Hearing in hooktip moths (Drepanidae: Lepidoptera)

This study presents anatomical and physiological evidence for a sense of hearing in hooktip moths (Drepanoidea). Two example species, Drepana arcuata and Watsonalla uncinula, were examined. The abdominal ears of drepanids are structurally unique compared to those of other Lepidoptera and other insects, by having an internal tympanal membrane, and auditory sensilla embedded within the membrane. The tympanum is formed by two thin tracheal walls that stretch across a teardrop-shaped opening between dorsal and ventral air chambers in the first abdominal segment. There are four sensory organs (scolopidia) embedded separately between the tympanal membrane layers: two larger lateral scolopidia within the tympanal area, and two smaller scolopidia at the medial margin of the tympanal frame. Sound is thought to reach the tympanal membrane through two external membranes that connect indirectly to the dorsal chamber. The ear is tuned to ultrasonic frequencies between 30 and 65 kHz, with a best threshold of around 52 dB SPL at 40 kHz, and no apparent difference between genders. Thus, drepanid hearing resembles that of other moths, indicating that the main function is bat detection. Two sensory cells are excited by sound stimuli. Those two cells differ in threshold by approximately 19 dB. The morphology of the ear suggests that the two larger scolopidia function as auditory sensilla; the two smaller scolopidia, located near the tympanal frame, were not excited by sound. We present a biophysical model to explain the possible functional organization of this unique tympanal ear.