No. 1774. THORAX OF HYMEN0PTERA—SN0D0RA8S. 75 



terior upper angles of the epimera. In Apis the median part of the 

 postnotum is lacking, but the lateral parts remain as two arms attach- 

 ing the postphragma to the angles of the mesothoracic epimera. The 

 postphragma (PpJi^) is of variable size, but it is usually large and 

 often projects through the metathorax far back into the cavity of the 

 propodeum. 



4- The reduction of the metanotum to a simple transverse plate carry- 

 ing the hind wings. 



In the Tenthredinoidea and Siricoidea the metanotum (1, 6, 11, 17, 

 18, 19, iVg) is a plate with more or less differentiation in its various 

 parts and presents two little dorsal prominences called the cenchri (6, 

 11, p.). In the higher families, however, it is usually a very simple 

 narrow plate (20, 26, 28, 57, 60, 63, N^) lying between the mesonotum 

 (iVg) and the metapostnotum (PiVg), the latter being often indistin- 

 guishably fused with the front of the propodeum (IT). Except in 

 wingless forms the metanotum remains an individually separate 

 plate of the dorsum, and may always be identified by the fact that it 

 carries the hind wings laterally. 



5. The fusion of the metapostnotum with the propodeum or first 

 abdominal segment. 



In the Tenthredinidse the postnotum of the metathorax is a narrow 

 transverse sclerite (11, 18, 19, PN^) lying between the metanotum 

 (iVg) and the first abdominal tergum (IT), though usually attached to 

 the latter. In Arge (11) and Trichiosoma (19) it is continuous later- 

 ally with the metathoracic epimera {Epm.^. In Tremex (1, 6, PN^ 

 it consists of two narrow plates associated with the front of the first 

 tergal plates of the abdomen {IT). In most of the other Hymenop- 

 tera, however, it constitutes a simple dorsal transverse yoke between 

 the posterior angles of the metapleura. {Trichiosoma, 19; Euuro- 

 Iracon, 20; Erymotylus, 23; Cryptus, 26; Proctotrypes, 57; and Pep- 

 sis, 61.) While in such cases the metapostnotum is a distinct though 

 often narrow sclerite between ^he metanotum in front and the propo- 

 deum behind, it is nearly always fused with the latter. In the highest 

 phase of its evolution it becomes indistinguishably merged into the 

 front of the propodeum {Odontaulacus , 22 ; Syntomaspis, 34 ; Catolac- 

 cus, 44; Coccophagus, 45; DimmocMa, 48; Rhodites, 51; Telenomus, 

 60; Ajns, 63). In such cases the dorsum of the thorax consists of 

 five plates (see Syntomasjns racemarise, 34, or Dimmockia incongruus, 

 48) — the pronotum (iVj), the mesoscuto-prescutum {Psc^ and /Sd,), 

 the mesoscutellum (S'cZg), the metanotum {N^), and the propodeum 

 {IT), including the metapostnotum. This suppression of the meta- 

 postnotum as an individual plate, together with the concealment of the 

 mesopostnotum, has led to a very erroneous nomenclature on the 

 part of Hymenopteran systematists. For example, according to the 

 ordinary application of names to the back plates of any such species 



