de- 

 termination of insect forms can be done intelligently. The 

 study of insect anatomy in a comparative way will also give 

 the student some understanding of the significance of 

 homology and specialization. 



The peculiarly flattened form of many insects, by which 

 the lateral aspects of thorax and abdomen are reduced to a 

 mere ridge or margin, is accompanied by a change in the 

 position of many of the body sclerites, in particular the 

 pleural sclerites of the thorax. This condition is well ex- 

 emplified among the predaceous ground-beetles (Carabidae) 

 and almost any species may be selected for illustration. We 

 have chosen the species Pterostichus calif ornicus , as the 

 representative of a widely spread genus, and the description 

 following applies especially to this form ; however, the notes 

 will serve as a guide for the examination of any member of 

 the genus. 



The meso- and metathoracic segments are closely joined 



