The fate of persisting thoracic neurons during metamorphosis of the meal beetle Tenebrio molitor (Insecta: Coleoptera).

A set of motor neurons and interneurons in the thoracic nervous system of the meal beetle Tenebrio molitor L. is described that persist during metamorphosis. The motor neurons under discussion innervate the thoracic ventral longitudinal muscles and were identified by retrograde transport of intramuscularly injected horseradish peroxidase. Persisting motor neurons exhibit a complex repetitive pattern that changes only slightly during development. Additionally, the characterization of serotonin-immunoreactive neurons defines a complex set of interneurons that also persist throughout development. The fate of these identified neurons is outlined in detail with special reference to variations in their dendritic arborizations. All motor and interneurons are affected by a similar change in their shape during development. The larval neurons lack the contralateral arborization that is found in the adult beetle and is already distinguishable in the prepupa. Essentially only quantitative changes of the neuronal shape were observed during the pupal instar. No pupa-specific degeneration of certain axo-dendritic structures of these neurons was found. Removal of descending interneurons by sectioning the promesothoracic connectives causes specific degeneration of the dendritic tree of an identified serotonin-immunoreactive interneuron.