584 EXTERNAL ANATOMY OF INSECTS. 



Bclostoma a right angle, and in Notonecta an obtuse 

 one. In the two first the angle of the mesophragm sends 

 two short diverging ridges to the metaphragm ; and in 

 the last only a single one: in this also the posterior 

 chambers together are nearly as large as the anterior. 

 From this structure it should seem that in flight the 

 Hemelytra are more important than the wings. In the 

 Homopterous section the anterior chamber is the 

 smallest, at least in Fulgora candelaria ; and the meso- 

 phragm is lofty and bipartite. In the Lepidoptera the 

 anterior chamber is the largest, and the part in question 

 conspicuous a . In the Libellulina and Hymenoptera it 

 is merely represented by a low ridge, and in the Diptera 

 it seems evanescent. 



4. Metaphragma b . This, in many cases, is the largest 

 and most remarkable of the three partitions of the upper 

 portion of the cavity of the alitrunk, which separates it 

 from that of the abdomen ; it is attached to the posterior 

 margin of the metathorax, and is nearly vertical : in sub- 

 stance it may be stated as rather firmer than the two 

 preceding partitions. In the Coleoptera it is commonly 

 of the width of the posterior orifice of the alitrunk ; and 

 its centre is cleft so as to form a deep sinus c for the 

 transmission of the intestines, — a circumstance which 

 also, though less conspicuously, distinguishes the meso- 

 phragm d : from this sinus it slopes gradually towards the 

 sides, and is sometimes armed with an intermediate pro- 

 cess on each side e . This structure you will find exem- 



* Plate IX. Fig. 2. s'. 



b Plate XXII. Fig. 10, 11. x . Comp. Linn. Trans, xi. t. ix. 

 /. 16. g. c Plate XXII. Fig. 10, 11. d. 



d Ibid. Fig. 9. c. « Ibid. Fig. 10. a. 



