Origin and development of unusual insect muscle tension receptors

This work describes the origin and development of about 200 tension receptor cells located around the anterior attachment site of the locust ovipositor muscle and their migration to their final position on the muscle fibres. The locust ovipositor muscle is the only insect system in which more than 100 tension receptor cells are associated with a single muscle. Neuronal precursors of tension receptors are first detectable by horseradish peroxidase immunohistochemistry in fourth instar larvae. Precursors consist of cell clusters (doublets, triplets and quadruplets) located on the anterior attachment site of the muscle. In the early fifth larval stage, cell clusters are absent, although a few sensory neurons that lie embedded between the muscle fibres are apparent. These neurons send their dendrites towards the anterior end of the muscle fibres and their axons posteriorly. By the fourth day of the fifth larval stage, a large number of cell clusters appears on the anterior muscle attachment site. In addition to these assemblies, cells have been identified that extend long processes running exactly along the lateral margin of the attachment site. These cells are thought to provide navigating cues for migrating tension receptors, since they are absent in later stages. By the end of the fifth larval stage, most of the clusters gradually disappear and increasing numbers of differentiated neurons embedded between the muscle fibres become visible. We conclude that the majority of tension receptors develop during the last larval stage from precursors situated on the muscle apodeme. They then migrate from the apodeme to their final place on the muscle fibres where they assume an appropriate orientation.