EXTERNAL ANATOMY OF INSECTS. 573 
to that below the ridge of the part just named in Coleo- 
ptera, which descends either vertically or obliquely to the 
abdomen*, A similar space, though often nearly obso- 
lete, may be seen in the Hemiptera and Lepidoptera. 
But the Orders in which this part is most conspicuous 
are the Hymenoptera and Diptera, and in these its aérial 
vessels are connected with a spiracle. In the saw-flies 
and other Serr/era, what Linné named grana, from their 
situation, should be regarded as belonging to the pnystega, 
and whether there is any part representing the meta- 
pnystega Iam not quite satisfied ; perhaps the membrane 
at the base of the abdomen in Tenthredo, and the bipar- 
tite piece, apparently its first segment, in Szrex>, may be 
its analogues: but in the great majority of the Order, the 
convex or flat piece that intervenes between the post- 
dorsolum and its adjuncts and the abdomen, and which 
bears a spiracle on each side, is the metapnystega °. This 
part is often remarkable, not only for its size, but for the 
elevated ridges that traverse it, as in Jchneumon, Chlorion, 
&c. In the last genus it is of a pyramidal shape, with 
the anterior part horizontal and the posterior vertical ; 
it is altogether vertical in Vespa, Apis, &c. Amongst 
the Diptera, in Tipula it is nearly horizontal, and shaped 
like a cushion; but in general in this Order it is vertical, 
and concealed under the postdorsolum *. 
We are now to consider the parts that constitute the 
postpectus or under-side of the metathorax, and which 
bears the posterior pair of legs. 
See above, p. 570. > Prate IX. Fic. 15. k". 
* Ibid. Fie, 11. #’. 4 Tpid. Fie. 20. #’. 
