在与蝙蝠的超声波大战中，飞蛾“隐形”了
新的研究发现，飞蛾的翅膀能消音。
飞蛾翅膀是一种“超声学材料”。来源：Thomas Neil, University of Bristol
蝙蝠能够利用回声定位来捕猎佳肴，而飞蛾则常常出现在它们的菜单中。在这场捕食者与猎物的声学军备竞赛中，飞蛾也有两手绝活——准确地说是“两翼”绝活：一项新的研究显示，飞蛾翅膀上的鳞片能够吸收声波，尤其是蝙蝠最爱的超声波。
“飞蛾和蝴蝶的翅膀都被一层层鳞片覆盖着，这些鳞片由一种叫做甲壳素（chitin）的天然聚合物组成。大多数昆虫和甲壳动物的外骨骼中都存在甲壳素。”英国布里斯托大学（University of Bristol）的托马斯·尼尔（Thomas Neil）说，一开始，他用声波对飞蛾的翅膀进行轰击，观察回弹的情况。
“我们发现， 被超声波击中时飞蛾鳞片会与之共振，共振频率与蝙蝠用于回声定位的超声波频率几乎是一致的。”振动将超声波的能量转化成了机械能，这会减弱传回到蝙蝠的回声。
“这可能不是巧合。由于具有特定的形状和大小，这些鳞片的共振频率恰到好处，让它们能够吸收来自狩猎蝙蝠的声波能量。”
接下来，尼尔和同事对一系列不同鳞片的声波阻尼性能进行了建模。
“飞蛾身上很有趣的一点是，所有的鳞片都有着不同的形状和大小。我们发现，每个鳞片的共振频率都有细微的差别——合在一起，它们能吸收的（声波）频率范围就很大了。”这个范围包括了蝙蝠回声定位的频率，尼尔在美国声学学会会议（Meeting of the Acoustical Society of America）上报告了这一发现。这说明，即使在野外遇到了一大波蝙蝠，飞蛾受到的保护也应该相当周全。
不过，这个策略真能奏效吗……[查看全文]
Moths Have an Acoustic Invisibility Cloak
to Stay under Bats' Radar
Hopkin: Bats use echolocation to hunt for their meals, and moths are often on the menu. But in the acoustic arms race between predator and prey, moths also have a trick or two up their sleeve—or, actually, on their wings, because a new study shows that moth wings are covered with scales that absorb sound, particularly the ultrasonic variety preferred by bats.
Thomas Neil: So moth and butterfly wings are covered in layers of scales. These are made of a naturally occurring polymer called chitin, which is a polymer you find in most insect and crustacean exoskeletons.
Hopkin: That’s Thomas Neil of the University of Bristol. He started out by bombarding bits of moth wings with sound and seeing what bounced back.
Neil: We discovered that moth scales actually resonate in response to being hit with ultrasound. And they resonate at frequencies that pretty much perfectly match the frequencies that bats use for echolocation.
Hopkin: That vibration converts sound energy to mechanical energy, which muffles the echo that gets back to the bats.
Neil: That probably hasn’t happened by accident, that these scales are such a shape and size that they’re resonating at just the right frequencies that they can absorb sound energy from hunting bats.
Hopkin: Next, Neil and his colleagues modeled the sound-dampening capabilities of an array of different scales.
Neil: The really cool thing about moths is their scales are all different shapes and sizes. So what we found is that each individual scale will resonate at slightly different frequencies—and that, collectively, they actually absorb a really broadband range of frequencies.
Hopkin: That range covers the frequencies of bat echolocation calls—findings Neil presented at the Meeting of the Acoustical Society of America.
Neil: So it means that the moths should be pretty well protected from a whole host of bats that they might interact with out in the wild.
Hopkin: But does the strategy actually work…[full transcript]
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Neil, T.R., Shen, Z., Robert, D., Drinkwater, B.W. and Holderied, M.W., 2020. Moth wings are acoustic metamaterials. Proceedings of the National Academy of Sciences, 117(49), pp.31134-31141.
封面图片来源：Pixabay
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