174 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



The eiMtrochlear sclerite lies in front of the descending process of the 

 hypotreme, and is prothoracic, whilst the lateral plate and the plastron 

 lie behind it, and are both mesothoracic. The hypotreme is a cylindrical 

 rod, between the manubrial suture and the lower margin of the spiracle, 

 and has probably become detached from the posterior margin of the coxo- 

 sternal foramen by the atrophy of the inflected hypodermis, from which it is 

 developed. I therefore regard the hypotremata the manubrial suture and 

 the spiracles as representing the primary suture between the pro- and meso- 

 thorax. 



The Metasternum is the largest sclerite of the metathorax. It 

 is shaped somewhat like the sphenoid bone of the human sktill. 

 It consists of a median longitudinal inflection of the integu- 

 ment supporting a metafurca, of a narrow transverse inflection, 

 and of two lateral plates — the metapleura of Osten-Sacken. 

 These must not be confounded with Audouin's metapleura ; 

 I term them ' metaplastra ' (Plate VIII. , Fig. 4, 7 ?). 



The metaplastra are the only parts of the metasternum which 

 are easily seen externally, but when the intermediate and 

 posterior coxas are carefully separated, a narrow ridge can be 

 distinguished between them : this is the lower edge of the 

 transverse inflected plate ; it unites the metaplastra with each 

 other, and is continuous with their anterior and posterior 

 margins. 



Each metaplastron is separated from the mesosternum in 

 front by the transverse ventral suture and the intermediate 

 coxa ; behind it is in relation with the metapleuron and the 

 first abdominal ring, and externally it surrounds the posterior 

 spiracle. 



Seen from its internal surface, the m.etasternum is cruci- 

 form. The transverse vertical plate is the inflection between the 

 intermediate and posterior coxae ; it is joined behind, in the 

 middle line, by a median plate, which is triangular ; the upper 

 edge of the latter supports the metafurca, its lower edge articu- 

 lates with the posterior coxae. At its junction with the transverse 

 vertical plate it supports a capitellum (yp) with which the four 

 coxae articulate. Its posterior angle bears two divergent capitate 

 processes (/cV), which articulate with the posterior coxae. 



The Metafurca" (^^) is a long hollow trough from which the 



