For years, scientists assumed that because moth ears have a simple structure, their mechanical response to sound might also be quite simple,despite the fact that they can code complicated sounds. But appearances can be deceptive: locusts' ears are also structurally simple but respond to sound in a more complex way than previously thought. So James Windmill and his colleagues James Fullard and Daniel Robert decided to investigate in detail how the ears of four noctuid moth species physically respond to sounds(p. 2637).
Because moths are preyed on by bats, researchers have intensively studied their behavioural and neural response to bats' ultrasonic calls, to find out what moths can hear and how they could avoid predation. But relatively little is known about how the ear actually moves to code sounds. A moth's ear consists of two regions of membrane, called the conjunctivum and the tympanum,that are separated by a piece of cuticle. In the centre of the tympanum there is a small circular opaque zone, where the receptor cells that pick up sound vibrations attach, surrounded by a transparent and thinner region of membrane. Researchers had assumed that the whole tympanum would move much like the skin of a drum.
To find out how the conjunctivum and the tympanum move during hearing, the team used a technique called microscanning laser Doppler vibrometry. They glued captured moths upside down onto a stand, and focussed a laser beam at a set frequency onto the moths' ears as they played them sounds within their hearing range. As they moved the laser beam over the ear's surface, they recorded the frequency of the laser beam as it bounced off the membranes. Because vibrations in the membrane affect the laser beam's frequency, the team could calculate the ear's movements according to the changing frequency of the returning laser beam.
They found that the pattern of the membranes' mechanical responses to sound were similar in all four moth species, although each species responded best to a different set of ultrasonic frequencies. Comparing the response of the conjunctivum and the tympanum, they found that they moved in anti-phase with each other. The conjunctivum moved more at lower sound frequencies, although what role these movements play in hearing is still a mystery, since the conjunctivum doesn't have any receptor cells attached to it to pick up the vibrations.
Focussing their attention on the tympanum, the team found that it responded best to higher frequencies. Rather than behaving like the skin of a drum, the team discovered that the central opaque zone vibrated in response to sound,but that the surrounding circular zone vibrated very little. The team suspect that this response could somehow focus the sound onto the receptor cells.
They also found that the tympanum moved as little as 100 pm (a minuscule 0.0000001 mm) in response to sounds just above the moths' auditory threshold. To confirm that these tiny movements caused the receptor cells and auditory neurons to respond, they recorded from the auditory neurons, finding that they did respond to 100 pm movements. Not only does this show that the moth ear is incredibly sensitive, but that it is also far more complex than previously thought.