Morphological coupling of the distal organ in the Peruvian walking stick (Oreophoetes peruana): Structural and functional aspects

In insects, the detection of mechanical stimuli from body movements, airborne sound, substrate vibration, medium flow, or gravity by mechanosensory organs plays an important role. These mechanosenory organs can have complex morphologies with numerous sensilla, and the functional morphology with specific attachments of the sensory neurons to surrounding tissues and structures determines the stimulation. In stick insects, the subgenual organ complex in the tibia of all legs is an elaborate system of two chordotonal organs, which respond to substrate vibrations, and associated tibial campaniform sensilla, which respond to cuticular strain. One chordotonal organ, the distal organ, is characterized by a linear set of sensilla. This distal organ has not been studied for its physiological characteristics in detail, but the attachment or mechanical coupling is functionally important. Here we characterize two aspects of attachment or mechanical coupling of the distal organ: At the dorsal side, the organ is connected to the inner side of the dorsal cuticle by connective tissue, which is shown to also contain the axons of campaniform sensilla. At the proximal end, a fine membrane runs to the adjacent chordotonal organ, the subgenual organ. This membrane spans the tibia in transverse direction. It does not contain neuronal elements, but as a connection between the subgenual and the distal organ, it may influence the mechanosensory activity of these organs. Such a connection is not present in other insects such as locusts or cockroaches and could affect the sensory function in stick insects (e.g., in vibration detection by the subgenual organ) or even couple the two organs, resulting in similar mechanical responses.