6533b856fe1ef96bd12b2f0a

RESEARCH PRODUCT

The onset of metamorphosis in Tenebrio molitor larvae (Insecta, Coleoptera) under grouped, isolated and starved conditions

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Abstract The post-embryonic development of the beetle Tenebrio molitor presents a variable number of larval instars. Several parameters (instar length, time of apolysis and cuticle deposition) were compared during the larval-larval and larval-pupal cycles of mealworms over 50 mg, reared in grouped or isolated conditions. In grouped conditions comparable to mass breeding, larval-larval and larval-pupal apolyses were found to occur at the same time, but instar duration was longer in the case of prepupae. However, isolation was found to accelerate larval-pupal (but not larval-larval) apolyses and to reduce the number of larval instars, whereas starvation inhibited larval-larval (but not larval-pupal) development by delaying or preventing larval apolysis. Our observations confirm that the onset of metamorphosis is linked to a loss of sensitivity to starvation and to an increased requirement of isolation. In isolated conditions, larval-pupal apolysis was found to be preceeded by a characteristic behaviour (transitory period), homologous to the wandering stage of Lepidoptera. Ecdysteroid titres, measured with a radioimmunoassay, showed a very different pattern in a larval-larval cycle (a single high peak beginning at apolysis) compared to a larval-pupal one (several smaller peaks, the first one during the transitory period). The involvement of ecdysteroids and other hormones in the control of developmental variability is discussed.