The Mechanosensory Subgenual Organ Complex in the Stick Insect Bacillus rossius (Phasmatodea): Neuroanatomy and Functional Morphology.

The subgenual organ complex is an elaborate mechanosensory complex in the insect leg containing chordotonal organs. In stick and leaf insects (Phasmatodea), it includes the subgenual organ and the distal organ. This study documents the neuroanatomy and functional morphology of the subgenual organ complex in the stick insect Bacillus rossius (Bacillidae: Bacillinae) by axonal tracing and micro-computed tomography. It also considers the first report on the subgenual organ complex in stick insects that reported a relatively simple organization of the sensory organ and its nerves on the basis of histological sections. Our findings show the neuroanatomy and nerve pattern of the subgenual organ complex in B. rossius with a subgenual organ and a distal organ. The subgenual organ is placed in the hemolymph channel. The distal organ is also located in the hemolymph channel, and it has several attachment elements, linking it to the cuticle of the tibia, the tibial tracheae, and the subgenual organ. The connections to the tibia may form an input pathway for vibrations transmitted over the cuticle, whereas the position in the hemolymph channel and the connection to the subgenual organ indicate a mechanical activation by vibrations transmitted via the hemolymph. Overall, the axonal tracing preparations document neuroanatomical details for B. rossius and resolve the numbers of sensilla in the sensory organs, the length of the distal organ, and confirm a single nerve branch for the subgenual organ. The data provide support for a consistent organization of the subgenual organ complex within stick insects.