Postembryonic development of an insect sensory system: ingrowth of axons from hindwing sense organs in Locusta migratoria

Axon counts have been made from electron micrographs of the hind­wing sensory nerves 1C1 and 1D2 in the adult locust and during develop­ment. In the adult, nerve 1C1 contains approximately 1000 axons. At least a quarter have diameters over 1 µm, more than forty 5-12 µm. Seventy large axons come from the tegula, the rest from the wing. Nerve 1D2 contains 400 axons, 64 between 1 µm and 6.5 µm in diameter. Large axons are assumed to come from the wing base chordotonal organ and stretch receptor, the remainder from thoracic hair fields. During development, axon numbers in nerve 1C1 rapidly increase at the 4th instar, corresponding to the development of the wing bud. By the final moult there are over 2000 axons, half of which disappear in the two weeks after fledging. In nerve 1D2 the stretch receptor and chor­dotonal axons are present from the first instar. Small fibres increase in number mainly in the 5th instar. In contrast to nerve 1C1 there is no change in numbers after fledging. In both nerves, diameters and glial wrapping of axons increase in the two weeks after fledging, although the changes are more marked in nerve 1C1. The large input from the tegula suggests an important rôle in the phasic control of flight. The post-fledging increase in diameter and glial wrappings of tegula axons may influence the increase in wingbeat frequency with age.