NO. I INSECT THORAX SNODGRASS 9 



to its various parts. The morphologist is often tempted to throw off 

 the nomenclatural shackles that bind him to the past, for new ideas 

 could be much more freely stated if they did not have to be expressed 

 in the terminology of former errors, or, at least, in the language of 

 what we now regard as the misconceptions of a less enlightened 

 earlier age. Yet, a discarding of old names might only set an example 

 for a future generation, which, most likely, would proceed in turn to 

 reject our terms along with our ideas, and adopt a new orismology 

 expressive of its own ideas. After all, even a scientific term usually 

 meets with the fate common to all names, and soon becomes ac- 

 cepted as a label for an object, without significance of derivation, and 

 without respect to the original conception it implied. 



GROUND-PLAN OF A THORACIC SEGMENT 



We have already seen that a primitive limb-bearing segment pre- 

 sents a dorsum and a venter, in each of which is developed a chitinous 

 plate, the tergum and the sternum, respectively, and a membranous 

 pleural area between the two on each side, in which the limb is 

 implanted, and in which also there are usually present a number of 

 chitinizations, the pleurites, or collectively the pleuron. 



The tergum, or notum as the dorsal segmental plate is frequently 

 called, in its typical form is a simple sclerite covering the back of 

 the segment (fig. 3 A, T) and in wingless segments often produced 

 downward on the sides, sometimes overlapping the bases of the legs. 

 The tergum of the mesothorax, and usually that of the metathorax 

 is marked anteriorly by a submarginal antecostal suture (ac) and 

 corresponding internal antecostal ridge (B, Ac), the two setting ofif 

 a narrow precosta (Pc) from the anterior margin of the principal 

 tergal plate. The tergum of the prothorax, however, always lacks a 

 true antecosta and precosta, these parts apparently having been lost 

 in the " neck." The dorsal muscles of the prothorax that arise pos- 

 teriorly on the antecosta of the mesotergum thus come to be inserted 

 anteriorly on the head, and to act as muscles for moving the head. 

 The metatergum also sometimes lacks an antecosta and a precosta, but 

 in such cases these parts are found to have been transferred to the 

 mesothorax, and, in the same way. the corresponding elements of the 

 first abdominal tergum may become a part of the metathorax. The 

 antecostae of the mesotergum, metatergum, and first abdominal tergum 

 commonly develop plates, called phraginata (fig. 23 C, iPh. 2PJ1, sPh), 

 that extend into the thoracic cavity to give increased surfaces for the 

 attachment of the dorsal longitudinal muscles. The rear margfin of 



