Low-frequency vibration in cave crickets
Vibrational communication is common in insects and often includes signals with prominent frequency components below 200 Hz, but the sensory adaptations for their detection are scarcely investigated. We performed an integrative study of the subgenual organ complex in Troglophilus cave crickets (Orthoptera: Rhaphidophoridae), a mechanosensory system of three scolopidial organs in the proximal tibia, for mechanical, anatomical and physiological aspects revealing matches to low frequency vibration detection. Microcomputed tomography shows that a part of the subgenual organ sensilla and especially the accessory organ posteriorly in this complex are placed closely underneath the cuticle, a position suited to evoke responses to low-frequency vibration via changes in the cuticular strain. Laser-Doppler vibrometry shows that in a narrow low-frequency range the posterior tibial surface reacts stronger to low frequency sinusoidal vibrations than the anterior tibial surface. This finding suggests that the posterior location of sensilla in tight connection to the cuticle, especially in the accessory organ, is adapted to improve detectability of low-frequency vibration signals. By electrophysiological recordings we identify a scolopidial receptor type tuned to 50–300 Hz vibrations, which projects into the central mechanosensory region specialised for processing low-frequency vibratory inputs, and most likely originates from the accessory organ or the posterior subgenual organ. Our findings contribute to understanding of the mechanical and neuronal basis of low-frequency vibration detection in insect legs and their highly differentiated sensory systems.