EXTERNAL ANATOMY OF INSECTS. 379 
~terior cavity of the alitrunk, and with the meta- 
phragm it forms the posterior cavity. 
b. PostporsoLum (the Postdorsolum). The middle- 
piece between the mesophragm and the Postscu- 
tellum. In Coleoptera it consists of a tense elastic 
membrane, which is quite covered by the Meso- 
thorax *. 
c. PostscuTELLUM (the Posétscutellum). A narrow chan- 
nel running from the Dorsolum to the Abdomen in 
Coleoptera, forming an isosceles triangle reversed. 
In other orders it is either a triangular elevation of 
the middle of the posterior part of the Postdorsolum, 
or a distinct triangular piece ». 
d. Postrr#£num (the Postfrenum). In Coleoptera the 
part of the Metathorar in which the Postscutellum 
lies, at first nearly horizontal, but posteriorly it takes 
a vertical direction towards the abdomen. In ge- 
neral it may be defined, the part that intervenes 
between the Postscutellum and the Abdomen ; and 
which in many cases is connected with the posterior 
basal margin of the under-wings, and prevents their 
being pushed too far forwards ©. 
e. PLEeuR# (the Pleure). The space behind the scapu- 
lars, on which the lower organs of flight are fixed ¢. 
They include the Ale Inferiores. 
A, Aum Inrreniores (the Under-wings). The lower or 
secondary pair of organs for flight ©. They include 
the Commissura, Tendo, Hamuli, Pterygium, Alula, 
and Halteres. 
@ Prave VIII. Fic. 3, 12, 2f and IX. Fic. 7, 10—12, 15, 20. ¢'. 
> Thid. x’. © Prates VIL. IX. XXVIIL v’. 
4 Prates VIII, and IX. w’. ¢ Pirate X. Fie, 4, 10. 
