Innervation of the subgenual organ in the stick insect Trachyaretaon echinatus: Neuroanatomical differences within Oriophasmata (Phasmatodea)

Neuronal features including the nerve pattern are relatively conserved during evolution and are therefore useful for comparative studies. In stick insects (Phasmatodea), neuronal elements are rarely analysed for different groups. As the morphology of this group is characterised by convergent phenotypes, traits of the nervous system may provide additional information on phylogenetic relationships. Here, the innervation for a mechanosensory chordotonal organ, the subgenual organ in the proximal tibia, is documented by axonal tracing. For species of Neophasmatodea, previous studies showed an innervation for the subgenual organ with a single nerve branch in Occidophasmata (New World stick insects) compared to the Oriophasmata (Old World stick insects) where two separate nerve branches occur at the subgenual organ. Here, the innervation pattern is analysed in Trachyaretaon echinatus (Heteropterygidae: Obriminae), as Heteropterygidae are the sister-group to the remaining Oriophasmata. This species was chosen as representative of Heteropterygidae to test for a homogenous and thus likely conserved nerve pattern within Oriophasmata. The innervation of the subgenual organ shows a single nerve branch. Therefore, different innervation patterns occur within Oriophasmata. The innervation with one single nerve documented for T. echinatus is discussed as potentially plesiomorphic for Neophasmatodea.