568 EXTERNAL ANATOMY OF INSECTS. 
metathorax is the postdorsolum, which presents itself 
under very different forms and circumstances in the dif- 
ferent Orders. In the Coleoptera it is intirely covered 
by the dorsolum and scutellum ; it is generally more or less 
of a membranous substance, or partly membranous and 
partly corneous, which enables it to yield more to the 
action of the wings in flight; it is usually an ample 
transverse piece with tumid sides*; but in the Scara- 
beide it is short though very wide; and in Cychrus, 
and probably other apterous beetles, it is extremely mi- 
nute and almost obsolete. In the Orthoptera Order, I 
observe once for all, the part in question, as well as 
the postscutcllum and postfrenum are mere counterparts 
of the dorsolum, scutellum, and frenum, except that in 
some cases they are larger’. In the Heteropterous He- 
miptera at first sight it would appear that all the parts of 
the metathorax were altogether wanting or absorbed in 
the ample scwéellum ; but if you remove this with care, 
you will find under it their representatives, its lower sur- 
face being hollowed out to receive them. The postdorso- 
lum appears in these as a transverse obtusangular band; 
in the Nepide, Notonectide, &c. the three parts of the 
metathorax seem united into a single plate, emerging la- 
terally from under the scutellum below the frenum ; in 
which, however, some traces of a distinction between 
them may be discovered. In the Homopterous section 
the Fulgoride exhibit these pieces very distinctly, cover- 
ed only at the base by the mesothorax: but in Cicada 
they are not so easily detected ; they exist however as a 
narrow strip or band, almost concealed by that part. 
® Prare Vil. Fie. 3.°¢. 
> Ibid. Fic, 12. Comp. 7, A’, U’, with ¢, uv, v'. 
