Male crickets and katydids produce calling songs to attract females by rubbing their forewings together (a mechanism known as wing stridulation). The songs are usually musical (i.e., have a single frequency). One of the wings bears a serrated vein, while the other harbours a plectrum on its edge. In a manner analogous to a bow being moved across the strings of violin, the wings open and close in rhythmic cycles.

But in contrast to playing a violin, these species usually produce sound only during the closing phase as the plectrum moves along the file. The opening wing phase is silent. This is because the teeth of the serrated file are arranged in a special way to maintain sound purity and musicality, so the contact of every tooth results in a tone of similar frequency, but only occurs during the closing wing cycle.

Using the morphology of the file we can determine certain features of the sound that would have been produced by museum specimens and fossils. From this analysis we know that extinct species (from the Jurassic) also produced calls during the closing phase, so it seems this adaptation emerged early on in the evolution of wing stridulation.

Using a sensitive optical diode and high-speed video to examine wing motion, and Laser Doppler Vibrometry, in his paper in the Journal of Insect Physiology Fernando Montealegre-Z from the School of Biological Sciences at the University of Bristol (UK) describes the mechanics of stridulation used by males of Ischnomela gracilis, an exotic neotropical katydid from a remote island in the pacific ocean in Colombia. Males sing with a pure tone at ca.15 kHz and in contrast to most species of crickets and katydids, they produce a highly tonal sound during the opening phase of the wings (click here for a video of the katydid in action). The teeth of the serrated file are adapted to maintain a constant tone during opening and not during closing. In addition, the file exhibits a lump or stopper that indicates when the sweep of the plectrum over the file has ended and prevents the wings from undue separation. Similar features also occur in other species of the same genus.

These findings offer a unique opportunity to help answering the question of why most crickets and katydids evolved to produce sound during the closing phase and not during the opening phase of the wing cycle. Wing closing obviously provides an obligatory stop as the plectrum moves towards the body (i.e., the plectrum can go no further). The opening silent phase (without frictional forces) on the other hand offers no such obligatory halt, and wing halt on silent opening is controlled by sensorial feedback in most species.

In Ischnomela gracilis the opposite occurs. The closing phase is silent while the opening phase is effective in sound production (frictional forces involved). As the scraper moves in reverse, a lump on the proximal region of the serrated file is necessary to arrest its motion, preventing the wings from undue separation and avoiding unwanted wing overlapping for the next closing. Overlapping of the wings is critical because the wings are asymmetrical and only one has a functional plectrum, so if positioned the opposite way, it would end in the animal failing to produce songs.

An important question remains: as evidenced in the evolutionary history of the vast majority of the crickets and katydids, making sound during the closing phase of the wings offers many advantages for control and effectiveness sound purity, so why has a more complicate system also evolved?