564 EXTERNAL ANATOMY OF INSECTS. 
pressed and sometimes membranous part: in Vespa, &c. 
a small subtriangular piece, just below the base of the 
upper wing, is probably its analogue?. 
8. Mesosternum®. The central part of the medipectus, 
or that which passes between the mid-legs when ele- 
vated, protended, or otherwise remarkable, is called the 
mesosternum or mid-breast-bone. In the Coleoptera Or- 
der it exhibits the most numerous variations, and is 
usually the most strongly marked of any of the three 
portions of the sternum, affording often important cha- 
racters for the discrimination of genera and subgenera. 
It may be said to be formed upon three principal types 
—the first is, where it is a process of the posterior part 
of the peristethium, and points towards the anus or the 
head ;—the second, where it is a process of the anterior 
part of the mesostethium, and points only towards the 
head: in this case there is no suture to separate the 
medipectus from the postpectus ;—the last type is where it 
is a ridge formed by a process both of the pertstethium 
and mesostethium meeting between the legs; an example 
of this you will see in the common dung-chafer, (Geo- 
trupes). Upon the two first of these cases I shall offer 
a few remarks ;—-the last affording no variation need 
only be mentioned. 
If you examine the terrestrial Predaceous beetles 
(Geodephagena) you will find that the peristethium is 
usually flat, terminating towards the postpectus in a kind 
of fork, the sinus of which receives the anterior point of 
the mesostethium—this is the mesosternum; but in the 
aquatic insects of this tribe, at least in Dytiscus margi- 
@ Prate IX. Fie, 12, 0’. » Prarte VIII. Fic. 3, 13. p’. 
