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A beetle is an insect whose adult forewings are modified into protective covers over the hindwings and abdomen. Its larva and adult can occupy different ecological roles, linking metamorphosis to changes in feeding and habitat.

J. F. Gudernatsch tested the effects of feeding mammalian organs to tadpoles. His 1912 report compared diets containing thyroid, liver, thymus and other tissues. Thyroid feeding caused rapid differentiation toward a frog-like form, including early hindlimb development, rather than simply making larger tadpoles.^[ref]

The experiment helped separate bodily growth from developmental transformation and implicated a substance in the thyroid in metamorphosis. It used crude tissue as food, so it demonstrated biological activity without isolating a pure thyroid hormone or identifying its molecular structure.

Metamorphosis is a developmental transformation between substantially different life stages, such as a tadpole becoming a frog or a caterpillar becoming an adult butterfly. It can reorganize feeding structures, movement, habitat use and internal tissues, rather than merely increasing size. Regulated cell differentiation, tissue remodelling and cell death contribute to the transition.

An early experiment showed that feeding tadpoles thyroid tissue accelerated differentiation relative to comparison diets. ^[ref] The intervention connected a secretory organ to the timing of a whole-body transformation. It did not isolate a pure hormone or establish that growth and differentiation are the same process. Modern thyroid hormone biology explains this amphibian example; insect metamorphosis uses a different hormonal system.

A salamander is an amphibian with a typically elongated body and a tail retained in the adult. Development varies from metamorphosis to retention of larval features. This variation makes salamanders useful for comparing biological development and life-history evolution.