Developmental neuroethology of insect metamorphosis.

During metamorphosis, the insect nervous system must change to accomodate alterations in body form and behavior. Studies primarily on moths have shown that these changes involve the death of some larval neurons, the conservation and remodeling of others, and the maturation of new, adult-specific cells. The motor and sensory sides of the adult CNS vary in this regard with the former being constructed primarily from remodeled larval components, whereas the latter arises primarily from new neurons. Neuronal remodeling has received considerable attention. Larval-specific dendritic fields are pruned back during the larval-pupal transition, followed by the sprouting of adult-specific dendrites. Simple reflexes have been used to correlate these neuronal changes with the acquisition or loss of particular behaviors. The loss of the proleg retraction reflex is associated with the regression of the dendritic arbors of the proleg motoneurons. By contrast, expansion of axon arbors of the gin-trap afferents is necessary, but not sufficient, for the assembly of the gin-trap reflex in the pupal stage. The stretch receptor reflex provides a third example in which a new dendritic field in the adult form of a neuron is associated with new adult-specific connections. Interestingly, these connections are masked by persisting larval contacts until the emergence of the adult moth. For the metamorphosis of more complex behavioral circuits, some, such as that for flight behavior, seem to be assembled de novo, whereas others, like that for adult ecdysis behavior, show conservation of some circuit elements from the larval stage but with the superposition of some adult-specific components.