72 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 89. 



noidea it presents two little oval lobes on the dorsal surface called the 

 cenchri (6, 11, p). The postnotum is always narrow, but in the 

 Tenthredinoidea it is continuous across the back and is usually 

 fused laterally with the epimera (11, 17, 18, 19, PN^). In all of 

 the Hymenoptera it is more closely attached to the first abdominal 

 tergum than to the metanotum, and, in the higher forms, is often 

 indistinguishably fused with the former. 



The metapectus of Tremex (7) is very similar to the mesopectus (5) 

 except that it is smaller. As in the latter, there are no sternopleural 

 sutures, though in some of the Tenthredinoidea such sutures are 

 present (11, 5). The interior of the pleurum (8) is identical in struc- 

 ture with that of the mesothorax (9). Its external appearance has 

 already been sufficiently described (1, Eps^, Epm.^). The metafurca 

 (7, Fu) is somewhat simpler in structure than the mesofurca (5, Fu). 



The first abdominal tergum would scarcely be deserving of a special 

 description in the Siridoidea and Tenthredinoidea were it not for the 

 fact that it is intimately fused into the thorax in all the other Hymen- 

 optera and constitutes the co-called ''median segment," ''propo- 

 deum," or "epinotum." In Tremex (1, 6, IT), as already described, 

 it consists of two plates, but in most forms it is continuous across the 

 back and always carries the first abdominal spiracles {ISp) laterally, 

 in the Tenthredinoids often in a special lateral subdivision (11, 17, 

 18, 19, It). 



2. MODIFICATION OP THE THORAX. 



In the general study of the Hymenopteran thorax it is found that 

 the structural departures from the comparatively simple thorax of 

 Tremex and the Tenthredinoidea consist of progressive modifications 

 along several lines. The chief of these mav be stated under the fol- 

 lowing nine heads: 



1. The separation of the pronotum from the propectus and its attach- 

 ment to the front of the mesothorax. 



The disassociation of the pronotum from the rest of the prothorax 

 is evident even in the sawflies, as shown by Bactroceros (17), Lygsene- 

 matus (18), and TricMosoTna (19). In the higher families, such as the 

 bees {Apis, 63), it appears to be an integral part of the mesothorax. 

 Its lateral parts nearly always reach far down on the sides, fitting into 

 the angle between the base of the procoxa and mesopleurum. In 

 the honey bee the lower ends extend mesally over the ventral surface 

 till they meet on the midline so that the pronotum forms a complete 

 collar about the front of the mesothorax. In Proctotrypes caudatus 

 (57) the lower parts of the pronotum {N^) not only meet each other, 

 but they fuse into a wide ventral plate between the prosternum and 

 the mesosternum, above the bases of the front coxae. The pronotum 

 in this case forms an entire annulus (54, N^) surrounding the front of 

 the mesothorax and inclosing the propectus (53, 8^ and Eps^). 



\ 



