STRUCTURES IMPLICATED IN METAMORPHOSIS. 
7 
second and third rings support the wings. The presence of legs 
and wings involves a considerable development of the segments 
of the thorax, and particularly of the second and third, for there 
must be abundance of space within for the passage and attachment 
of the muscles, which, influenced by the will of the insect, move 
the legs and the organs of flight. 
The description of the parts of the thorax is not difficult to 
understand. 
The first segment of the thorax which articulates with the 
back of the head is called the prothorax ; the second, to which 
the first pair of wings is attached, is the mesothorax ; and the 
third, bearing the second pair of wings, is the metathorax. The 
method of the formation of the abdominal segments is the key 
to that of those of the thorax. The great development of the 
mesothorax and metathorax does not usually take place until 
chrysalis life has progressed. 
The larva of the Calosoma has its thorax well armoured. 
Above, the three segments are clothed — as is the case with the 
abdominal rings — with a double dorsal plate with the small lateral 
piece on either side. This is the upper arch. The dorsal plate 
is very large, but it does not exhibit any cross markings or 
any transverse division, but in the adult beetle four distinct parts 
are added to the two segments that support the wings. The 
rings of the under part of the thorax of the same larva are prin- 
cipally membranous. A small plate which is to be observed in 
the centre of each is the sternal piece. It is very small and 
rudimentary, but it becomes very large and fully developed in 
the beetle. 
The mesothorax and metathorax become very large in pro- 
portion to the rest of the body in the insects which have wings, 
and are often so united by growth that they cannot be separated. 
The mesothorax, which supports the first pair of wings, is almost 
always better developed than the metathorax, and the constituent 
parts of the first ring are generally more distinguishable than 
in the last. 
The dorsal piece, so simple in the larva, has four transverse 
divisions in the beetle, marked out either by ridges or grooves. 
They can be readily distinguished in the Calosoma sycophant a 
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