Presynaptic terminals persist following degeneration of “flight” muscle during development of a flightless grasshopper

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Abstract
We have studied the development of a neuromuscular system for which mature function has been lost through evolution in the grasshopper, Barytettix psolus (Cohn and Cantrall, 1974). Barytettix is flightless throughout life and has only vestigial wings that are incapable of active movement. Adult Barytettix lack muscles homologous to the indirect flight muscles of locusts and grasshoppers that fly, while other thoracic muscles are similar. We have found, using light and electron microscopic examination of tissues from various developmental stages, that the metathoracic dorsal longitudinal muscle is present and is innervated during nymphal life but is absent in adults. Yet its nerve persists and, in the adult, contains axonal presynaptic specializations opposite inappropriate targets such as glial processes and basal lamina. Our findings indicate that selective muscle death during development is one mechanism underlying the reduction of the flight system of Barytettix through evolution. The finding that presynaptic terminals persist in the absence of the muscle indicates that the muscle and its innervation follow programs of development that are at least partially independent and reinforces the concept that in insects motorneurons, and perhaps neurons in general, are not dependent upon trophic influences from their targets for survival and maintenance of their differentiated phenotype.