How Do Moths Use Ultrasound-Absorbing Scales Against Bats?

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Bats find prey by listening for echoes of their own ultrasonic calls, so a moth that returns a weaker echo is harder to find. The question how do moths use ultrasound absorbing scales against bats concerns one such defence. Two research articles describe scales on moth bodies and wings that absorb ultrasound rather than reflecting it. This article explains the mechanism the sources describe, the measurements they report, and the parts that remain unproven.

Quick answer

• Bats hunt by echolocation, and moths can reduce the echo. The first source says that “Moths lacking such ears could still gain protection from bat biosonar by using stealth acoustic camouflage, absorbing sound waves rather than reflecting them back as echoes.”
• The scales do the absorbing. The first source reports that “the thorax scales of earless moths absorbs an average of 67 ± 9% of impinging ultrasonic sound energy.”
• The effect is in the scales’ structure. The second source explains that “The scales have interstitial spaces between them that create a network of interconnected pores similar to that found in porous sound absorbers.”

How we chose

Two research articles hosted on PubMed Central were selected because both measure ultrasound absorption by moth scales or wings. Both pages returned HTTP 200 and their article text was read for the quotes in this piece. Claims are limited to what those pages state.

Why bats make moths vulnerable

The threat is set by the bat’s way of hunting. The first source explains that “As bats are auditory rather than visual hunters, any mechanism that reduces the detectability of moth echoes returning to the bat would confer a survival advantage.” An echo that comes back weaker gives a bat less to work with.

Many moths have a direct defence against this threat. The first source notes that “Many moths are endowed with ultrasound-sensitive ears that serve the detection and evasion of echolocating bats.” The second source describes the wider set of responses: “Moths have developed a series of measures to avoid detection from the echolocation cries of bats (primary defences) and to promote survival once detected (secondary defences).”

Moths without ears face a different problem. The first source states that they could still gain protection “by using stealth acoustic camouflage, absorbing sound waves rather than reflecting them back as echoes.” This is the route the scales are thought to serve.

What makes the thorax an echo target

An insect’s body reflects sound in a particular way. The first source says that “Unlike their thin cuticular wings, an insect’s thorax contains its nutritious protein-rich locomotor apparatus encased in thick and hence acoustically highly reflective cuticle.” The thorax is therefore a strong reflector in the first place.

Its shape adds to the echo. The same source states that “Its shape determines the echoes it produces, and because the thorax is roughly spherical it sends consistently strong echoes in many directions, creating an omnidirectional target for bat attacks.” A target that reflects in many directions can be detected from many angles.

The scale layer is the feature that changes this. The first source notes that “Unlike most other insects, much of the thorax of moths and butterflies is covered in hair-like (piliform) scales.” It adds that the layer “is remarkably similar in structure and arrangement to natural fibrous materials commonly used in sound insulation, such as hemp and kenaf.”

What the measurements showed

The first source tested the thorax scales directly. It reports that “From tomographic echo images, we find that the thin layer of thoracic scales of diurnal butterflies affects the strength of ultrasound echoes from the thorax very little, while the thorax scales of earless moths absorbs an average of 67 ± 9% of impinging ultrasonic sound energy.” The contrast with diurnal butterflies is part of the result.

The same source describes the frequency range. It states that “We show that the thorax scales of moths provide acoustic camouflage by acting as broadband (20–160 kHz) stealth coating.” That range is wider than the dominant calls of many insectivorous bats, which the source describes as lying “between 20 and 60 kHz” for most species.

The authors then describe the scales as a sound absorber. Their wording is “Such scales, despite being thin and lightweight, constitute a broadband, multidirectional and efficient ultrasound absorber that reduces the moths’ detectability to hunting bats and gives them a survival advantage.” The source states this as its finding, and it is the central claim of the first article.

How the scales absorb sound

The physical model comes from fibrous materials. The first source explains that “Modelling results suggest the scales are acting as a porous sound absorber; however, the thorax scales of moths achieve a considerably higher absorption than technical fibrous porous absorbers with the same structural parameters.” This suggests the scales work like a porous material but do better than the man-made fibers the authors compared.

The second source describes the pore structure. It says that “The scales have interstitial spaces between them that create a network of interconnected pores similar to that found in porous sound absorbers.” It then explains the energy loss: “when a sound wave propagates through this network, thermal and viscous effects cause the dissipation of its acoustic energy.”

