34 AN INTRODUCTION TO ENTOMOLOGY 



d. THE SEGMENTATION OF THE BODY 



The cuticular layer of the body-wall, being more or less rigid, 

 forms an external skeleton; but this skeleton is flexible along certain 

 transverse lines, thus admitting of the movements of the body, and 

 producing the jointed appearance characteristic of insects and of 

 other arthropods. 



An examination of a longitudinal section of the body-wall shows 

 that it is a continuous layer and that the apparent segmentation is due 

 to infoldings of it (Fig. 43), 



The body-seg- 

 ments, somites, or 

 metameres . — Each 



section of the body „. ^. , , . ,. . ^. , ^, 



r 1 Fi?- 43- — Diagram of a longitudinal section of the 



between two of the body-wall of an insect. 



infoldings described 



above is termed a body-segment, or somite, or metamere. 



The transverse conjunctivae. — The infolded portion of the body- 

 wall connecting two segments is termed a conjunctiva. These con- 

 junctivae may be distinguished from others described later as the 

 transverse conjunctivce. 



The conjunctivae are less densely chitinized than the other portions 

 of the cuticula; their flexibility is due to this fact, rather than to a 

 comparative thinness as has been commonly described. 



e. THE SEGMENTATION OF THE APPENDAGES 



The segmentation of the legs and of certain other appendages is 

 produced in the same way as that of the body. At each node of an 

 appendage there is an infolded, flexible portion of the wall of the 

 appendage, a conjunctiva, which renders possible the movements of 

 the appendage. 



/. THE DIVISIONS OF A BODY-SEGMENT 



In many larvae, the cuticula of a large part of the body-wall is of 

 the non-chitinized type ; in this case the wall of a segment may form 

 a ring which is not divided into parts. But in most nymphs, naiads, 

 and adult insects, there are several densely chitinized parts in the wall 

 of each segment; this enables us to separate it into well-defined 

 portions. 



The tergum, the pleura, and the sternum. — The larger divisions of 

 a segment that are commonly recognized are a dorsal division, the 



