1. How a female parasitic fly finds a singing cricket by sound alone
In the grass at night, male crickets sing. A female of the parasitic fly species Ormia ochracea finds a singing male by sound, lands nearby, walks toward it, and deposits her larvae on it[1]. She does not search with her eyes but finds the location by sound. The challenge lies in the spacing of her ears. The distance between her left and right eardrums is about 520 micrometers, or roughly 0.5 millimeters [2]. Some sources cite a distance of about 1.2 millimeters, so the exact figure varies slightly depending on the reference [3]. Human ears are separated by the width of the head. The fly’s ears are much closer together. How does it know the direction with such a small gap?
2. How tiny the time difference between the fly's two ears is
Sound travels at about 343 meters per second. Calculating the time it takes to cross the 0.52-millimeter gap gives about 1.5 microseconds (1.5 millionths of a second). This crossing time is about the same as the natural time difference reported for the fly’s ears[2]. If a blink takes 0.1 seconds, about 66,000 instances of this 1.5-microsecond difference fit into that blink (author’s calculation). Distinguishing such a short difference with nerves alone seems difficult. Another clue is the wavelength of the sound wave. If the cricket’s song is 5 kilohertz, the wavelength is about 6.9 centimeters. The 0.52-millimeter ear spacing is about 1/130th of this. Another paper states the ear spacing is about 1.2 millimeters, or about 1/100th of the wavelength, but these are approximate figures [3]. If the ears are too small compared to the wavelength, sound reaches both ears almost equally. So, what trick does the fly use to utilize this tiny difference?
3. How a bridge connecting the fly's eardrums amplifies the difference
The fly’s left and right eardrums are connected by a small bridge in the center, acting like a rotation axis [2]. This is a major difference from the ears of other insects. Thanks to this connection, the two eardrums rock in opposite directions while shifting relative to each other. An article from Cornell University compares this to a playground “seesaw” [1]. When one side is pushed, the other moves in the opposite direction through the bridge. A paper studying microphones states that near this rocking mode, the time difference increases from about 1.5 microseconds to about 50 microseconds, and the difference in sound intensity increases from about 1 decibel to about 12 decibels [2]. In terms of time difference, this is about a 33-fold increase (author’s calculation). 50 microseconds corresponds to the time it takes for sound to travel about 1.7 centimeters (author’s calculation). Thus, ears spaced 0.5 millimeters apart function as if they were 1.7 centimeters apart. However, this amplification occurs under specific rocking conditions and is not constant for all sound directions or frequencies.
4. How accurately the fly can tell which direction a sound comes from
How accurately can the direction be known? The Cornell University article states that the fly can distinguish source position changes of about 2 degrees [1]. This is a difference as small as moving your fingertips slightly while keeping your arm straight. However, there are conditions. Within about 30 degrees to either side of the front, a paper reports that the fly can judge direction to within about 2 degrees, which is where it works best [3]. Models also show that accuracy drops at larger angles [2]. It is not 2 degrees in all directions. Given the small ear spacing, this accuracy is notable. In the Cornell article, researcher Hoy said that considering body size, this fly is an expert at knowing sound direction [1]. The fly’s directional accuracy is similar to that of humans (about 2 degrees), but the key point is the small ear spacing of around 1 millimeter. So far, this has been the story from the fly’s side. What is happening on the cricket’s side, where they are being found?
5. Why crickets in Hawaii stopped singing after the parasitic fly arrived
This parasitic fly is native to places like Florida in the United States but was introduced to Hawaii about 30 years ago [4]. The crickets in Hawaii (Teleogryllus oceanicus) became targets for this fly. On the island of Kauai, within about 20 generations, many male crickets with flat wings that do not sing became common [4]. Males without singing wings are thought to have become better at hiding from flies that rely on song. This change is reported to have arisen from different genetic changes on the islands of Kauai and Oahu separately. The fly’s ear, which precisely knows direction, and the speed of the cricket’s change are two sides of the same event. Next, let’s look at how the idea from the fly’s ear is approaching human life.
6. Whether microphones copying the fly's ear have reached hearing aids
The idea from the fly’s ear is being used in microphone research. Miles and others at Binghamton University created a microphone with a small diaphragm (1 millimeter by 3 millimeters) that rotates around a central axis. Measurements of the prototype microphone showed that noise was about 17 decibels lower than the conventional two-microphone method, as reported in 2013 [5]. Hearing aids are considered the largest application field for directional microphones that select sounds from specific directions. MEMS directional microphones inspired by the fly’s ear are being studied by many research groups [2]. It is important to note that the examples introduced here are at the prototype or research stage. The sources do not state that they are widely used in current hearing aids. Humans are in the process of gradually incorporating the fly’s ear idea into engineering.
7. How to try guessing a sound's direction with your own ears
Try this sound guessing game at home. In a quiet room, close your eyes and face straight ahead. Have an adult ring a small bell in front, to the side, or diagonally in front, and point to where the sound came from. Is it easier to guess near the front or directly to the side? Try it while comparing it to the story about the fly’s high accuracy within about ±30 degrees of the front. Looking at 0.5 millimeters on a ruler is also recommended. Seeing where the cricket’s song wavelength (about 7 centimeters) falls on the ruler helps you feel the smallness of the fly’s ears.