No. 1774. THORAX OF HYMENOPTERA—SNODGRASS. 79 



9. The obliteration of the metapleural suture resulting in the formation 

 of a single meta'pleural sclerite, which becomes divided again into an 

 upper and a lower plate. 



The evolution of the metapleurum is parallel with that of the meso- 

 pleurum. In the Tenthredinoidea and Siricoidea (1, 8, 11, 17, 18) it 

 consists of two approximately equal plates, the episternum {Eps^) 

 and the epimerum (Epm^) separated externally by the plural suture 

 (1, 11, PS3) and internally by the pleural ridge (8, PR). In all the 

 other Hymenoptera, however, the metapleural suture is obliterated, 

 and the metapleurum consists either of one single plate (34, 40, 44, 45, 

 48, 49, 50, 51, 57, 59, 60, 62, PI2) or it becomes more or less divided into 

 a dorsal wing bearing part and a ventral leg-bearing part (20, 23-28, 

 61, 63, PI3 and pl^). In Trichiosoma lanuginosa (19) there is a sug- 

 gestion of this dorsal and ventral subdivision even before the pleural 

 suture has disappeared, resulting in the formation of four subsclerites 

 (EpSs, epSg and Epm^, epm^). It has already been shown that the 

 upper parts of the metapleura are nearly always fused with the 

 lateral parts of the metapostnotum (PN^), but besides this they are 

 nearly always fused also with the sides of the propodeum (IT). In 

 many cases, therefore, all four of these parts, the metapostnotum 

 (PN^), the propodeum (IT), and the two metapleura {PI3) are fused 

 into one large piece in wliich sometimes all traces of sutures are 

 obliterated (21, 50, 51). 



3. SUMMARY OF THORACIC CHARACTERS. 



As a result of these various modifications the thoracic division of 

 the body in the higher Hymenoptera looks very different in its com- 

 position from that of all other insects. By the rearrangement of 

 some of the parts and the consolidation of others the original meta- 

 meral structure is obscured, and the thoracic walls come to be made 

 up of seven distinct chitinous plates having but little evident relation 

 to the original four segments. This remodelled structure is well 

 shown by the Proctotrypid, Helorus paradoxus (58). The parts may 

 be specified as follows: (1) the propectus, consisting of the pro- 

 sternum and the proepisterna {Eps^), which supports the head and 

 carries the front legs; (2) the protergum, or pronotum {N^), forming 

 a cap over the front of the mesothorax; (3) the scuto-prescutal plate 

 of the mesonotum {Psc^ and Sct^) carrying the anterior dorsal articu- 

 lations of the front wings; (4) the scutellar plate of the mesonotum 

 {Scl^), separated from the preceding by the suture (k) and carrying 

 the posterior articulations of the front wings; (5) the mesopectus, 

 consisting of the fused mesosternum {S^) and mesopleurites (EpSo, and 

 Epm^), supporting the front wings from below and carrying the mid- 

 dle legs; (6) the metanotum {N^) carrying the dorsal attachments of 



