Michael E. Adams
University of California, Riverside
The majority of insects undergo embryonic development within an egg and then advance through a series of immature larval stages culminating in metamorphosis to the adult, reproductive form. It has been suggested by Lynn Riddiford and James Truman that the stunning evolutionary success of insects is attributable to complete metamorphosis, whereby the immature larva, essentially a gut covered with cuticle, is exquisitely adapted for resource exploitation, rapid growth, and avoidance of competition with its conspecific adult, reproductive stage. Metamorphosis is a magnificent transformation of one body form into a completely different one under the control of hormones. Upon reaching the requisite body size, precisely timed hormonal signals are released, committing the animal to a postembryonic rebirth. Carroll Williams summarized this basic developmental process with the following anecdote:
The earth-bound stages built enormous digestive tracts and hauled them around on caterpillar treads. Later in the life-history these assets could be liquidated and reinvested in the construction of an entirely new organism—a flying machine devoted to sex.
Conversion of the energy accumulated by the larval form into the adult during metamorphosis is a fascinating process of organismal remodeling. It is accomplished via programmed cell death of larval-specific cells, reprogramming of others, and postembryonic birth of new cells from imaginal disc tissues upon receipt of precisely timed and coordinated hormonal signals.
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