THE THORAX. 83 



-which is to be ascribed chiefly to the circumstance of their having both 

 wings and legs attached to them, whereas the first has but legs alone ; 

 consequently greater compass was required for the reception of the 

 muscles of the wings, and which explains the reason of their much more 

 artificial construction. We likewise observe the fullest development in 

 the number and situation of the parts to occur here, also, in the Cole- 

 optera, as was to be expected in the highest order. The third seg- 

 ment, likewise, consists of seven pieces, which are similar to those of the 

 second. The superior central piece, the METANOTUM (PI. IX., &c. p, F), 

 occupies the whole superior part of the metathorax ; it is generally an 

 oblong quadrangle, with the anterior angles advanced : it is frequently 

 hollowed in front. A somewhat arched partition (mesophragma of 

 Kirby and Spence), which descends into the cavity of the thorax, sepa- 

 rates the cavity of the meso- from that of the metathorax, and serves 

 for the insertion of the muscles of the back, as well as of the legs. The 

 membrane which connects this segment with the preceding passes over 

 this partition, but which is, however, no longer apparent in the Hymen- 

 optera, and in all those orders wherein the corneous plates are attached 

 together. In general, the posterior edge of the SCUTELLUM projects 

 somewhat over the anterior margin of the metathorax ; it often (Diptera 

 and Cicadaria) conceals its centre though rarely its entire surface 

 (Tdbanus, Pi. XIV. No. 1. c; Cheh/phus, Tetyra). Sometimes a 

 straight furrow, which, however, occasionally runs concentrically with 

 the scutellum, separates from the remainder an anterior portion of the 

 metathorax, which has been called POSTSCUTELLUM. In the saw-flies 

 (Tenthredonodea) this portion, particularly laterally, very strongly 

 projects, and displays two small, very generally white, points, which 

 are called CENCHRI. 



The posterior wings are placed at the anterior angles, and often 

 occupy the whole sides of the metathorax. This occurs through the 

 medium of a peculiar organisation, the description of which belongs to 

 the anatomical division ; thus much may stand here the strong cor- 

 neous nervures are attached to the metathorax by articulation, and the 

 membrane is formally affixed to it, and is supported, upon the expansion 

 of the wing, by the horny plates contained within it. 



A pergamenteous partition at the posterior margin, and called the 

 METAPHRAGMA, and which descends in a perpendicular direction, 

 bowing in its middle towards the abdomen, separates the latter from 



G 2 



