Synopsis The problem of insect metamorphosis has inspired naturalists for centuries. One question that often arises is why some insects, such as butterflies and bees, undergo a fairly radical metamorphosis while others, such as crickets and lice, do not. Even before the concept of homology emerged scientists speculated which stage found in more direct-developing insects would correspond with the pupal stage of metamorphosing insects. William Harvey (1651) considered the pupal stage to be a continuation of embryonic events, calling it a "second egg." Since then variations of this idea have emerged over the centuries of scientific research and have been supported by a wide variety of methods and rationales. This review will follow those ideas and the ideas that emerged in opposition to them to the present state of the field.
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