8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



a fold, or posterior reduplication (Rd), which overlaps the anterior 

 part of the segment following. 



The segmental appendages, which in their origin are simple out- 

 growths of the body wall, have their bases in the pleural membranes, 

 one on each side of the segment. In the Chilopoda and the Insecta, the 

 pleural areas also contain skeletal plates, the pleurites, but most of 

 these plates are probably derived from a basal part of the appendage. 

 In pterygote insects the pleurites constitute a highly organized pleuron. 



Entomologists have usually described the various small chitiniza- 

 tions that occur between the principal segmental plates of insects as 

 " intersegmental " sclerites. The only true intersegmental elements, 

 however, are the antecostse or parts derived from them, such as the 

 phragmata of the dorsum and the intersegmental muscle processes 

 of the venter. Most other so-called " intersegmental " chitinizations 

 belong either to the anterior or the posterior parts of the true seg- 

 mental areas. Examples of sclerites of this sort probably are the 

 neck plates, or cervical sclerites, though their exact morphological 

 status has not yet been determined. Since the anterior ends of the 

 principal dorsal and ventral muscles of the prothorax are attached 

 anteriorly on the back of the head, it would appear, at first thought, 

 that the neck-plates belong to the anterior part of the prothorax. But 

 the post-occipital ridge of the head, and the tentorium, to which the 

 prothoracic muscles are attached, are formed by invaginations between 

 the maxillary and labial segments. It follows, then, that an inter- 

 segmental line has been lost somewhere between the anterior margin 

 of the prothorax and the posterior margin of the labial segment ; per- 

 haps its position is indicated by the ends of certain muscles attached 

 on the neck membrane in some insects. It is possible, therefore, that 

 the cervical sclerites are derived from both segments. The two lateral 

 plates constitute an important part of the mechanism for moving the 

 head : their muscles extend to the back of the cranium, and to the 

 tergum of the prothorax. The cervical sclerites of various insects 

 have been described by Verhoeff (1903), \^oss (1905), Martin 

 (1916), and Crampton (1917, 1917a, 1926). 



II. ELEMENTAL STRUCTURE OF A THORACIC SEGMENT 



Though the form of the insect thorax most familiar to entomolo- 

 gists does not present to the eye the basic structure of its parts, it is 

 that on which our nomenclature has been established, and, therefore, 

 it will be necessary to consider first a typical thoracic segment in its 

 definitive state in order to explain the terms in common use applied 



