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Diapause is a physiological state of dormancy with very specific triggering and releasing conditions; there are various definitions and contexts in which the term is used, but its most common application is in arthropods, especially insects. One of the most explicit definitions, covering many of the important features, is the following:
"a neurohormonally mediated, dynamic state of low metabolic activity. Associated with this are reduced morphogenesis, increased resistance to environmental extremes, and altered or reduced behavioral activity. Diapause occurs during a genetically determined stage(s) of metamorphosis, and its full expression develops in a species-specific manner, usually in response to a number of environmental stimuli that precede unfavorable conditions. Once diapause has begun, metabolic activity is suppressed even if conditions favorable for development prevail."
Of primary importance is that diapause is not only induced by specific stimuli, but - once initiated - only certain stimuli are capable of releasing the organism from this state; the latter is essential in distinguishing diapause as a different phenomenon from hibernation, for example.
A similar phenomenon occurs in the seeds or other resting stages of various plants. In the eggs of various vertebrates there is a phenomenon sometimes known as "embryonic diapause", which is also termed "delayed implantation," and is not directly equivalent to the phenomenon in arthropods, though in both cases there is a cessation of metabolic activity.
The definition above specifically includes the phrase "altered or reduced behavioral activity" to accommodate for the related phenomenon of reproductive diapause; this is a physiological condition where the organism itself will remain fully active, including feeding and other routine activities, but the reproductive organs experience a tissue-specific reduction in metabolism (with characteristic triggering and releasing stimuli). This is widespread in insects, including common and well-known species such as the Monarch butterfly.