Moth wingtips an `acoustic decoy' to thwart bat attack

Date:
September 9, 2021
Source:
University of Bristol
Summary:
Wingtips of certain species of silkmoth are structured to reflect
sound and throw off attackers, according to a new study.



FULL STORY ========================================================================== Wingtips of certain species of silkmoth are structured to reflect sound
and throw off attackers, according to a new study.


========================================================================== Researchers at the University of Bristol have discovered that the tips
of some saturniid moth forewings are curiously rippled and folded. They
found that these unique structures strongly reflect sound, meaning that
a bat hunting using echolocation is more likely to attack the wingtip
region of the moth over the body, potentially saving the moth's life.

They also discovered that the ripples and folds of the forewing
tips have evolved to act as hemispheric and corner retroreflectors respectively, meaning that they reflect sound strongly back to its point
of origin. Coupled together, the folds and ripples of these wingtips
cover a huge range of incident sounds angles, meaning that over the
entire wingbeat cycle of a flying moth and most possible positions of
an attacking bat, the wingtip would consistently produce the strongest
echoes. The acoustic protection of wingtips is even stronger than that
of common hindwing decoys.

Prof Marc Holderied of Bristol's School of Biological Sciences explained:
"We have demonstrated that the folded and rippled wingtips on the
forewings of some silkmoths act as acoustic decoys.

"Structurally, the wingtips act as acoustic retroreflectors, reflecting
sound back to its source from numerous angles, meaning a bat would be more likely to strike the wingtip over the more vulnerable body of the moth."
The findings, published today in Current Biology,are the latest revelation
in the bat-moth acoustic arms race -- the battle between bats which
hunt moths using echolocation, and the subsequent evolution of different defensive strategies amongst moths to increase their chances of survival.

Towed acoustic decoys are a well-established defense amongst some
silkmoths.

These species have evolved elongated hindwings which terminate in a
coiled and twisted end. The morphology of these elongated hindwings
means that they generate very strong echoes, so much so that they will
often divert a bat's acoustic gaze towards them, away from the exposed
body of the moth, causing the bat to strike the expendable tail of the
moth or miss the moth all together.

Lead author Dr Thomas Neil said: "There are many silkmoths that do
not have these elongated hindwings, and we were interested in how they
might protect themselves from bats. Through our research we discovered
that there are many silkmoths that have rippled and folded structures
not on the tips of their elongated hindwings but on the tips of their forewings. These resembled the twisted hindwing structures seen in other
moths and so we wanted to know if they might also serve as an acoustic
decoy to thwart a bat's attack.

"To test this theory, we used innovative acoustic tomography analysis. We recorded echoes from moths from over 10,000 angles, to compare whether
the echoes coming from the wingtips of these moths were stronger than the echoes from the body. If the echoes coming from the rippled and folded
wingtips were stronger than that of the body, this would indicate that
they were indeed acoustic decoys.

"Conclusive support for the idea that the forewing reflector is an
acoustic decoy comes from our finding that acoustic forewing decoys always evolved as an alternative to acoustic hindwing decoys, with there being no species known to possess both." Now the researchers will try and collect behavioral data to corroborate their findings in the lab. They plan to
monitor bats and moths with varying levels of folded wing morphologies
to see how much of a survival advantage it really gives them.

Prof Holderied added: "The results of this study introduce another
exciting aspect to the story of the bat-moths acoustic arms race. We
have identified a novel form of acoustic defense amongst silkmoths
which may give them an advantage over hunting bats. Wider implications
might include improved man-made anti radar and sonar decoy architectures." ========================================================================== Story Source: Materials provided by University_of_Bristol. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Thomas R. Neil, Ella E. Kennedy, Brogan J. Harris, Marc
W. Holderied.

Wingtip folds and ripples on saturniid moths create decoy
echoes against bat biosonar. Current Biology, 2021; DOI:
10.1016/j.cub.2021.08.038 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210909124217.htm

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