The moth specialist spider Cyrtarachne akirai uses prey scales to increase adhesion

Contaminants decrease adhesive strength by interfering with substrate contact. Spider webs adhering to moths present an ideal model to investigate how natural adhesives overcome contamination because moths' sacrificial layer of scales rubs off on sticky silk, facilitating escape. However, Cyrtarachninae spiders have evolved gluey capture threads that adhere well to moths. Cyrtarachne capture threads contain large glue droplets oversaturated with water, readily flowing but also prone to drying out. Here, we compare the spreading and adhesion of Cyrtarachne akirai glue on intact mothwings, denuded cuticle and glass to the glue of a common orb-weaving spider, Larinioides cornutus, to understand how C. akirai glue overcomes dirty surfaces. Videos show that C. akirai 's glue spreading accelerates along the underlying moth cuticle after the glue seeps beneath the moth scales—not seen on denuded cuticle or hydrophilic glass. Larinioides cornutus glue droplets failed to penetrate the moth scales, their force of adhesion thus limited by the strength of attachment of scales to the cuticle. The large size and low viscosity of C. akirai glue droplets function together to use the three-dimensional topography of the moth's scales against itself via capillary forces. Infrared spectroscopy shows C. akirai glue droplets readily lose free-flowing water. We hypothesize that this loss of water leads to increased viscosity during spreading, increasing cohesive forces during pull-off. This glue's two-phase behaviour shows how natural selection can leverage a defensive specialization of prey against themselves and highlights a new design principle for synthetic adhesives for adhering to troublesome surfaces.

Biomechanics of a moth scale at ultrasonic frequencies

The wings of moths and butterflies are densely covered in scales that exhibit intricate shapes and sculptured nanostructures. While certain butterfly scales create nanoscale photonic effects, moth scales show different nanostructures suggesting different functionality. Here we investigate moth-scale vibrodynamics to understand their role in creating acoustic camouflage against bat echolocation, where scales on wings provide ultrasound absorber functionality. For this, individual scales can be considered as building blocks with adapted biomechanical properties at ultrasonic frequencies. The 3D nanostructure of a full Bunaea alcinoe moth forewing scale was characterized using confocal microscopy. Structurally, this scale is double layered and endowed with different perforation rates on the upper and lower laminae, which are interconnected by trabeculae pillars. From these observations a parameterized model of the scale’s nanostructure was formed and its effective elastic stiffness matrix extracted. Macroscale numerical modeling of scale vibrodynamics showed close qualitative and quantitative agreement with scanning laser Doppler vibrometry measurement of this scale’s oscillations, suggesting that the governing biomechanics have been captured accurately. Importantly, this scale of B. alcinoe exhibits its first three resonances in the typical echolocation frequency range of bats, suggesting it has evolved as a resonant absorber. Damping coefficients of the moth-scale resonator and ultrasonic absorption of a scaled wing were estimated using numerical modeling. The calculated absorption coefficient of 0.50 agrees with the published maximum acoustic effect of wing scaling. Understanding scale vibroacoustic behavior helps create macroscopic structures with the capacity for broadband acoustic camouflage.

Supersaturation with water explains the unusual adhesion of aggregate glue in the webs of the moth-specialist spider, Cyrtarachne akirai

Orb webs produced by araneoid spiders depend upon aggregate glue-coated capture threads to retain their prey. Moths are challenging prey for most spiders because their scales detach and contaminate the glue droplets, significantly decreasing adhesion. Cyrtarachne are moth-specialist orb-weaving spiders whose capture threads adhere well to moths. We compare the adhesive properties and chemistry of Cyrtarachne aggregate glue to other orb-weaving spiders to test hypotheses about their structure, chemistry and performance that could explain the strength of Cyrtarachne glue. We show that the unusually large glue droplets on Cyrtarachne capture threads make them approximately 8 times more adhesive on glass substrate than capture threads from typical orb-weaving species, but Cyrtarachne adhesion is similar to that of other species after normalization by glue volume. Glue viscosity reversibly changes over 1000-fold in response to atmospheric humidity, and the adhesive strength of many species of orb spiders is maximized at a viscosity of approximately 10 5 –10 6 cst where the contributions of spreading and bulk cohesion are optimized. By contrast, viscosity of Cyrtarachne aggregate glue droplets is approximately 1000 times lower at maximum adhesive humidity, likely facilitating rapid spreading across moth scales. Water uptake by glue droplets is controlled, in part, by hygroscopic low molecular weight compounds. NMR showed evidence that Cyrtarachne glue contains a variety of unknown low molecular weight compounds. These compounds may help explain how Cyrtarachne produces such exceptionally large and low viscosity glue droplets, and also why these glue droplets rapidly lose water volume after brief ageing or exposure to even slightly dry (e.g. < 80% RH) conditions, permanently reducing their adhesion. We hypothesize that the combination of large glue droplet size and low viscosity helps Cyrtarachne glue to penetrate the gaps between moth scales.

