Development of the wing discs of Zophobas atratus under natural and experimental conditions: occurrence of a gradual larval-pupal commitment in the epidermis of tenebrionid beetles.

Using light and electron microscopy, we studied the development of the wing discs in the large beetle Zophobas atratus, under natural and experimental conditions. A reversible differentiation of the wing discs is usually observed during supernumerary instars of crowded larvae. Juvenile hormone analog (JHA) application during the wandering period or compelled experimental crowding during the larval-pupal switchover - or commitment - inhibits the onset of metamorphosis. Isolation, followed by recrowding, also induces the disc cells to secrete unusual cuticular material. Recrowding is able to trigger the reversal of metamorphosis during the 4-day period when larval-pupal commitment is taking place. Likewise, feeding behaviour which normally stops at commitment often recovers. Ecdysis of intermediate instar animals (prothetelic larvae) corroborates the occurrence of a temporal and spatial variation to commitment, unique to each organ. All these data lead us to consider this 4-day period, which we have called the C period or commitment period, extending from the wandering stage (the previous T period) to the crooked posture stage (i.e. from eyestage 4 to 7) as the physiological time during which the larval organs are gradually committed to differentiate into pupal organs.