DECEMBER 8, 2020

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www.pnas.org/cgi/doi/10.1073/iti4920117 PNAS | December 8, 2020 | vol. 117 | no. 49 | 30861–30863

Moth scales act as acoustic metamaterials

Bats track prey using ultrasound-mediated echoloca-tion, and moth species without ultrasound-sensitive ears must rely on other strategies to avoid predation. Thomas Neil, Zhiyuan Shen, et al. (pp. 31134–31141) report a form of acoustic camouflage in two species of earless moths—Antheraea pernyi and Dactyloceras lucina—that allows them to evade echolocating bats. A thin layer of paddle-shaped scales that reduce ultrasonic echoes and absorb impinging sound at the relevant frequencies carpets the moths’ wings. Unlike butterfly scales, the moth scales reduce the average target strength across the frequency range bats use to detect flying prey and absorb up to 72% of sound intensity at 78 kHz. The scales dampen sound at frequencies as low as 20 kHz, acting as ultrathin, deep-subwavelength metamate-rial absorbers; porous absorbers used in sound insula-tion technology are typically much thicker. Analysis of moth scale models borne on stalks inserted into a wing membrane revealed that the scales owe their sound- absorbing properties to a unique resonance geometry resulting from a combination of scale morphologies and tiling patterns. In contrast, the authors note, a meta-material absorber built using butterfly scales, which are arrayed uniformly and exhibit clustered resonances, would not dampen sound over a similarly broad range. According to the authors, the findings could inspire the design of high-performance acoustic panels and ultra-thin, lightweight noise-canceling devices. — P.N.

Zircons in meteorite yield clues to Mars’ tectonic past Although the mineral zircon is not expected to be a significant component of most rocky planets in the Solar System, some meteorites from Mars contain zir-con crystals dating back to the origins of the planet. Maria Costa et al. (pp. 30973–30979) extracted zir-con crystals from the Northwest Africa (NWA) 7533 meteorite and analyzed the crystals’ ages and isoto-pic compositions. The oldest zircons in the sample ranged in age from around 4.485 to 4.331 billion years, possibly representative of heavy bombard-ment on the planet’s surface around the time of the early migration of gas giant planets. Another group of zircons returned ages of around 1.548 billion to 299 million years, and the authors suggest that the

False-color 3D representation of a wing section of the moth Lasiocampa quercus showing structure, diversity, and arrangement of base scales (orange) and cover scales (blue and yellow). Image credit: Simon Reichel, Thomas R. Neil, Zhiyuan Shen, and Marc W. Holderied (University of Bristol, Bristol, United Kingdom).

The NWA 7533 zircon-rich meteorite, representing a fragment of the ancient crust of Mars.

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zircons likely formed as part of the Elysium and Thar-sis volcanic provinces. These provinces are expres-sions of deep-seated plumes of material from the mantle, and the isotopic composition of the zircons suggest that their source is a previously unrecog-nized primitive mantle reservoir, with implications for the tectonic regime of Mars throughout its history. According to the authors, zircons may be ubiquitous on the surface of Mars, and the zircons in the mete-orite sample may provide a record of the planet’s vol-canic activity. — P.G.

How bumblebees avoid collisions during flightVertebrates account for their body size while nav-igating obstacles as they move, but the ability of insects to do so is unknown. Sridhar Ravi et al. (pp. 31494–31499) observed bumblebees flying in a tun-nel as the bees encountered walls containing gaps

of varying width. The authors report that when bees were presented with narrow gaps, they first assessed the gap visually through side-to-side peering flights before reorienting their angle of flight to minimize their frontal profile and safely pass through the gap. In extreme cases, in which the gaps were smaller than the bees’ wingspan, bumblebees passed through the gap, flying entirely sideways. As bumblebees vary in size, the time spent assessing the gap and the reori-entation performed to safely navigate it was deter-mined not by the absolute size of the gap, but by the size of the gap relative to each bee’s own wingspan. The results suggest that flying bumblebees navigate their environment with an implicit awareness of their own body size and shape in relation to their environ-ment. According to the authors, this ability in insects, which have much smaller brains than vertebrates, sheds light on the neural capacity needed for per-ception of individuals’ own body sizes. — P.G.

