20 
POPULAR SCIENCE REVIEW. 
dissection, however, this piece can scarcely be regarded as a 
plate forming a portion of the crust of the segment, so much as 
a process of the same, bent down behind the coxa, to assist in 
forming the acetabulum, or place of lodgment for the latter, and 
anchylosed with the posterior extremity of the ventral plate 
between the legs. 
It will be impossible, within the limits of this paper, to fully 
illustrate all the parts of which the thorax is composed. The 
two following segments are covered by the elytra and wings, 
which must be removed in order to reveal them. Having done 
this, we may find some difficulty at first in separating the 
mesothorax from the segment which follows, so firmly are they 
united to give solidity and strength to the body of the insect. 
By carefully bearing in mind, however, that this segment bears 
the elytra and intermediate legs, the separation can be effected ; 
and it will then be found that it is perfectly distinct. Its most 
prominent feature on the dorsal surface is the small triangular 
plate, which is just visible between the elytra before these are 
removed, and is termed the scutellum (figs. 1 and 3, scl.) We 
cannot dwell upon this segment further than to observe that, in 
consequence of the great development of the prothorax in front, 
and of the metathorax behind, it is, so to speak, almost crushed 
out between them, in conformity with a general law of animal 
organization, which requires that the excessive enlargement of 
any one part shall be accompanied by a corresponding diminu- 
tion in the parts adjoining it. In the Metabola, as has already 
been noticed, precisely the opposite of this occurs, the meso- 
thorax being predominant at the expense of the other segments. 
This segment, it should be noted, is composed of the same con- 
stituent plates as the preceding and following ones. 
The metathorax is the most highly developed segment of 
the thorax in the Coleoptera ; to it is assigned especially the 
function of flight, and the segment itself, and the great system 
of muscles contained within it, are both specially designed for 
this end. The dorsal plate is shown in PI. II. fig. 8. And 
here we may take occasion to notice the subdivision of its sur- 
face into four transverse parts, which Audouin, to whom we 
owe so much, was the first to point out. He regarded them as 
evidences of the ultimate subdivision of the elemental insect- 
ring into so many subsegments. These parts are perhaps 
better seen in the mesothorax of the Metabola, but are capable 
of demonstration here. They received from Audouin the fol- 
lowing names, viz. the pra3scutum, scutum, scutellum, and post- 
scutellum. Of these the scutum is that broad portion to which 
the wings are attached; following this is the scutellum, the 
anterior limit of which is not very obvious in this segment. 
This portion, however, is very well marked in the mesothorax, 
