ANATOMY AND PHYSIOLOGY. 3S 
the different bones. And not a little additional 
difficulty has been thrown over the subject by con- 
fusion in the nomenclature of the various parts of 
the thorax. This difficulty pervades even the most 
elementary descriptions, the same word being often 
used in a different sense by different writers; and I 
can scarcely expect to have escaped altogether from 
this source of confusion, although it is only with the 
chief divisions, about which nearly all are agreed, 
that we are here concerned. 
Fig. 2.—Diagram of the divisions of the thorax traced on its dorsal 
aspect. 
a, prothorax, 3 scutellum. 
6, mesothorax, 2 scutum. 
Se 3 scutellum. 
c, metathorax, 2 scutum. 
=f 3 scutellum. 
The darkest lines indicate the divisions of the 
segments. The next shade denotes the subdivi- 
sions. The finest lines mark the boundaries of 
the yellow patches which are seen on the smaller 
wasps. 
The first larval segment, as we have already 
seen, is absorbed in the formation of the head of the 
perfect insect.. The thorax accounts for the three 
next segments, namely the second, third, and fourth, 
which constitute severally the pro-, meso-, and meta- 
thorax. Each of these segments is subdivided into 
four rings more or less distinct in different species, to 
which the names of pree-scutum and scutum, scutel- 
lum and post-scutellum have been given. And each 
of these has again been divided into a dorsal and 
a sternal, that is to say, as the insect stands on its 
legs, an upper, and a lower or lateral portion. Alto- 
gether, on the simplest mode of arrangement, there 
are twenty-four distinct pieces into which the insect 
