50 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



In most other insects a postnotuni is present in each of the wing- 

 bearing segments, as given at D of the figure. In this case the meso- 

 thorax is provided with an anterior phragma (iPh) and a posterior 

 phragma (2Ph), while the metathorax has only a posterior phragma 

 (jPh). The first phragma (iPh) is never transferred to the pro- 

 terguni. If the base of the middle phragma (2Ph) retains its con- 

 nection also with the metatergum, the mesotergum and metatergum 



ISg- Se-^ ISg- Se^ ISg- 



/\ Wa s \ \'\'\K''w ^^ '\' ^^ Mi m iimiii!i ii> s ^sssm^ ^^^s 



Pc. acl L^^^^ Mb| Pc ac T5 >|b_i |Pc IT 



B 



C 



D 



/ I "Ph 

 T^ Mb -PN-t' mb IT 



the 



Fig. 23. — Diagrammatic lengthwise sections showing 

 phragmata and the thoracic postnotal plates. 



A, primary segmentation in non-chitinous segments {Seg), with longitudinal 

 muscles (LMcl) attached at intersegmental rings (Isg). 



B, secondary segmentation: each tergal plate (T) including the chitinized 

 antecosta (Ac) and narrow precosta (Pc) ; antecostse bearing phragmata (Ph) ; 

 the antecostal sutures (ac) are the original intersegmental grooves (A, Isg). 



C, thorax with metathoracic wings chief organs of flight : third phragma 

 (sPh) separated from first abdominal tergum (IT) by secondary membrane 

 (mb), and its precosta (B, Pc) enlarged to form the pre-phragma part in a 

 postnotal plate (PNs) of metathorax. 



D, thorax with mesothoracic wings chief organs of flight : each wing-bearing 

 segment with a postnotal plate (PN2, PNx) and a posterior phragma; the 

 postnotal plates prevent telescoping' of the segments (fig. 2 C). 



are said to be " fused." Comparing C and D with A in figure 23, 

 it is clear that the part of postnotal plate before the phragma belongs 

 to the posterior part of the primary segment in which it occurs, 

 and that only the narrow posterior lip of the phragma base has come 

 from the following segment. It is thus true, as Berlese (1909) has 

 shown, that the postnotum of a winged segment is the " acrotergite " 

 of the tergum following. 



The variation in the connections of the phragmata is correlated with 

 the specialization of one pair of wings or the other as the chief organs 



