78 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



In some of the Aculeata (Pepsis, 61) in which the mesepimerum 

 (E^^mj) is still distinct, the episternum alone is divided into dorsal 

 and ventral plates {Eps2, eps^). In the honey bee {Apis, 63) the 

 epimerum {Epm^) is well developed above, but reaches only about 

 half way down from the wing process ( TfPj) to the base of the middle 

 leg. In a worker ant (Pogonomyrmex, 62) the upper pleural plate 

 (PI2) is continuously fused with the mesoscutum (Sct^) and, in this 

 case, might just as reasonably be called a part of the mesonotum, as 

 may the lower plate (pl^) in other forms be called a part of the meso- 

 sternum. In Pogonomyrmex (62) the latter plate (5*2) is, however, 

 demarked from the pleurum by a suture. 



8. The formation of a prepectal plate in the mesothorax cut off from 

 the anterior parts of both the mesosternum and the mesopleura. 



This character reaches its highest development in the Chalcidoids. 

 The plate in question (Ppct2) is specially well shown in such species as 

 Catalaccus incertus (44), Coccophagus lecanii (45), and DimmocTcia 

 incongruus (48), in all of which species it forms a conspicuous plate 

 on the side of the thorax lying between the pronotum (iVJ and the 

 mesepisternum (Eps^). An examination of the ventral aspect of the 

 thorax, however, shows that this pleural sclerite {Ppct2) on each side 

 is only the lateral part of a plate that is continuous across the ventral 

 surface in most cases. This is specially well shown by Prospaltella 

 herlesii (47), where the plate (Ppct2) forms an anterior subdivision of 

 the entire mesopectus. Hence the writer has given it the name of 

 prepectus, signifying that it is derived from the anterior parts of both 

 the "sternum {S2) and the episterna (EpSj). 



The beginning of the mesoprepectus is to be found in the Ichneu- 

 monidsp, in nearly all members of which the anterior part of the 

 mesopectus is differentiated as a subsclerite (23-28, 31, 32, Ppdj); 

 marked off by a suture from the sternum {S2) and the episternum 

 {EPS2). In the Chalcid, Leucospis affinis (35, 37, 39), the median part 

 of the prepectus is not entirely cut off from the front of the meso- 

 sternum (S2). In Microterys, sp. (43) there are two prepectal plates, 

 one on each side (Ppct2, Ppct2), which are not connected in front of 

 the sternum, but this is most evidently a secondary reduction. In 

 Ceramhycohius cushmani (40, 41) the prepectus occurs in a distorted 

 condition {Ppct2) on account of the curious shape of the mesopleurum. 

 Though the prepectus has something of the appearance of the preepi- 

 sternum (see p. 47) of the more generalized orders of insects, espe- 

 cially if we assume a continuity between the preepisterna and the 

 presternum, yet the phylogenetic gap between them is too great to 

 permit of the homologizing of one with the other. The prepectus of 

 the Hymenoptera appears to be a purely secondary production 

 within this order. 



