THE ANATOMY OF INSECTS. 



V 



Fig. 31. — Diagram of the 

 fixed parts of a thoracic 

 segment. 



The Fixed Parts of the Thorax.* 



Each segment of the thorax is composed of several sclerites. The shape 

 and relative position of these sclerites afford characters which are much used 

 in classification. Fig. 31 is a diagrammatic representation 

 of what is considered the typical arrangement of these 

 parts in eacli of the thoracic segments. Each segment 

 of the thorax is a ring, which is divided into four parts : 

 a dorsal, a ventral, and two lateral. The dorsal part 

 is named the notum or tergum ; each lateral part the 

 pleilrum. ; and the ventral part the sternum. 



When the notum or sternum of a particular thoracic 

 segment is to be indicated, it is done by the use of one of the prefixes pro, 

 mesa, or meta. In this way are formed the terms pronotum, mesonotum, tneta- 

 notu/n, presternum, mesosternum and metasternum ; which are applied to the 

 notaand sterna of the prothorax, mesothorax and metathorax respectively. 



By some writers the entire dorsal part of an insect is termed the tergum ; 

 the lateral part, the pleilrum ; and the ventral part, the sternum. These 

 writers apply the terms tcrgite, pleurite, and sternite respectively to the dorsal, 

 lateral and ventral regions of each segment. 



The tergum of each thoracic segment is composed typically of four scle- 

 rites. These are arranged in a linear series. They are named, beginning with 

 the first or most cephalic, prasciitum (a), scut fan (d), scutellum (c), and postscu- 

 tellum (d). (Fig. 32.) In the prothorax the sutures between these four scle- 



Fig. 32. — Dorsal aspect of the thorax of a beetle, 

 Dysticus, dissected. (After Audouin.) 



Fig. 



-Necrophorus, to show scutellum. 



* The more important works on the nomenclature of the parts of the thorax are the 

 following : 



Audouin, J. V. Recherches anatomiquessur le thorax des animaux articules et celui 

 des insectes hexapodes en particulier. Annales des Sciences Naturelles, T. I. (1824). 



The works of Kirby and Sprue,-, MacLeay, Straus-Durckheim, Newman, and 

 port cited on p. 12. The description of the anatomy of the thorax by MacLeay was 

 republished in the Ann. des Sci. Nat. t. 25 (1S32). 



