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

## TL;DR

This study examines the structure and function of a sensory complex in the leg of a stick insect, revealing how it detects vibrations through specialized organs.

## Contribution

The study provides the first detailed neuroanatomical and functional analysis of the subgenual organ complex in the stick insect Bacillus rossius using axonal tracing and micro-CT.

## Key findings

- The subgenual and distal organs in B. rossius contain about 70 sensilla and are located in the hemolymph channel.
- The distal organ connects to the tibia and subgenual organ, suggesting dual pathways for vibration detection.
- The subgenual organ has a single nerve branch, confirming a consistent sensory organization in stick insects.

## Abstract

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.

The subgenual organ complex in orthopteroid insects consists of few mechanosensory organs. In the stick insect Bacillus rossius, the subgenual organ and distal organ include ∼70 sensilla, and both organs are located in the hemolymph channel with several connections to the leg cuticle. The neuroanatomy and functional morphology of the sensory organs resemble that of other stick insects and support a shared vibrosensory function.