Functional fibrillar interfaces: Biological hair as inspiration across scales.

Hair, or hair-like fibrillar structures, are ubiquitous in biology, from fur on the bodies of mammals, over trichomes of plants, to the mastigonemes on the flagella of single-celled organisms. While these long and slender protuberances are passive, they are multifunctional and help to mediate interactions with the environment. They provide thermal insulation, sensory information, reversible adhesion, and surface modulation (e.g., superhydrophobicity). This review will present various functions that biological hairs have been discovered to carry out, with the hairs spanning across six orders of magnitude in size, from the millimeter-thick fur of mammals down to the nanometer-thick fibrillar ultrastructures on bateriophages. The hairs are categorized according to their functions, including protection (e.g., thermal regulation and defense), locomotion, feeding, and sensing. By understanding the versatile functions of biological hairs, bio-inspired solutions may be developed across length scales.

Trichoid sensilla on honey bee proboscises as inspiration for micro-viscometers.

Sensing physical properties of liquids, such as viscosity, is of great significance for both biological organisms and industrial applications. For terrestrial organisms feeding on liquids, such as honey bees that forage nectar, sensing viscosity may help to determine the quality of food sources. Previous experiments showed that honey bees exhibit strong preferences for less viscous nectar; however, the physical mechanism underlying how they perceive viscosity remains unexplored. In this study, we propose that the western honey bee (Apis melliferaL.) is capable of distinguishing viscosity using the slender trichoid sensilla emerging from a ball and socket-like joint on the proboscis. Observations of the trichoid sensilla using transmission electron microscopy reveal physical characteristics that are typical of mechanosensory structures. Additionally, we found that bees actively alter the rate at which they feed based on the liquid's viscosity and not its sugar content, hinting at their sensing of viscosity. Through mathematical modeling, we found that the sensitivity of the biological viscometer was determined by its length, and the optimal sensitivity for a western honey bee occurs when the tongue interacts with nectar with a viscosity of 4.2 mPa·s, coinciding with the viscosities typically found in the wild. Our findings broaden insights into how honey bees adapt to varying-viscosity nectar from the perspective of mechanical sensing, and how the bee-flower partnership may be based around the optimal nectar viscosity for feeding. By understanding how bees may sense viscosity at the micrometer scale, we may motivate new technologies for micro-viscometers.

Feeding behavior of honey bees on dry sugar.

The feeding habits of insects can be influenced by food abundance, nutrition, physical forces, and many other variables, which is why this topic is multidisciplinary and perennially fascinating. Although honey bees primarily feed on liquid nectar, they also can feed on dry sugar; however, the feeding mechanism for feeding on dry substances by a primarily fluid-feeding insect remains unexplored. We observed that, when honey bees are accessible to both dry sugar and liquid nectar, they prefer to feed on the latter. To elucidate the diet preference, we conducted a comparative study between feeding on dry-sugar and drawing up liquid-nectar, from the tongue kinematics and dynamic configuration, friction force, glossal durability, and feeding efficiency. Using a high-speed camera, we discovered that the hairy tongue of the honey bee uses back-and-forth movements to furrow a groove on pieces of dry sugar, with saliva simultaneously dissolving the sugar. We found that the lapping frequency of the tongue on dry sugar reduces from 4.5 Hz to 1.6 Hz when compared to feeding on the liquid diet; a 64% decrease in average tongue speed. Through tribological tests, we revealed that the friction forces when feeding on dry sugar is approximately 5 times that of dipping nectar, and the glossal hairs wear 4 times faster when feeding on dry sugar compared to the sucrose solution. We built a mathematical model to bridge the gap between energy intake rate and tongue dynamics of these two feeding modes. The theoretical net energy intake rate of feeding on dry sugar is 50% lower than when feeding on sucrose solutions. Both experimental and theoretical discoveries revealed that although honey bees can feed on dry substances, natural selection has forged their tongue structures primarily for a liquid diet. This study combined behavioral and mechanical tests with mathematical modeling, which highlights the advantages of using multidisciplinary approaches for uncovering the feeding physiology of insects.