572 EXTERNAL ANATOMY OF INSECTS. 
13. Metapnystega*. This part, although in the table I 
have placed it as anappendage of the pleure, is not always 
confined to them, as you will soon see. It either covers 
aérial vesicles, or is the seat of a spiracle. In the Order 
Coleoptera it is of the former description. If you exa- 
mine the metathorax of the common dung-chafer (Geo- 
trupes stercorarius), in the horizontal part of the pleura 
you will see a sublanceolate or subelliptical rather mem- 
branous silky tense plate, with its point towards the head 
—this is the part we are considering ; something similar 
you will find in most beetles ; but in some, as Cerambyx 
moschatus, it is less conspicuous. ‘This part, as far as I 
have observed, is not so situated in any other Order, ex- 
cept in some Heteropterous Hemiptera: in Belostoma 
the channel lately mentioned is filled up at its posterior 
end by a red organ with an anterior vertical fissure, ter- 
minating behind in a conical bag: in Notonecta the pleura 
has something of a plate like that of Coleoptera, but of 
a horny substance. In the 'Orthoptera and Neuroptera 
this part changes its situation, if it be indeed synony- 
mous; and as the pnystega follows the frenum, so the 
metapnystega succeeds the postfrenum. In the Libellulina 
M. Chabrier found that this as well as the other covered 
aérial vesicles’, and it probably does the same in the 
other cases in which it occurs. In Mantis and Phasma 
in the Orthoptera it is very minute; but in Locusta 
Leach, it is more conspicuous under the form of a tense 
membrane, the surface of which is depressed below that 
of the abdomen: in Acrida viridissima it fills the sinus 
of the postfrenum, and is vertical, as it is in Ashna. It 
is worthy of remark that this piece bears some analogy 
° Pirate XXIX. Fic. 25.; Prate VIII. Fic. 12; and Prats 
IX.Fic. 7. &” ’ Sur le Vol des Ins. c. iii, 354. 
