EXTERNAL ANATOMY OF INSECTS. 577 
so remarkable nor important as the mesosternum. In 
Bolbocerus it is a rhomboidal elevation: in Gyrinus a 
ridge; as also in many Hydrophili, in which it passes 
between the hind-legs to the abdomen, and terminates in 
asharp point*; and in Dyfiscus its two diverging lobes 
cover the base of the posterior trochanters>. In the 
Orthoptera Order this part is not remarkable; but in 
Acrida viridissima it consists of three triangular pieces, 
the lateral ones being erect, and the intermediate one ho- 
rizontal: in Locusta Leach it resembles the mesosternum®. 
In the Heteropterous Hemiptera the whole mesostethium 
is elevated, and terminates at both ends in a fork, the 
anterior one receiving the point of the promuscis, and the 
posterior one that of the epigastrium : in the Homopte- 
rous section, the Cicade have usually a distinct meta- 
sternal point between their hind-legs. In the remain- 
ing Orders there is no metasternum, or no remarkable 
one, except in one singular Hymenopterous genus, 
Ewania, the parasite of the Blatte 4, in which there is a 
forked posterior process of the mesostethium with recurved 
points. 
17. Opercula®. By this term I distinguish those 
plates, before largely described ‘, which cover the drums 
of male Cicade ; and likewise those called also by the 
same name by M. Chabrier £, which cover, in many 
* Prate VIII. Fic. 8. a ft. b. De Geer iv. z. iv. f..3. dd. ec. 
©, Piare VIII. Fic. 13. @ fF. 
* The history of this parasite has been traced by Dr. Reid; but 
alas! this learned and acute observer of nature did not live to give 
his discoveries to the world: it is hoped, however, they will not be 
lost, being in most able hands. 
© Prate VIII. Fic. 18. and XXII. Fic. 13. e+. 
£ Vor. II. p. 401—. ®& Sur le Vol des Ins. c. i. 459, 
VOL. III. 2p 
