STRATEGIES FOR HEARING IN NOISE - PERIPHERAL CONTROL OVER AUDITORY-SENSITIVITY IN THE BUSH-CRICKET SCIARASAGA-QUADRATA (AUSTROSAGINAE, TETTIGONIIDAE)

The carrier frequency of the call of the Australian bushcricket Sciara saga quadrata is unusually low for tettigoniids at 5 kHz. The sound tr ansmission measured ire the habitat of the insect reflects the advanta ges of producing low-rather than high-frequency signals; attenuation i s explained almost entirely by the spherical spreading of sound. The n atural vibration frequency of the wings is controlled by size and stif fness. The small tegmina are unusually fleshy, with an order-of-magnit ude higher water content than in comparable sagine species. Reduced st iffness allows the insect to call at low carrier frequencies, albeit a t lower intensity levels (60 dB SPL at 1m), than bushcrickets of compa rable size (80-90 dB SPL). The responses of the tympanic nerve and a f irst-order interneurone (omega neurone) in the afferent auditory pathw ay showed that the hearing system is most sensitive to frequencies of 15-20 kHz, an effective mismatch to the conspecific call resulting in a reduced sensitivity of approximately 20 dB at the carrier frequency of the call. S. quadrata can occlude its spiracular opening, which inc reases the sensitivity of the ear to lower frequencies. Under such con ditions, the best frequency of the ear matched that of the carrier fre quency of the call. We measured the activity of auditory neurones in t he field, noting the ability of the open and partially closed ear to f ilter out potentially masking calls of congenerics. At the same time, the directionality of the system was only slightly reduced in the clos ed relative to the open spiracle status. We discuss the evolutionary a dvantages of an insect both calling with a low carrier frequency and h aving the ability to close down the tracheal system to avoid the effec ts of masking. In addition, we consider the advantages of such a signa lling system in avoiding the most significant predator of the male, th e ormine fly Homotrixa alleni.