Physiological development of a monosynaptic connection involved in an adult insect behavior.

Locust flight is an exclusively adult behavior whose neural basis has been extensively studied. The coordinated neural pattern underlying this behavior appears rapidly at the end of postembryonic development. This paper examines the ontogeny of elements of the nervous system involved in the behavior. Alternative extreme hypotheses are: 1) the neurons and synapses involved develop concomitant with the behavior, or 2) they are constructed early in development, and are activated at the appropriate time by, for example, the release of inhibition. These hypotheses were evaluated by selecting a synapse that is important in adult flight, and monitoring its physiological features during postembryonic development. The synapse between the forewing Stretch Receptor (SR) and the First Basalar (BA) motor neuron, two uniquely identified neurons, mediates a monosynaptic reflex which operates only in flight. The EPSP, initiated by SR in BA, was recorded intracellularly during the last four of six postembryonic instars. As early as third instar, the monosynaptic EPSP is present and appears to be as effective as in the adult. It also decrements and summates similarly in younger animals and adults. Therefore, some flight system synapses are present and effective throughout most of postembryonic development, and thus do not develop concomitant with the behavior.