582 EXTERNAL ANATOMY OF INSECTS. 
lina and Hymenoptera it is merely represented by a low 
ridge, and in the Diptera it seems evanescent. 
4. Metaphragma*. ‘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 metathoraz, 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” for the 
transmission of the intestines,—a circumstance which 
also, though less conspicuously, distinguishes the meso- 
phragm‘: from this sinus it slopes gradually towards 
the sides, and is sometimes armed with an intermediate 
process on each side?’. This structure you will find 
exemplified in the common cockchafer and many others 
of the Order. I have not, however, discovered traces of 
it either in the Silphide, Staphylinide, or the vesicatory 
beetles (Cantharis); or even in such species of ground- 
beetles (Eutrechina) and tiger-beetles (Eupterina) as I 
have examined; while in the water-beetles (Zunechina) 
it is very visible. In the Orthoptera it is nearly obsolete; 
but in Locusta, under the metapnystega, one on each 
side, is a pair of seemingly pneumatic pouches which 
may be mistaken for it. It is almost equally inconspi- 
cuous in both sections of the Hemiptera. As to the 
Lepidoptera,—in Pontia Brassice it resembles in some 
degree, though in miniature, the metaphragm of the 
= Prate XXII. Fic. 10, 11. 2’. Comp. Linn. Trans. xi. t. ix. 
f. 16. g. > Prate XXII. Fic. 10, 11. d. 
© Ibid. Fic. 9. e. 4 Tbid. Fic. 10. a. 
