EXTERNAL ANATOMY OF INSECTS. 575 



nating 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 frcenum, so the 

 metapnystega succeeds the postfrcenum. In the Libellulina 

 M. Chabrier found that this as well as the other covered 

 aerial 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 mridissima K. it fills the 

 sinus of the postfrcenum, and is vertical, as it is in 

 JEshna. It is worthy of remark that this piece bears 

 some analogy to that below the ridge of the part just 

 named in Coleoptera, which descends either vertical- 

 ly or obliquely to the abdomen b . A similar space, 

 though often nearly obsolete, may be seen in the Hemi- 

 ptera and Lepidoptera. But the Orders in which this 

 part is most conspicuous are the Hymenoptera and Dipte- 

 ra, and in these its aerial vessels are connected with a 

 spiracle. In Tenthredo L. and Sirex L., what Linne 

 named grana, from their situation, should be regarded as 

 belonging to the pnystega, and whether there is any part 

 representing the metapnystega I am not quite satisfied ; 

 perhaps the membrane at the base of the abdomen in 

 Tenthredo, and the bipartite piece, apparently its first 

 segment, in Sirex c , may be its analogues : but in the great 

 majority of the Order, the convex or flat piece that in- 



a Surle Vol dcs Ins. c. iii. 354. b See above, p. 572. 



c Plate IX. Fig. 15. k" . 



