CHAPTER V. 



THE EXTERNAL ANATOMY OF A BEETLE. 

 Pterostichus calif ornicus. 



With the knowledge of the external anatomy of the locust 

 as a basis the student may successfully attempt to examine 

 comparatively some of the various conditions of the body 

 exhibited among different orders of insects. With the varied 

 habits of insects there are necessarily correlated various 

 modifications of structure, internal and external. The modi- 

 fications 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 determination of insect forms can be done in- 

 telligently. The study of insect anatomy in a comparative 

 way will also give the student an understanding of the sig- 

 nificance of homology and specialization. 



As an example of simple work of this character which 

 may be undertaken by the student, the external anatomy of 

 a beetle (order Coleoptera) may be studied. 



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 {Caralndce) 

 and almost any species may be selected for illustration. We 

 have chosen the species Pterostichus calif ornicus, as the repre- 

 sentative of a widely spread genus, and the description fol- 



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