Aquatic adults seldom have terrestrial juveniles. Respiratory problems may lead to tropical species being smaller than related temperate ones. Suctorial feeding on higher plants seems unknown. Cryptic coloration is common, aposematic, and epigamic rare. Flightlessness is common, but potentially flying forms must be generally available; this restricts tracheal gills to juveniles.
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G. Evelyn Hutchinson (1981) studied this question.
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