The scales may also work in a second way. The second source notes that “the ultrastructure of the scales resembles a perforated panel backed by air,” which “could act as a microperforated panel absorber.” This is a separate model from the porous one, and the source presents both as possibilities rather than a single settled mechanism.

Wings and the proposed echo effect

The wings have been studied too. The first source reports that “the wings of nocturnal moths reportedly absorb slightly more ultrasound (1–2 dB) than wings of diurnal butterflies” over a narrow band of frequencies. The effect is small, so the thorax and wing results are reported separately.

The second source describes the proposed effect in simple terms. It says that “several studies have proposed that the wings of some moths absorb a large portion of the sound energy contained in a bat’s ultrasonic cry; as a result, the bat receives a dampened echo, and the moth becomes invisible to the bat.” The word “proposed” marks this as a hypothesis.

The second source tested the hypothesis with a reverberation chamber. It reports that “the wings of the chalcosiines absorb more ultrasound than the wings of some diurnal butterflies.” The authors linked this to exposure to bats, and they wrote that “To test the hypothesis that greater exposure to bat predation drives the development of higher ultrasound absorbance, we used a small reverberation chamber.” The result supports the idea, but it is one set of species.

Scales can serve more than one purpose. The second source notes that “The absorption of ultrasound is not the sole defensive function of the moth wing scales.” It gives another example: “a moth can release itself from a spider web by shedding some of its wing scales.” Scales are therefore part of a broader defence system. For more on insects in the garden, see how to create a pollinator garden for butterflies.

Other defences that moths use

Some moths avoid bats by changing how they fly. The first source notes that “Earless moths tend to fly less” and that they “achieve acoustic crypsis by remaining close above vegetation in order to blend into the echoes of surrounding clutter.” The scales and the flight behaviour are separate tools.

Hearing is another defence. The first source notes that “Eared moths detect the echolocation calls of bats via tympanic organs and take defensive action.” The first source ties the scale mechanism to moths that lack such ears, which is the group it describes as able to gain “stealth acoustic camouflage.” For a related garden-insect read, see how to attract pollinators to your garden naturally.

What we could not verify

This article could not verify how much a real bat is fooled in the wild. The first source states that “the effect of thorax scales on moth echoes has never been characterized” before its own work, and the measurements it reports were made under test conditions. The second source’s chamber results also come from a lab setup rather than free flight.

The evidence for a survival benefit is indirect. The first source describes the scales as giving moths “a survival advantage,” but the sources report absorption and echo measures, not counts of captured moths. The benefit is therefore inferred from the physics.

What the sources do not say

The sources do not give a figure for how much a moth’s detection range shrinks in the field. They also do not compare every moth family. The second source notes that the absorption peaks in a particular frequency range, and readers should not assume the same peak applies to every species.

Frequently asked questions

Do all moths have ultrasound-absorbing scales?

The sources do not say that all moths do. The first source describes the thorax scales of “earless moths” in its measurements, and the second source compared “nocturnal (Bombycoidea: Saturniidae) and diurnal moths” in its chamber tests.

How much ultrasound do the scales absorb?

The first source reports that “the thorax scales of earless moths absorbs an average of 67 ± 9% of impinging ultrasonic sound energy.” This is a measurement from its test setup, and it is an average with a stated spread.

Why are the scales like sound insulation?

The first source notes that the layer “is remarkably similar in structure and arrangement to natural fibrous materials commonly used in sound insulation, such as hemp and kenaf.” The similarity in structure is the basis for the porous absorber model.

Do the scales protect moths from all bats?

The sources do not address that question. The first source says the scales reduce “the moths’ detectability to hunting bats,” but it does not test each bat species. Its own frequency range is “20–160 kHz,” so the effect is described across a broad band rather than against every call.

Are the scales only for hiding from bats?

No, according to the second source. It states that “The absorption of ultrasound is not the sole defensive function of the moth wing scales,” and it adds that “the microstructure of the scales is responsible for some of the colours found in moth wings.”

Sources

• Thoracic scales of moths as a stealth coating against bat biosonar. https://pmc.ncbi.nlm.nih.gov/articles/PMC7061704/
• The anti-bat strategy of ultrasound absorption: the wings of nocturnal moths (Bombycoidea: Saturniidae) absorb more ultrasound than the wings of diurnal moths (Chalcosiinae: Zygaenoidea: Zygaenidae). https://pmc.ncbi.nlm.nih.gov/articles/PMC5278430/