Perhaps you've never wondered what sounds permeated the nighttime in a Jurassic forest, but there is now a partial answer. With the good fortune to obtain a well-preserved fossil specimen, Jun-Jie Gu, Fernando Montealegre-Z, Daniel Robert, Michael Engel, Ge-Xia Qiao, and Dong Ren were able to reconstruct the sounds that would have been made by a katydid of that geologic time [Reference Gu, Montealegre-Z, Robert, Engel, Qiao and Ren1].
Katydids produce “songs” that are specific to each species by rubbing a toothed vein on one wing against a structure (the plectrum) on the other wing. This produces a sound by a process called stridulation, which is essentially a ratcheting motion that occurs at an audible frequency. In this case, this is a biophysical mechanism that results in a resonant (musical) or non-resonant (broadband) sound that varies with the species. Which of these sounds was developed first is a key question in the evolution of insects' acoustic communication.
The specimen was collected from a formation in Inner Mongolia (China) that dates to the Middle Jurassic period, about 165 million years ago. Importantly, the specimen was found among other fossils that established the site as a forest of coniferous trees and giant ferns. Such a forest would be expected to contain reptiles, amphibians, and mammalian insectivores that could have heard the song of this katydid. Gu et al. named this specimen Archaboilus musicus, referring to the extinct family Haglidae, a group thought to precede all extant katydids, and musicus, derived from the fact that this insect was equipped to make tonal calls.
Using scanning electron microscopy to examine extant species and ultra-high resolution light microscopy to examine A. musicus (Figure 1), Gu et al. found that A. musicus had veins of nearly identical dimensions on both wings (which were large, about 72 mm long) and identical stridulatory files with exceptionally well-preserved and elaborate cuticular teeth. They concluded that this insect had bilaterally symmetric wings. This is significant because most extant katydids exhibit asymmetric wings and make their call by overlapping the left wing over the right (producing either musical or broadband calls), whereas a few others have symmetrical wings that allow swapping wing overlap and produce only musical calls. The evidence strongly suggests that A. musicus is more closely related to the latter group.
Figure 1: SEM images of three extant katydids and a light microscope image of a fossil of a Jurassic period katydid ancestor (lower right). The tooth patterns in each case are used in the production of characteristic katydid “songs.”
Using innovative comparative analysis and current knowledge of the biomechanics of wing stridulation of extant species, Gu et al. reconstructed the main properties of the acoustic signals of these ancient insects. It is remarkable that they reproduced the sound of what they concluded this long-extinct katydid sounded like. Furthermore, they included an image of what the Jurassic forest may have looked like when this chirping occurred. The image and sound can be accessed at: www.pnas.org/content/suppl/ 2012/02/03/1118372109.DCSupplemental/sm01.mov
The main purpose of katydid songs is to attract mates, yet attempting to avoid predators. Bats are an example of predators that use the acoustic signal of an insect for predation, but interestingly, bats appeared after the Jurassic period, nearly 100 million years after the time of A. musicus. Contrary to previous scenarios, Gu et al. appear to have established that musical songs evolved early, preceding the broad-bandwidth songs of extant katydids.
