68 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



abdominal spiracles (ISp). Both the metapostnotiim and the first 

 abdominal tergum are divided along the median dorsal line into two 

 lateral plates (6, PN^ and IT). The metasterniim, like the mesoster- 

 num, is continuously fused with the episterna of its segment. 



The thoracic homologies, as presented in the above account, cer- 

 tainly seem indisputable when the parts of each segment are compared 

 with those of any of the generalized orders of insects. For example, 

 compare the mesothorax of Tremex with either segment of an adult 

 stonefly such as Tsenioj^teryx frigida (tig. 5). The two pleural 

 plates (Eps and Epm) of Tseniopteryx, separated by the pleural suture 

 (PS), are identical with those of the mesothorax of Tremex (1). 

 The pleural suture (PS) in each case extends from the coxal articula- 

 tion into the wing process (WP). The parapterum (P) lies before 

 the latter in both, though it is attached to the episternum in the 

 stonefly. The notum (N) is unquestionably the same plate in each 

 case, and the postnotum (PN) in each is connected with the posterior 

 angles of the epimera (Epm). The ventral parts are different in 

 that the sternum (S) of the stonefly is separated from the episternum 

 (Eps) by a suture (q), and the coxa (Cx) is articulated below to a 

 sclerite, the trochantin (Tn), which does not occur in Tremex. It 

 will be shown later that many Hymenoptera, however, possess a 

 sterno-pleural suture on each side corresponding with that of Txnio- 

 pteryx. The structure of the metathorax of Tremex but duplicates 

 that of the mesothorax, the differences being simply in the size and 

 the shape of the parts. 



Marlatt (1896) has described and figured the thorax of a sawfiy, 

 Lygsenematus (Pachynematus) ericlisonii. He calls the large mesepi- 

 sternum (18, Eps2) the '^epimeron" of the mesothorax, while he calls 

 the true mesepimerum (Epm^) a "posterior plate of the epimeron." 

 In the metathorax he calls the episternum (Eps^) the "epimeron," 

 while he does not name the true epimerum (Epm^) of this segment. 

 The plates of the first abdominal tergum he calls the "scutellum" 

 of the metathorax, but does not say how they come to carry the first 

 abdominal spiracles (ISp). The writer can produce no argument 

 against these interpretations so effective as that to be derived from 

 a comparison of the sawfiy (18) with a stonefly (fig. 5) or a grass- 

 hopper (fig. 13). Systematists in general, who have attempted to 

 explain the thoracic anatomy of the Hymenoptera, have made so 

 many inconsistent applications of anatomical names that space can 

 not be given here to a review of their works. 



We may now more conveniently make a detailed study of each part 

 of the thorax separately and at the same time note the modifications 

 that occur in the Tenthredinoidea, for some of them are, in some ways, 

 more generalized than Tremex. 



