76 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



as the examples cited above, the metanotum is called the "postscu- 

 tellum of the mesothorax," while the propodeum, the true first 

 abdominal segment, is called the "metathorax." Many systematists, 

 of course, recognize the impropriety of such a nomenclature from an 

 anatomical standpoint, but are still constrained from making a 

 change on account of the confusion it would create in taxinomic 

 literature. The object of the present paper is simply the determina- 

 tion of the true morphology of the plates as far as tliis can be done 

 by a comparative study. As already pointed out in the introduction 

 and elsewhere, morphological terms may often be too awkward for 

 use in systematic descriptions. For example, in those species in 

 which the metapostnotum and the first abdominal tergum are fused 

 this combined plate may for convenience still be called simply the 

 propodeum, though in other forms the first abdominal tergum alone 

 receives this name. 



6. The fusion of the first abdominal segment with the metathorax and 

 its complete incorjjoration into the thoracic diinsio7i of the body. 



This character of the Hymenoptera is now so well known that it 

 scarcely needs any discussion here. It is interesting to observe, how- 

 ever, that in such a form as Sirex (1) the first abdominal tergum (IT) 

 is but slightly separated from the second segment (//), and the same 

 is true in the Tenthredmidse (18, 19). In Bactroceros (17) it is more 

 distinctly separated from the rest of the abdomen, but is still most 

 evidently the first abdominal tergum (IT). In all the Hymenopteran 

 families above the Tenthrfedinoidea and Siricoidea, however, it cer- 

 tainly appears to be a part of the thorax, though it of course always 

 carries the first abdominal spiracles (ISp). Its fusion with the meta- 

 postnotum has already been described. Laterally it fuses with the 

 metapleura (Pl^ and pla) in most of the higher families (20, 22, 24, 45), 

 the line of separation being sometimes entirely obsolete (21, 50, 51). 



This transferred abdominal segment was first called the "segment 

 mediaire" by Latreille (1821), but the name "propodeon" given to 

 it by Newman (1833) is more convenient to use. Emery (1900), and 

 Wheeler (1910) following him, call it the "epinotum" in ants. One 

 of the few modern authors who have argued that it belongs to the 

 thorax is Marlatt (1896), who, following Westwood (1838) calls it the 

 "metascutellum." A voluminous account of the history of the dis- 

 cussion concerning this plate is given by Gosch (1883). 



7. The formation of a single large mesopleural plate on each side by 

 the narrowing of the mesepimerum and the suppression of the mesopleural 

 suture, and its secondary division into an upper and a lower plate. 



In many of the Hymenoptera the pleurites are developed in a most 

 typical form in both the mesothorax and the metathorax. Such is 

 the case in all of the Tenthredinoidea and Siricoidea (1, 17, 18, 19) 

 where a distinct episternum {Eps2, Eps^) and epimerum (Epm^, 