Insectivory in Fijian flying foxes (Pteropodidae)

We used scat and isotope analyses to assess insectivory in Fijian flying foxes (Pteropodidae), seeking insights into niche partitioning of co-occurring bat species with high plant diet overlap. Moth scales were most common in scats of Notopteris macdonaldi (87%; P. tonganus: 62%; Pteropus samoensis: 36%) and may indicate shared resources. The small and highly manoeuvrable N. macdonaldi exploited nectar-rich flowers also favoured by moths (e.g. Barringtonia spp.). Other invertebrate remains were most frequent in scats of P. tonganus (69%). On the basis of scat results and ecological observations, P. tonganus uses a combination of insectivory and a highly varied plant diet to obtain sufficient nutrients. Scats of P. samoensis contained few invertebrate remains, but abundant protein-rich plant species (including Freycinetia spp.), and juveniles seemed to consume moths frequently. Clustered δ15N and δ13C for N. macdonaldi and P. samoensis indicated a narrower dietary breadth than that of P. tonganus. P. tonganus juveniles appeared at a significantly higher trophic level than did adults, probably the result of milk consumption and/or higher rates of protein synthesis. The methods used detected little evidence that bats partitioned resources vertically. This study generates hypotheses for the further examination of flying-fox diets.

Diet of the western pygmy possum, Cercartetus concinnus Gould (Marsupialia: Burramyidae), at Innes National Park, South Australia, and evaluation of diet sampling methods

The diet of a population of western pygmy possums, Cercartetus concinnus Gould (Marsupialia: Burramyidae), at Innes National Park, South Australia, was examined using faecal and fur pollen swab samples in relation to monthly plant phenological data. Eucalyptus pollen was the most abundant in both faeces and in fur swab samples, followed by Melaleuca pollen; plant exudates could not be examined by this study. Moth scales were found in 26% of the scat samples. Faecal samples comprised most plant species identified (15 of 17), but up to 25% of plant species recorded from fur pollen swabs were not recorded from faeces. The relatively high frequencies of plant species represented in fur pollen swabs indicates that this method is valuable for supplementing faecal analysis used to determine plant visitation by nectarivorous animals.

Foraging ecology of three species of hipposiderid bats in tropical rainforest in north-east Australia

We studied the foraging ecology of three species of hipposiderid bats – Hipposideros diadema (mean forearm length: 82 mm), H. cervinus (47 mm) and H. ater (41 mm) – in tropical, lowland rainforest in north-east Queensland, Australia. H. diadema foraged by perching within gaps and flying out to intercept slow-flying insects. The two smaller species typically foraged during flight, in undisturbed forest and gaps, and captured insects by aerial hawking. Seven arthropod taxa were identified in faeces of H. cervinus, with Coleoptera and Lepidoptera being present in most faeces. Percentage volume of moth scales was generally low: 35 of 60 faeces had a volume of <10%, whereas all faeces (n = 60) of H. ater had a moth scale volume of >90%. No other taxa were frequently present in faeces of H. ater. Differences in foraging ecology between H. diadema and the smaller species were related to its large size and low manoeuvrability. The dietary differences we found between H. ater and H. cervinus were unexpected, because both species have high-frequency echolocation calls (160–164 and 144–145 kHz, respectively), which suggested that both would capture predominantly moths. Our data show that pairs of hipposiderid species with only small differences in call frequencies may consume different prey taxa; however, we contend that dietary variation is more likely to result from differences in body size, wing morphology, and tooth, jaw, and cranial morphology.