OF INSECTS. 
195 
it is to retain that portion of the central canal in its 
place. The brief enumeration now given refers to 
masticating insects alone; it will readily be understood 
that the muscles are greatly modified in sucking insects, 
since in these the parts of the mouth have undergone 
an important change. 
Muscles of the Thorax . — The muscular system of 
the thorax has for its office to unite the component 
segments of that portion of the body, and to furnish 
a motive apparatus for the organs of locomotion, the 
wings and legs. They differ somewhat according as 
the prothorax is free or connate with the succeeding 
segments. In the former case, (as among the Cole- 
optera, for example,) the principal mass of the muscles 
is concentrated in the prothorax, and they are almost 
wholly employed in effecting its free movements, there 
being no wings to he supplied. In general, four pairs 
can be detected, which have their posterior attachment 
in the mesothorax, and an orbicular one accompanies 
each spiracle. The mesothorax, in the kind of insects 
of which we now speak, is provided with three pairs 
of muscles, all of which have greater or less influence 
on the motions of the anterior wings. A like number, 
and acting mediately in a similar manner, is found in 
the metathorax ; the largest, which occupies the upper 
part, assisting to depress the posterior wings, the 
second, placed at its side, to draw them backwards, 
and the third to elevate them. Besides these principal 
muscles, numerous others of smaller size occur in the 
metathorax, consolidating the various pieces of which 
it is composed, and also acting for other purposes. 
