No. 1774. THORAX OF HYMENOPTERA—SNODGRASii. 67 



student of any one group is almost sure to be misled in his interpre- 

 tations of the morphology. The writer has selected the horntail, 

 Tremex columba, as the subject of a preliminary description, both 

 because its thorax is very generalized in structure and because it is a 

 large and widely distributed species. 



Figure 1 on plate 1 shows a side view of the thorax and the base 

 of the abdomen, the wings being entirely removed and the legs 

 detached from their basal joints or coxas {Cx^, Cx^, Cx^). Since the 

 latter are unmistakable landmarks, they make good starting points 

 for a morphological orientation. The plate to which the first coxa 

 (Cxj) is attached is the propleurum, consisting in Tremex entirely of 

 the proepisternum (EpSi) . Each curves mesally over the ventral sur- 

 face of the prothorax, so that the two almost meet along the midline 

 (2, E'pSi) in front of the small prosternum (Sj) . Above these episternal 

 plates is the large protergum (1, N^), forming a cap over the anterior 

 end of the mesothorax. Just behind its lateral margin on each side 

 is a small sclerite containing the anterior thoracic spiracle (ISp). 

 It will be noticed that the pronotum is associated much more closely 

 with the mesothorax than it is with the pleural and sternal parts of 

 its own segment, these latter, which together constitute the pro- 

 pectus, forming a loose suspensorium for the front legs. 



Between the front coxa and the middle coxa on each side are the 

 plates of the mesopleurum — the mesepisternum {Eps^} and themesep- 

 imerum {Epm^). They are separated by the distinct mesopleural 

 suture {PS2) extending upward and forward from the articulation 

 of the coxa into the mesopleural wing process ( WP^) which supports 

 the front wing from below. The small irregular sclerite lying before 

 the wing process is the parapterum of the mesothorax (Pj). The 

 episternum {Eps^ is not separated in Tremex from the mesosternum 

 (xSj), though in many other species the two are divided by a distinct 

 suture. Above the mesopleura is the mesonotum {N^), consisting 

 principally of the scutum (-S'c^j) ^^^ ^^^ scutellum {Scl^. Beneath 

 the posterior edge of the latter is seen a part of the mesopostnotum 

 {PN^, whose lateral parts are attached to the mesepimera {Epm^. 



Between the mesocoxa {Cx^ and the metacoxa {Cx^ are the two 

 plates of the metapleurum, the metepisternum {Eps^) and the metep- 

 imerum (Epm^), separated by the oblique metapleural suture (PS^). 

 The last ends above in the slender metapleural wing process ( WP2) 

 which supports the hind wing. In front of it is the small parapterum 

 of the metathorax (P3). Between the latter and the mesepimerum 

 {Epm^ is a small sclerite containing the posterior thoracic spiracle 

 {2Sp). Above the metapleura are three dorsal sclerites (iVg, PN^, 

 and IT). The first is the metanotum (iVg) and carries the hind 

 wings. The second is the metapostnotum (PN^), though it is more 

 closely attached to the plate following it than to the metanotum. 

 The third is the first abdominal tergum {IT) containing the first 



