THE THORAK AMD ITS APPENDAGES. 



57 



ordinary tergal divisions seem to be entirel}^ obliterated. The meta- 

 plenrum consists of a dorsal plate (fig. 21, PI,,) supporting the hind 

 wing and of a ventral jilate {p^-,.) carrying the hind leg. These two 

 functions certainly identify these two plates as constituting together 

 the mctapleuruni, but there is absolutely no trace of a (livii?ion into an 

 episternum and an epimennn. Once more, therefore, we have to go 

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



Aph 



Fig. 23. — A, thoracic terga of worker spparated from ono another, showing protergum 

 (7'i), mesotergum (7'2» and ilh inlcrnal postseiitclluni ( posliiotiim J'Nn) and phragma 

 {Pphn), metatergum (Ts) and propodeum or first abdominal tcrgum (IT) ; B, ventral 

 view of principal or notal plate of mesotergum. 



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

 an episternum and eijimerum 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 wdng-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 coxw are articulated ventrally to knobs 