Spontaneous hydrogen peroxide formation from water vapor When water is atomized into micron-sized droplets, water molecules can become electrochemically active and catalyze a variety of reactions. Previ-ous research has shown that water molecules can spontaneously oxidize to form hydrogen peroxide (H2O2). Expanding on the finding, Jae Kyoo Lee, Hyun Soo Han, et al. (pp. 30934–30941) report that spontaneous H2O2 production occurs when micro-droplets are formed by condensing water vapor onto low-temperature substrates, suggesting that oxida-tion of water into H2O2 is a general phenomenon associated with microdroplet formation, regardless of the mechanism. Through experiments on a variety of surfaces, including silicon, plastic, glass, and metal, the authors found that H2O2 yield depends on drop-let nucleation and growth and on conditions at the substrate surface, such as temperature and relative humidity. The authors also suggest potential mecha-nisms of H2O2 formation, noting that the established presence of a strong electric field at the periphery of water microdroplets may create OH radicals that subsequently recombine. The finding that water spontaneously oxidizes into H2O2 as a general prop-erty of microdroplet formation carries implications for a broad array of fields, from environment-friendly surface disinfection technology to research in atmo-spheric sciences and the origin of life on Earth. — T.J.

Circadian regulation of muscle growthMuscle physiology and metabolism vary between day and night, but whether the diurnal variation is dependent on the circadian clock or due to differ-ences in nutrition or physical activity remains unclear. To examine circadian regulation of muscle growth independent of exercise and feeding, Jeffrey Kelu

30862 | www.pnas.org/cgi/doi/10.1073/iti4920117 In this issue

A bumblebee foraging among shrubs.

Cross-sections of the body muscle of a growing zebrafish, with labeled plasma membranes (cyan) and actin (red) arranged as a clock and whole fluorescent larvae depicting the clock hands.

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www.pnas.org/cgi/doi/10.1073/iti4920117 PNAS | December 8, 2020 | vol. 117 | no. 49 | 30863

et al. (pp. 31208–31218) measured muscle growth over 12-hour periods in live pre-feeding larval zebrafish. The authors found that larval zebrafish muscle grows more during the day than night and that the differences are independent of muscle activity. Muscle anabolism was greater in daytime, promoted by physical activity, and correlated with TORC1 activity, whereas catabolism was greater at night and correlated with the accumulation of Mus-cle RING Finger (murf) mRNAs, which are markers of protein degradation. Rapamycin, a TORC1 inhibi-tor, reduced muscle growth during the day, whereas proteasome inhibitors enhanced muscle growth at night. Inhibiting molecular clock function abolished circadian differences and reduced muscle growth. After entrainment of larvae, diurnal variations in muscle growth and protein turnover persisted under constant conditions. According to the authors, mus-cles in zebrafish larvae show circadian differences in growth and are regulated by the circadian clock inde-pendent of physical activity and food intake. — S.R.

Native Californian rock art suggests hallucinogen useA long-established hypothesis holds that creators of rock art may have been in altered states of con-sciousness, but conclusive evidence of consumption of hallucinogens at rock art sites is lacking. David Robinson et al. (pp. 31026–31037) analyzed fibrous bundles called quids, found in the ceiling of Pin-wheel Cave in California. Because a pinwheel-like design painted on the cave resembles the flower Datura wrightii, which has known hallucinogenic properties and was used by Native Californians to induce trance states, the authors explored whether the quids might have contained Datura. Three- dimensional analysis of the quids suggested that they had been chewed, potentially inside the cave and under the paintings. Further analysis revealed the presence of the hallucinogenic compounds

scopolamine and atropine in the quids, and scan-ning electron microscopy confirmed that the fibers in the quids came from Datura. The authors report that the paintings in the cave were likely not rep-resentations of the visual phenomena induced by Datura but, rather, representations of the plant itself that may have served to convey knowledge about the plant in preparation for a communal experience. According to the authors, the results both confirm the use of hallucinogens in rock art and challenge the previous model of the hallucinogens’ influence on the form of rock art. — P.G.

Digital image of Pinwheel painting, processed with an image-enhancing technique.

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