No. 1774. 



THORAX OF HYMEN0PTERA—SN0DGBAS8. 



45 



though in the stonefly (fig. 1) a prescutal region (Psc) is distinctly 

 marked off from the rest. The cord-Uke thickenings of the basal 

 membranes of the wings (figs. 1, 2, 3, and 4, AxC) arise from the pos- 

 terior angles of the notum, at the ends of the posterior reduplication. 



Postnotum (PiV). — The posterior transverse postalar sclerite of the 

 mesotergum and metatergum (figs. 1, 4, and 5, PN), developed 

 best in those segments that have the wings best developed as organs 

 of flight, though not present in either segment of the Isoptera. It 

 is absent in the mesothorax of Orthoptera, Euplexoptera, and 

 Coleoptera, and is greatly reduced or absent in the metathorax of 

 species having the hind wings reduced. That of the metathorax is 

 generally fused with the first abdominal tergum in Orthoptera, 

 Euplexoptera, and Hymenoptera. 



The postnotum is ordinarily called the 

 " postscutellum," since it lies immediately 

 behind the scutellum of the notum. How- 

 ever, it is not one of the divisions of the 

 notum, since it is formed independently 

 as a secondary tergal chitinization in the 

 dorsal membrane behind the notum. Lat- 

 erally it is connected with the epimera of 

 the same segment (fig. 5, PN and Epm), 

 while posteriorly it carries the succeeding 

 phragma, which thus becomes a post- 

 phragma (figs. 1 and 4, Pph) of the seg- 

 ment. (See special discussion of the post- 

 notum and the phragmas, pp. 53 to 64.) 



Phragmas (PJi). — The internal, verti- 

 cal, transverse plates developed from the 

 intersegmental folds between the terga 

 (figs. 15 and 16, SPh, SPh). There are 

 commonly three phragmas present, the 

 anterior, the middle, and posterior, orig- 

 inating between the prothorax and tlie 

 mesothorax, between the mesothorax and metathorax, and between 

 the metathorax and the first abdominal segment respectively. In 

 the adult stage the phragmas are not independently intersegmental, 

 each being connected with either the tergum behind it or the one in 

 front of it. The first, when present, is always fused with the front 

 edge of the mesonotum. The second is likewise fused with the front 

 of the metanotum in Orthoptera, Euplexoptera, and Coloeptera, but 

 when present in the other orders it is connected with the postnotum 

 of the mesotergum. The third is always connected with the meta- 

 postnotum even when this plate is fused with the first abdominal 

 tergum. If the postnotum is absent there is likewise no phragma. 



Fig. 4.— Mesotergum of Holorusia 

 grandis (cranefly), showing divi- 

 sion of notum into three parts 



{Psc, Set, AND Scl), BACK OF WHICH IS 



POSTNOTUM {PN): AxC, axillary 

 cord; a NP, anterior notal winq 

 process; PiV,pn, postnotum; PNP, 



POSTERIOR notal WING PROCESS; 



Pph, postphragma; Psc, prescu- 

 tum; Ed, posterior reduplication 

 of notum; Scl, scutellum; Set, 

 scutum; u, lobe of prescutum 

 before base of wing. 



