By Leslie Mertz, Ph.D.

The relaxing chirp of crickets and katydids are familiar summer sounds, but what is that hushed “twittering” coming from the nursery at the butterfly house? It turns out that the pupae of certain butterflies also make noise, and new research shows that some produce their sounds with a never-before-known mechanism.

In a study published in August in the Annals of the Entomological Society of America, a research team found that the pupae of dozens of brush-footed and swallowtail butterflies—in the families Nymphalidae and Papilionidae, respectively—do some modified twerking, an abdominal wiggling movement that triggers sound from tiny structures located at the membranes between their abdominal segments. Children and young adults have sufficiently sensitive hearing to pick up the twittering at close range, and mobile phones can record it. But, to analyze the soft sound, the researchers had to employ sophisticated ultrasound equipment.

The twittering arises from a pair of so-called “sound plates” within each structure. Each of the two sound plates is covered with bumps and dips so that every bump on one plate fits into an associated dip on the other, thereby allowing the plates to dovetail snugly. “It is the unlocking of these inversely sculptured and interlocking surfaces that produces the sound,” explains lead author Michael Boppré, Ph.D., an ecologist and chair of the Forest Zoology and Entomology Department at the University of Freiburg in Germany. “This is a completely new mechanism of sound production in insects, and so far this mechanism is not known for any other insects,” he adds. By comparison, crickets produce sound through stridulation, which is the rubbing of two morphologically different structures, known as a file and scraper, against one another.

To discover the secrets of the sound production in butterfly pupae, Boppré teamed up with a bat researcher who provided access to sensitive technology for recording the ultrasonic frequencies of bat echolocation. “I was already doing field work in Costa Rica, and I went with a student to Mariposario El Bosque Nuevo, a butterly farm there that produces plenty of pupae. Basically, we tickled the pupae mechanically (with a paintbrush or by hand) to start them wiggling and then recorded the sounds,” he says.

Next, they waited until the butterflies emerged from the pupae, looked at the shed hull (or exuviae) of the pupae with a standard microscope, and saw tiny structures, which only measured a few square millimeters in size. A follow-up examination with digital and scanning electron microscopes revealed the elaborate surfaces of the sound plates. The researchers discovered three varieties of sound plates—type I specific to the swallowtails, type II in the Epicaliini tribe of brush-footed butterflies, and type III in the Heliconiini tribe of the brush-footed butterflies—and, correspondingly, three types of twitters.

Study co-authors Michael Boppré (right) and Otti Fischer, of the Forest Zoology and Entomology Department at the University of Freiburg in Germany, review images of the sound plates found in butterfly pupae. Other co-authors (not pictured) included: Patrick Dolle and Philipp Klein, also of University of Freiburg; Hans-Ulrich Schnitzler of the Institute for Neurobiology at the University of Tübingen; and Lawrence E. Gilbert of the Department of Integrative Biology at The University of Texas at Austin. (Photo credit: Eva Schottmüller) Sound is produced when the sound plates disengage, as illustrated in this sketch. The authors describe this as “stick-slip friction”— the plates normally stick together, but under pressure from abdominal movements, slip apart to make the twittering sound. (Originally published in Dolle et al 2018, Annals of the Entomological Society of America) The tiny sound-producing structures are different in swallowtails: type I, which is shown in Pachliopta kotzebuea; type II in the Epicaliini tribe of the brush-footed butterflies, which is shown in the Catonephele numilia, and type III in the Heliconiini tribe of the brush-footed butterflies, which is shown in Heliconius hecale. The sound-producing structures are circled. Higher magnifications of each structure are provided in the right two images. (Originally published in Dolle et al 2018, Annals of the Entomological Society of America)

Asked the purpose of pupal sounds, 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.”

Boppré is interested in pursuing the finding, especially how the twittering results when the plates are pulled apart, but that will likely wait until he retires next year. “In the United States, you can go on at the university until you die, but in Germany, (my age dictates that) I have to give up my lab, unfortunately,” he says. “Nonetheless, I might carry on with this mechanism if I can find a collaborator from the physics department to help with things like making a 3-D print of these organs and using some other measurements to better understand how the sound is produced when these interlocking surfaces are opening.”

While he ponders his future, Boppré offers some advice for students entering research: Classic, old-fashioned, natural-history studies can yield fascinating insights. “Young students don’t need to use only genetic analyses, DNA barcoding or other (high-tech) approaches,” he says. “This work, for instance, is a nice piece of butterfly natural history that turned out to be scientifically very interesting when high-tech techniques were applied in addition.”

He adds, “Just by going to the field with curiosity and open eyes, there’s a lot to be discovered in entomology―but, most unfortunately, natural history studies in the tropics are not in the focus of grant-giving agencies, in particular when results cannot be fully foreseen in the beginning.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.