EXTERNAL ANATOMY OF INSECTS. 583 



solum, and metapnystega, the last but one of these being 

 usually less distinct, they seem not analogous to the three 

 partitions of the alitrunk in other Orders; so that in 

 these the mesophragm at least seems to have no repre- ' 

 sentative, and the prophragm and metaphragm include 

 between them only one ample chamber. In the Diptera, 

 wherever there is an external depression or suture there 

 is a corresponding internal ridge or seam, so that the 

 parts seem more distinctly marked out on the inside 

 than on the outside of the crust. 



3. Mesophragma a . This piece also, which forms the 

 middle partition of the upper part of the cavity of the 

 alitrunk, dividing it into two chambers, is most conspi- 

 cuous in Coleoptera. It is usually in them a vertical 

 piece, resembling the prophragm in substance, but twice 

 its height, of a quadrangular shape with a notch in the 

 middle ; it fills the sinus of the postdorsolum, the sides of 

 which sometimes descend below it b . In this Order the 

 chamber that it forms with the prophragm is very small c , 

 the motions of the elytra requiring no powerful apparatus 

 of muscles; but that which it forms with the metaphragm, 

 which is appropriated to the muscles moving the wings, 

 is very large d . In the Orthoptera the anterior chamber 

 is larger than in the preceding Order, which proves that 

 tegmina are more moved in flight than elytra. In the 

 Heteropterous Hemiptera a remarkable variation takes 

 place — the anterior being larger than the posterior 

 chamber; which last, in fact, consists of two, one for each 

 wing : in these the mesophragm towards the abdomen 

 forms an angle, which in Pentatoma, &c, is acute ; in 



• Plate XXII. Fig. 9, 11, s. b Ibid. Fig. 9. a a. 



c Ibid. Fig. 1 1 . a. rf Ibid. b. 



