THE THORAX AND ITS APPENDAGES. 



57 



ordinary tergal divisions seem to be entirely obliterated. The meta- 

 pleurum consists of a dorsal plate (fig. 21, Pl^) supporting the hind 

 wing and of a ventral plate {pl^) carrying the hind leg. These two 

 functions certainly identify these two plates as constituting together 

 the metapleurum, but there is absolutely no trace of a division into an 

 episternum and an epimerum. Once more, therefore, Ave have to go 

 back to the generalized Hymenoptera to find out what has happened. 



,,5ct 



Fig. 23. — ^A, thoracic terga of worker separated from one another, showing protergum 

 [T-l), mesotergum (Ta) and its internal postscutellum (postnotum PN2) and phragma 

 (Pp7»2), metatergum (Ts) and propodeum or first abdominal tergum (IT) ; B, ventral 

 ¥iew of principal or notal plate of mesotergum. 



The answer is simple. Sirex has a typical metapleurum consisting of 

 an episternum and epimerum separated by a complete pleural suture. 

 In the higher forms this suture simply disappears, and consequently 

 the pleurum shows no traces of its original component plates. The 

 division into a wing-bearing and a leg-bearing plate is, therefore, a 

 purely secondary one. 



None of the Hymenoptera has separate trochantinal sclerites (see 

 fig. 4, Tn), but, since the coxae are articulated ventrally to knobs 



