It has been suggested, therefore, that the
primitive insect returned to the water, as the whale and seal did
in the struggle for life of a later period. The fact that the
mayfly and dragon-fly spend their youth in the water is thought
to confirm this. Returning to the water, the primitive insects
would develop gills, like the Crustacea. After a time the stress
of life in the water drove them back to the land, and the gills
became useless. But the folds or scales of the tough coat, which
had covered the gills, would remain as projecting planes, and are
thought to have been the rudiment from which a long period of
selection evolved the huge wings of the early dragon-flies and
mayflies. It is generally believed that the wingless order of
insects (Aptera) have not lost, but had never developed, wings,
and that the insects with only one or two pairs all descend from
an ancestor with three pairs.
The early date of their origin, the delicacy of their structure,
and the peculiar form which their larval development has
generally assumed, combine to obscure the evolution of the
insect, and we must be content for the present with these general
indications.
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