Can evolution explain complete metamorphosis?
In a 1999 issue of Nature, two scientists (James Truman & Lynn Riddiford) presented their hypothesis of how complete insect metamorphosis evolved. In the article the authors tried to explain the evolution of four-stage metamorphosis from three-stage metamorphosis by proposing that the latter actually contains four stages.1 They called this arbitrarily-defined fourth stage the ‘pronymph’, and described it to be a period which always precedes the first moult, and which varies in duration among different species, sometimes ending as soon as the insect hatches out of its egg.2 This ‘pronymph’, they argued, evolved into our modern larva. In plainer terms, some ancient insect hatched out of its egg too soon and began groping around for food. It continued evolving until it could spend many weeks in this premature, caterpillar-like form, before finally metamorphosing into the long-belated nymph stage—which according to the authors had shortened and evolved into our modern pupa. One fatal probl
