They have largely
disappeared. The work of locomotion has been transferred from the
trunk to the legs.
The abdomen of the insect is as clearly composed of distinct
segments as the body of the annelid. Of these there are perhaps
typically eleven. The thorax is composed of three segments, distinct
in the lowest forms, fused in the highest. This fusion of segments
in the thorax of the highest forms furnishes a very firm framework
for the attachment of wings and muscles. These wings are a new
development, and how they arose is still a question. But they give
the insect the capability of exceedingly rapid locomotion.
The three pairs of jaws, modified legs, in the rear half of the head
show that this portion is composed of three segments. For only one
pair of legs is ever developed on a single segment. Embryology has
shown that the portion of the head in front of the mouth is also
composed of three segments. Possibly between the prae- and post-oral
portions still another segment should be included, making a total of
seven in the head. The head has thus been formed by drawing forward
segments from the trunk, and fusing them successively with the first
or primitive head segment. This is difficult to conceive of in the
fully developed insect, where the boundary between head and thorax
is very sharp.
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