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 præ- 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. But the ancestors of insects looked more like
thousand-legs or centipedes, and here head and thorax are much less
distinct. But in the annelid the mouth is on the second segment;
here it is on the fourth. It has evidently travelled backward. That
the mouth of an animal can migrate seems at first impossible, but if
we had time to examine the embryology of annelids and insects, it
would no longer appear inconceivable or improbable. And its backward
migration brought it among the legs which were grasping and chewing
the food. And in vertebrates the mouth has changed its position,
though not in exactly the same way. Our present mouth is probably
not at all the mouth of the primitive ancestor of vertebrates. Thus
in the insect three segments have fused around the mouth, and three,
possibly four, in front of it. This makes a head worthy of the name.