1909.] 



NATURAL SCIENCES OF PHILADELPHIA. 



13 



article on Polistes. By comparing the thorax of Polistes with that of 

 certain other less specialized Hymenoptera, such for example as 

 Macroxyela, Tenthredo, etc.. in which the mesonotum has retained a 

 ■comparatively primitive condition, it is an easy matter to apply 

 Audouin's terminology to insects in general. 



In the mesothorax of Macroxyela, Chrysopa, Myrmeleon, etc., the 

 portion of the tergum just behind the prsescutum is composed of two 

 regions : a median portion triangular in shape, with its apex directed 

 caudad (fig. 1, N 2& ), and a larger portion surrounding the first laterally 

 and posteriorly (A 7 ^). The triangular median region of the scutum 

 may be termed the mediscutum, and the remainder the parapsido- 

 scutum (from MacLeay's, '30, parapsides, applied to two pieces sepa- 

 rated off from this region in Polistes). 



In his description of Dytiscus, Audouin regards the mediscutum as 

 part of the prsescutum, while in his translation of MacLeay's work he 

 speaks of the region corresponding to the mediscutum as the scutum 

 proper, and likewise reckons the parapsides to the scutellar region. 

 The latter division is the only natural one, and has consequently been 

 adopted in this discussion. 



In the Hymenoptera, one can trace an extremely interesting series 

 of changes in the scutellar region of the mesothorax. Thus, if one 

 examine the thorax of Macroxyela, Abia, Odynerus and Chrysis, in the 

 order given, it will be seen that the apex of the triangular mediscutum 

 (figs. 1 and 2, N 23 ) becomes gradually lengthened out, and pushes 

 through that portion of the parapsidoscutum (N 2h ) behind it until it 

 reaches the scutellum (N 3 ). Its sides then begin to open out, and 

 become nearly parallel. By this 

 process the formerly triangular 

 mediscutum assumes a rectangular 

 form (fig. 2, N 2a ), and divides the 

 parapsidoscutum into two widely sepa- 

 rated halves (N 2 ^ and AT 2 b), the parap- 

 sides of MacLeay, '30. 



As shown by MacGillivray, '06, the 

 wing veins of the Xyelidse show that 

 this family is the most generalized of 

 the Hymenoptera. Consequently the 

 thorax of Macroxyela is more primitive 

 than that of Polistes — an opinion which 

 is confirmed by a comparison with cer- 

 tain lower insects, such as Myrmeleon, 

 etc. If this be true, one is justified in assuming that the " parapsides ' 



Fig. 2. — Eumenes. — Dorsal 

 view of the mesothorax. A 

 comparison with fig. 1 shows 

 that the mediscutum (N" 2 a) 

 has pushed through the par- 

 apsidoscutum (N" 2 b) and di- 

 vided it into the two "par- 

 apsides" (N" 2 b and N" 2 b). 





