No. 1774. THORAX OF HYMEN0PTERA—SN0DGRAS8. 73 



The propectus is freely movable on account of its membranous con- 

 nection with the rest of the thorax. It serves both as a suspensorium 

 for the front legs and as a support for the head, its lateral episternal 

 parts being produced forward in the side walls of the neck as two 

 processes which loosely articulate with the occiput (1, 2, 12, 13, 53, v). 

 The evolution of the pleurum has already been indicated in the 

 description of Tremex. In Arge (12) the episternum (Eps^) and the 

 epimerum (Epm^) are well differentiated and are separated by a dis- 

 tinct pleural suture (PS). The epimerum is also present in Dolerus 

 (13), Lygsenematus (18), and others, but in general it is either absent 

 or ver}^ rudimentary, the propleurum consisting of a single plate, the 

 episternum, as in Tremex (1, Eps^), Leucospis (36), and Proctotrypes 

 (53). In Dolerus (13) there is a posterior subdivision {eps^) of the 

 episternum (EpSi). 



2. The separation of the mesonotum into an anterior and a posterior 

 plate hy a transverse memhranous suture. 



The mesonotum of Tremex columha is divided into a scutum (3, iS'rtj) 

 and a scutellum (Scl.^) by the line (imr) of the entodorsal ridge (4, VNR). 

 In some of the Tenthredinidaj there is an indistinct sutural line on 

 each side, anterior to this ridge, extending toward the lateral emargi- 

 nations of the notum (16, 17, 19,1c). In nearly all the higher families 

 of the Hymenoptera these two lines are continuous over the dorsum 

 and constitute a distinct transverse suture (k) cutting the notum into 

 two parts. This is shown in all the figures representing the mesono- 

 tum of the families from the Braconidae (20) to the Apidae (63). It 

 is especially illustrated in Erymotylus (33, Tc), Leucospis (37, 1c), 

 Geramhycohius (41, Jc), Microterys (42, Ic), Eurytoma (46, Ic), and 

 Proctotrypes (55, Jc). This division of the mesonotum is so complete 

 that in most cases it actually comes apart along this suture into two 

 distinct plates, which are normally connected only by membrane- 

 The posterior plate in some species has two flat apodemes on its 

 front margin which slip under the posterior edge of the anterior 

 plate. The anterior notal wing processes always arise from the sides 

 of the first plate just in front of the lateral ends of the transverse 

 suture (33, 46, 55, ANP), while the posterior processes (55, PNP) 

 are situated on the edges of the posterior plate. The posterior notal 

 plate may be called the scutellum (Scl^), though it is clear that it is 

 more than the equivalent of the scutellar division of the mesonotum 

 of Tremex (3, /ScZ,), which is defined by the line (vnr) of the entodorsal 

 ridge (4, VNR). In a former paper the writer (1909) has demon- 

 strated the impossibility of drawing strictly homologous lines between 

 the subdivisions of the notum in different families and orders. The 

 posterior edge of the scutum is generally differentiated as a marginal 

 ridge or lobe bearing the axillary cords of the wing bases at its 

 extremities. Such a subdivision as this might appropriately be called 



