Initiation of Metamorphosis in the Pitcher‐Plant Mosquito: Effects of Larval Growth History

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Abstract
Variable growth and development were imposed on the pitcher—plant mosquito. Wyeomyia smithii, to test whether the initiation of metamorphosis was determined early in larval life or remained flexible throughout development. Like anurans, larval W. smithii are specialized for a suspension—feeding way of life and they metamorphose into dispersing, reproductive adults. We therefore used models originally formulated in an amphibian context to examine the effects of modified larval growth on the triggering of metamorphosis and then used our results to reexamine metamorphosis in amphibians. Response to enhanced and decreased food regimens showed that larval growth during the first two (of four) instars affected both the time to and mass at metamorphosis. The major effects of developmental inertia are prominent for about one to two instars and then abate, but do not necessarily disappear, with each succeeding instar. Despite evidence for developmental inertia in W. smithii, experiments transferring larvae from high food to starvation show that the physiological commitment to undergo metamorphosis does not take place until the last (fourth) instar and that nonzero growth during the last instar is required to trigger metamorphosis. All amphibian models for the initiation of metamorphosis involve the effects of rates: developmental rate and past or present growth rates. In W. smithii, there is no primacy of developmental rate and low growth rate does not stimulate metamorphosis of larvae having attained the minimum mass required for metamorphosis. An insect—derived model involving the effects of size—specific growth increments is more consistent than any amphibian model with our results in W. smithii. Testing the reciprocal ability of the insect model to predict amphibian metamorphosis is not possible with current data, because no study has considered zero growth in amphibians to identify the transition from growth—dependent to growth—independent development leading to metamorphosis. This transition marks the irrevocable commitment of the organism to an ontogenetic niche shift from an aquatic larva to a terrestrial, dispersing, and reproducing adult. Identifying the proximal causes of this mechanism is fundamental to understanding how flexible growth and ontogeny of complex life cycles have adapted to variable larval environments.

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