The scolopidial accessory organ in Gonatoxia bush-crickets (Tettigonioidea, Phaneropterinae, Holochlorini)

The scolopidial accessory organ is a relatively small and compact chordotonal organ consisting of mechanosensory scolopidial sensilla. The accessory organ is placed in the tibia next to the larger subgenual organ in several Orthoptera. In other orthopteroid insects, the accessory organ detects substrate vibrations in low frequency ranges. Among Tettigonioidea katydids or bush-crickets, the accessory organ has been described so far in only a few species. The peripheral nerve pattern for the two chordotonal organs usually includes two separate nerve branches for each sensory organ. Within the Phaneropterinae (leaf katydids or sickle-bearing bush-crickets), some differences occur as the accessory organ can share a single nerve branch with the subgenual organ. Here, the accessory organ in two phaneropterine species of the East African genus Gonatoxia , G. immaculata Karsch, 1889, and G. helleri Hemp, 2016, is investigated. The aim is to document the accessory organ and its nerve pattern for further tettigonioid species and to provide a first report for species from the Holochlorini. The accessory organ was present in both species in all leg pairs. The number of sensilla was 5–12. The sensillum numbers were slightly lower in G. helleri than in G. immaculata . The accessory organ has a nerve branch that is separate from the nerve branch for the posterior subgenual organ. This neuroanatomy, including the nerve pattern, is similar to most other tettigonioids. The findings support the occurrence of the accessory organ among the different subgroups of Phaneropterinae, and in Tettigonioidea at large, and raise the possibility of behaviorally relevant vibrational inputs.