Early postembryonic morphology of the stick insect subgenual organ complex

Mechanoreceptor organs in hemimetabolous insects are physiologically important not only in the adult insects, but also for sensory functions in the juvenile postembryonic stages. They provide proprioceptive and exteroceptive information, e.g., in locomotion or orientation. In stick insects (Phasmatodea), the subgenual organ complex in the tibia of all legs is an elaborate mechanoreceptor system. This complex contains two chordotonal organs, the subgenual organ and the distal organ. These organs have mainly been studied in adult insects for the neuroanatomy and functional morphology. Here, the sensory organs were investigated in newly hatched Ramulus artemis (Westwood, 1859) to indicate the functional organisation at the beginning of postembryogenesis, when the detection of mechanical stimuli becomes relevant for behaviour. The organs were investigated by axonal tracing for the organ neuroanatomy, innervation, and number of sensilla in the distal organ. In addition, the sensory complex was analysed for the connection by a membrane between the organs. The organs are present after hatching, indicating in particular a possible vibration detection by the subgenual organ. In most cases, the connection between the sensory organs was also present. This indicates the development of sensory neurons and additional tissues during embryogenesis. The sensory neurons in the subgenual organ show a re-organisation, as the dorsal sensilla change from orientation in distal directions to proximal directions. This finding is discussed for implications in vibration detection. The overall results indicate some neuroanatomical modifications during postembryonic development, while the main structures of the subgenual organ complex already originate during embryonic development.