Butterfly pupae 'twerk', but we don't know why
Researchers discover a sound-producing mechanism unique among insects. Tanya Loos reports.
Butterfly pupae create high frequency twittering sounds by twerking.
New research published in the Annals of the Entomology Society of America reveals that the sounds are created by tiny plates that zip and unzip together. The plates are unique in the insect world, and have only been recorded in one other animal group, the spiny lobster family.
The study was carried out in the rainforests of Costa Rica by a mostly German team of researchers, led by Michael Boppré from the University of Freiburg.
The pupae produce twittering sounds by making a wriggling or ‘twerking’ movement. They can be heard at close range heard by children and young adults, who still have the capacity to detect sounds in higher frequency ranges.
To better understand the sounds, Boppré teamed up with a bat researcher, who had all the necessary equipment for recording and analysing ultrasonic frequency sounds.
“I was already doing field work in Costa Rica, and I went with a student to Mariposario El Bosque Nuevo, a butterfly farm there that produces plenty of pupae,” he told the Entomology Today website.
“Basically, we tickled the pupae mechanically (with a paintbrush or by hand) to start them wiggling and then recorded the sounds.”
The recordings revealed that the twitters in human hearing range were between three and 13 kilohertz but extended into ultrasonic frequencies up to 100 kilohertz. And there were three types of sounds: Type I, specific to swallowtail butterflies of the genus Triodini; Type II in the Epicaliini tribe of brush-footed butterflies; and Type III in another group of of brush-footed butterflies, the Heliconiini.
To learn more about the mechanism producing the sound, the team examined the shed pupae husks the pupae under a microscope and found tiny paired structures – sound plates – on the membranes in between their abdominal segments.
A follow-up examination using tissue studies and an electron microscope revealed that the sculptured and interlocking surfaces of each plate appear structurally the same but are actually mirror-images of each other.
“The sound is produced when these inversely and interlocking surfaces separate during pupal wriggling, representing a stick-slip mechanism,” explains Boppré.
“This is a completely new mechanism of sound production in insects, and so far this mechanism is not known for any other insects.”
By comparison, crickets produce sound through stridulation, which is the rubbing of two physically different structures, known as a file and scraper, against one another.
The three types of twittering sounds are produced by three different types of sound plates. All use the same stick-slip mechanism, but have varying patterns. The study analysed the sounds of 35 species in 13 genera, but the authors expect that it is likely that similar sound plates will be found in many more taxa in the butterfly family, Papilionoidea.
Similar organs exist in the spiny lobster family (Palinuridae), and the sounds generated are thought to be used as predator deterrence. Are the twerking twittering pupae somehow defending themselves?
In answer, Boppré says, “We can only speculate. With many organisms, sound production is a defensive mechanism alerting some potential predator, but at this point we don’t have a proper natural history basis that we can use for an explicit interpretation.”