30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



occur at the bases of the wings. Anteriorly it is commonly marked 

 by a transverse groove, and the resulting three areas of the notum are 

 termed the prescutum, scutum, and scutellum. Close to the anterior 

 margin is another transverse groove that forms a deep internal ridge, 

 the prephragma, for the attachment of the longitudinal dorsal 

 muscles. These muscles, however, are intersegmental, their posterior 

 attachment being on the phragma of the following segment. The two 

 segments, therefore, must be solidly attached, and this is accomplished 

 by a lengthening of the precostal part of the following segment as a 

 postnatal plate firmly joined to the scutellar margin of the preceding 

 notum. The contracting muscles thus give a strong upward curvature 

 to the wing-bearing notum which effects the depression of the wings 

 in flight. Flattening of the notum results from the contraction of 

 vertical notosternal muscles, and produces the upward movement of 

 the wings. 



In the 4-winged insects, the two winged segments are essentially 

 alike, the postnotal plate of the second being derived from the first 

 abdominal segment, and the third phragma likewise. In the clistogas- 

 trous Hymenoptera, the first abdominal segment is so thoroughly in- 

 corporated into the thorax during early pupal development that it 

 becomes virtually a part of the thorax in the adult stage. The ab- 

 dominal pedicel is then formed from the second abdominal segment. 



The pleuron of a winged segment is marked by a deep vertical or 

 inclined groove from the leg base to the wing base. This forms a 

 strong internal pleural ridge to strengthen the pleural wall in its 

 double duty of supporting both the leg and the wing (see Pleuron). 

 The ridge forms the coxal articular process at its lower end, and 

 the wing fulcrum at its upper end. In the prothorax, the ridge sup- 

 ports the leg but usually does not extend on to the back. The pleural 

 ridge also usually gives off a strong apodemal arm directed inwardly. 

 The pleural sulcus divides the pleural surface into an anterior 

 episternum and a posterior epimcron, but these parts may themselves 

 be further differentiated into areas by sulci or by desclerotization. 



The sternal region of the thorax is generally continuously sclero- 

 tized in each segment except for small membranous areas between the 

 major plates. The latter is often differentiated by a transverse groove 

 into an anterior basisternum and a posterior sternellum. Between the 

 two parts at the ends of the groove arises a pair of sternal apodemes 

 which are commonly united at their bases to form a Y-shaped process 

 known as the furca. The arms of the furca turn outward and are 

 closely associated with the inner ends of the pleural process, the 

 two being usually connected by short muscle fibers. The pleural 



