For accurate data, we have had to design and build a small-scale reverberation chamber to conduct experiments with moth wings. Our design was based on a previous design (Zeng et al., 2011; Ntelezos et al., 2017) where they used Cox and D’Antonio-modified primitive-root diffusers that are based on the frequency we are going to work with, consisting of wells of the same width and different depths.
Working with sound always highlights noise issues
Ultrasound is tricky to work with; we don’t even realize that even keys jingling in our pockets, motion detectors to turn on lights and even some lights themselves, produce ultrasound. So, in order to work in a noise-free environment, we had to build an anechoic chamber in which to put our reverberation chamber. The chamber-in-chamber design also provides electric insulation since the local electric connections are not always properly isolated.
Survival of the stealthiest
We found that the moths present an acoustic startle response when presented with bat calls: they change the position of their body and wings and pause in-flight for a moment. This reaction causes a reduction of the amplitude modulation in the returning echo. For a moment, the moth is able to hide, acoustically, from hungry insectivorous bats.
Interestingly, after further examination of the videos, we notice that when the moths exhibit this response they will also turn their wings. This behaviour raises two questions: why do moths turn their wings, and do the wings play a role in the reduction of the acoustic modulation?
We are in the process of experimenting with and analyzing the wings of a number of different moth species. So far, we see that, in fact, moth wings are absorbing ultrasound and that the main frequency at which amplitude is decreased seems to be different between species. We have also started to look into the anatomy of the wings; it is amazing to see the structure of the scales under a microscope – they resemble acoustic foam used in anechoic rooms. This may be part of an anti-bat strategy that moths have evolved to avoid being detected and, more importantly to the moths, eaten by bats.