Sound production and hearing in the blue cracker butterfly Hamadryas feronia (Lepidoptera, Nymphalidae) from Venezuela

Certain species of Hamadryas butterflies are known to use sounds during int eractions,vith conspecifics, We have observed the behaviour associated with sound production and report on the acoustic characteristics of these sound s and on the anatomy and physiology of the hearing organ in one species, Ha madryas feronia, from Venezuela, Our observations confirm previous reports that males of this species mill take flight from their tree perch when they detect a passing conspecific (male or female) and, during the chase, produ ce clicking sounds. Our analyses of both hand-held males and those flying i n the field show that the sounds are short (approximately 0.5s) trains of i ntense (approximately 80-100 dB SPL at 10 cm) and brief (2-3 ms) double-com ponent clicks, exhibiting a broad frequency spectrum with a peak energy aro und 13-15 kHz, Our preliminary results on the mechanism of sound production showed that males can produce clicks using only one wing, thus contradicti ng a previous hypothesis that it is a percussive mechanism. The organ of he aring is believed to be Vogel's organ, which is located at the base of the forewing subcostal and cubital veins. Vogel's organ consists of a thinned r egion of exoskeleton (the tympanum) bordered by a rigid chitinous ring; ass ociated with its inner surface are three chordotonal sensory organs and enl arged tracheae, The largest chordotonal organ attaches to a sclerite positi oned near the center of the eardrum and possesses more than 110 scolopidial units. The two smaller organs attach to the perimeter of the membrane. Ext racellular recordings from the nerve branch innervating the largest chordot onal organ confirm auditory sensitivity with a threshold of 68 dB SPL at th e best frequency of 1.75 kHz. Hence, the clicks,vith peak energy around 14 kHz are acoustically mismatched to the best frequencies of the ear, However , the clicks are broadbanded and even at 1-2 kHz, far from the peak frequen cy, the energy is sufficient such that the butterflies can easily hear each other at the close distances at which they interact (less than 30 cm), In H. feronia, Vogel's organ meets the anatomical and functional criteria for being recognized as a typical insect tympanal ear.