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henryi males indeed pick larger leaves when given a choice, make holes that match their wing area, and position the holes centrally. Unlike mole crickets, which fine-tune their burrows through trial and error, the tree crickets seem to get it right on the first try (N. Mhatre, personal communication).
Enhancing Sound Reception
As mentioned earlier, the small size of insect ears limits their ability to capture acoustical energy from the environment. As for sound production, insects have evolved adaptations that mitigate this problem.
Resonance
The eardrums of crickets are resonantly tuned to the domi- nant frequency of conspecific song (Paton et al., 1977). Simi- larly, the antennae of male mosquitoes resonate at a frequency near that of the flight tones of conspecific females, which serve as mate recognition signals (Göpfert et al., 1999). Frequency- matched resonance ensures efficient transfer of acoustic en- ergy from air to ear, selectively increasing sensitivity to the sound frequency that matters.
Katydid Ear Trumpets
Unlike signals of mosquitoes and crickets, katydid songs are often rather broadband, spanning frequencies from a few ki- lohertz well into the ultrasonic range. Accordingly, katydid eardrums tend to be broadly tuned, limiting the utility of resonance as a means to enhance sensitivity. The eardrums receive acoustic input via two routes: directly from the sound source to the external surface of the eardrum and indirectly through a specialized portion of the respiratory system. In- sects breathe through a network of tracheae, which are tubes that deliver air from external openings called spiracles direct- ly to internal tissues. A specialized “acoustic trachea” extends from the exterior of the animal to the internal surface of the eardrum. In many species, the cross-sectional area of the tra- chea tapers exponentially from a relatively large spiracular opening, about 1-2 mm depending on species, to its smaller eardrum-associated end, about 0.1 mm, thereby concentrat- ing acoustic energy. The acoustic trachea thus functions as an ear trumpet and is the dominant source of acoustical input to the eardrum. An infinitely long exponential horn transmits sound in a frequency-independent manner above a cutoff fre- quency determined by the rate of flare of the horn (Beranek and Mellow, 2012). The cutoff frequencies of katydid trache- al horns are low enough not to impede the transmission of songs. The horns are, of course, not infinite, and as a result, there are ripples in the transmission function amounting to a few decibels in magnitude. Nevertheless, both calculations
and measurements show a gain in acoustical power at the eardrum, amounting to some 10-20 dB over the rather broad spectra of katydid songs (Hoffman and Jatho, 1995).
Active Mechanics
The resonances of eardrums or antennae and the ear trumpets of katydids are passive mechanisms to enhance sound recep- tion; they improve sensitivity but do not require energy to do so. A fundamentally different sort of mechanism, active am- plification via input of mechanical energy to the receiver, has recently been found in three groups of insects: mosquitoes, fruit flies, and tree crickets (Mhatre, 2015). Active amplifi- cation has long been known in the ears of vertebrates (Hud- speth, 2008), where it is sometimes manifest as otoacoustic emissions, which are the emission of sound from the ear. Antennae of mosquitoes and fruit flies and eardrums of tree crickets exhibit an analogous phenomenon, spontaneous os- cillations in the absence of acoustic stimulation.
Internally, insect ears, whether associated with eardrums as in tree crickets or antennae as in mosquitoes and flies, com- prise groups of structures called scolopidia, each of which includes one or two auditory nerve cells together with sev- eral supporting elements (Yack, 2004). Motion of the exter- nal sound-capturing structure results in deformation of the nerve cells that, in turn, causes mechanosensory ion chan- nels to open, thereby allowing the influx of ions that results in nerve action potentials. As in vertebrates, active ampli- fication works by using metabolic energy to produce force that adds to that exerted by the sound stimulus, effectively amplifying the latter. In mosquitoes, fruit flies, and tree crickets, the combined forces of their many scolopidia (a few dozen in tree crickets, nearly 500 in fruit flies, and more than 1,000 in mosquitoes) are sufficient to cause vibration of the external structures, detectable as spontaneous vibra- tions in the absence of stimulation or as boosted vibration amplitude in response to sound stimuli. Studies on the fruit fly Drosophila melanogaster, where genetic manipulations allow modification or elimination of specific proteins, point to molecular motors as the source of the active force (Albert and Göpfert, 2015), although the precise details are still un- der investigation.
Active force production may be sharply tuned with respect to sound frequency, providing another mechanism, besides passive tuning, for selective sensitivity to species-specific signals. The frequency tuning of active mechanics is revealed by the spectra of spontaneous oscillations. Comparisons across seven different Drosophila species with different song
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