The struggle to survive can produce some of nature’s strangest adaptations. When predators and prey pursue conflicting interests, each new advantage can provoke a countermeasure, locking both sides into an evolutionary arms race.

Hawkmoths and bats offer a particularly bizarre example. The chunky flying insects can provide bats with a substantial meal, but the moths have developed unusual ways to make an attack more difficult.

Bats use echolocation to hunt in darkness, while hawkmoths have evolved deterrents that operate within the same sensory world. Among the most remarkable is the ability of some hawkmoths to produce ultrasonic sounds using structures associated with their genitals.

The behavior is a form of stridulation, a sound producing defense also found among other invertebrates. Genitals are not always involved, since crickets, for example, produce sounds with their forewings.

What makes hawkmoths especially interesting is that bat deterrents appear to have evolved repeatedly across different lineages and through multiple mechanisms. A new study investigated how the moths behave when they believe a bat is attacking.

Researchers conducted experiments with captive hawkmoths during simulated bat attacks. Because the insects might otherwise fly away, the team tethered them using a fine piece of fishing line and closely watched their responses.

The researchers tested several hawkmoth species and found both shared patterns and notable exceptions. Species with known hearing organs responded to a recording of a bat and produced their own ultrasonic signal.

Surprisingly, some hawkmoths without known hearing organs also produced ultrasound in response to the recording. That finding complicated the apparent relationship between detecting a bat’s call and generating a defensive sound.

One group stood apart. Hawkmoths in the Smerinthinae group, which are not thought to possess hearing organs, did not respond to the recorded bat call, although around half produced sound after being physically touched.

Those different responses suggest that the sounds may serve more than one purpose. For some moths, ultrasound may interfere with an approaching bat’s sonar, while for others it may function as a final attempt to make a predator release them.

The team combined its behavioral experiments with microscopic image analysis and evolutionary trees. The results indicated that sound production against bats emerged multiple times during hawkmoth evolution rather than descending from a single common ancestor.

That pattern is known as convergent evolution, in which the same trait appears independently among distantly related animals. In hawkmoths, different lineages reached similar ultrasonic solutions while developing a striking variety of physical structures.

“We were amazed by the remarkable morphological diversity of structures we observed, both across and within the three stridulatory systems,” study author Paul Masonick of the University of California told IFLScience.

The repeated emergence of these systems may help explain why different hawkmoths use their sounds in different circumstances. It also helps account for the diversity of the anatomical equipment used to create those signals.

Humans rely heavily on vision to avoid hazards during daylight, but many hawkmoths face danger in darkness. Their unusual sound producing anatomy reflects the very different sensory demands created by nocturnal encounters with bats.

“I think the bat-moth system provides an excellent opportunity to study how interactions between predators and their prey can drive the evolution of complex and sometimes bizarre adaptations,” Masonick said.

“Bat predation has been a major driver of insect evolution. It has contributed significantly to the remarkable diversity of bioacoustic defenses among moths that we see today, including ears tuned to detect approaching bats and sound-producing organs that may deter attacks.”

The findings present hawkmoth genitals as more than reproductive anatomy. In an evolutionary contest shaped by sound, they can also become specialized instruments for surviving a bat attack.

The study was published in Proceedings of the Royal Society B: Biological Sciences. Its findings show how repeated pressure from predators can produce similar yet anatomically diverse solutions among separate hawkmoth lineages.