THE ANATOMY OF INSECTS. 



l-i<;. 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 

 in each 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 

 pleurum ; 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, 

 . or met a. In this way are formed the terms pronotum, mcsonotum, meta- 

 notum, prosternum, mesosternum and mctastcrnum ; which are applied to the 

 notaand sterna of the prothorax, mesothorax and metathorax respectively. 



me writers the entire dorsal part of an insect is termed the tcrgnm ; 

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

 writers apply the terms tergite, pleuritc. 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, prcescutum (a), scutum (/>). scutellum (<), and ;V 

 tcllum (if). (Fig. 32.) In the prothorax the sutures between these four scle- 



FIG. 3?. Dorsal aspect of the thorax of a beetle, 

 Dyttfctu, dissected. ' 



FIG. tt.Ntcrtfkor*t, to show tcutellum. 



important works on the parts 



follow 



Audouin, J. V. Recherches anatomiqucssur le thorax clcs animaux 

 des inscrtcs hexapodes en particulicr. Annalcs clcs Sciences Naturelles. I 



The work^ 



port cited on j the anatomy of the thorax 1 . was 



repu the Ann. des Sci. Nat. t. 25 (1833). 



