Superhydrophobicity and size reduction enabled Halobates (Insecta: Heteroptera, Gerridae) to colonize the open ocean

Abstract

Despite the remarkable evolutionary success of insects at colonizing every conceivable terrestrial and aquatic habitat, only five Halobates (Heteroptera: Gerridae) species (~0.0001% of all known insect species) have succeeded at colonizing the open ocean-the largest biome on earth. this remarkable evolutionary achievement likely required unique adaptations for them to survive and thrive in the challenging oceanic environment. For the first time, we explore the morphology and behavior of an open-ocean Halobates germanus and a related coastal species H. hayanus to understand mechanisms of these adaptations. We provide direct experimental evidence based on high-speed videos which reveal that Halobates exploit their specialized and self-groomed body hair to achieve extreme water repellence, which facilitates rapid skating and plastron respiration under water. Moreover, the grooming behavior and presence of cuticular wax aids in the maintenance of superhydrophobicity. further, reductions of their body mass and size enable them to achieve impressive accelerations (~400 ms −2) and reaction times (~12 ms) to escape approaching predators or environmental threats and are crucial to their survival under harsh marine conditions. These findings might also inspire rational strategies for developing liquid-repellent surfaces for drag reduction, water desalination, and preventing bio-fouling. The proliferation of invertebrate taxa in the ocean during the Cambrian explosion eventually led to their colonization on land, where insects first appeared ~479 million years (Myr) ago 1 to eventually dominate various terrestrial ecosystems 2,3. With an estimated ~5.5 million extant species 4 , insects comprise 80% of all known metazoans, making them the dominant animals on Earth 5. Despite their successful colonization of both terrestrial and aquatic habitats, only a small number of insects (~0.5% of total species) have been found in marine environments, mostly in nearshore habitats, with only a handful of Halobates (~0.0001% of insect species) ultimately colonizing the open ocean, the largest biome on Earth 6-8. Halobates is a member of the family Gerridae, which evolved nearly 55 Myr ago 6,9. Some 48 species of Halobates are known to occur in tropical and subtropical seas and oceans that cover more than 70% of the Earth's surface. Most of these species are found in mangrove and near-shore habitats and two even live in freshwater, whereas five live on the surface of the open ocean. The fact that only a mere 0.0001% of insect species were able to colonize the largest habitat on Earth indicates that there must have been formidable environmental challenges for oceanic Halobates to overcome in order to live there. Halobates most likely evolved from an estuarine or mangrove ancestor that was washed out to the sea and became adapted to survive in the open ocean 8,10. Freshwater relatives of sea-skaters are often found on placid water bodies such as small ponds, lakes or slow-flowing streams, whereas many tropical species can be found

How drain flies manage to almost never get washed away

“…As the fly drifts away from the collision site, it rolls on the pool making several attempts to stand ( –400 ms). The fly manages to stand in less than a second and quickly leaps into flight ( –600 ms) by moving its four hind legs in a rowing-like jump, similar to that of a water strider 11 , 28 (Supplementary video 8 ; the jumping motion is smoother in some other observations). In all observations, the flies never stay on the pool surface for more than a couple of seconds (Table 1 ) and do not exhibit any kind of water walking motion, seeming to prefer to locomote in the air and rest on solid surfaces.…”

“…Augmenting chemistry with roughness can increase the apparent contact angle 8 , and superhydrophobicity ( ) occurs when air becomes entrapped in the valleys of roughness elements (Cassie wetting) with increasing air fraction raising 9 . Many insects take advantage of this wetting physics and attain superhydrophobicity by both coating their bodies with oil or wax to optimize their surface chemistry 3 , 10 , 11 , and using hierarchical roughness structures to maximize the air fraction (minimize the solid-liquid contact) between the liquid and their bodies 4 . The roughness appears in many forms in insects and plants alike, including: nanopillars on drone fly 12 and dragonfly wings 13 , micropapillae on lotus leaves 14 , micropapillae with nanofolds on rose petals 15 , needle shaped setae with nanogrooves on water strider 11 , 16 and crane fly legs 17 , and arrays of hair with star-shaped cuticular projections on termite wings 18 .…”

“…Many insects take advantage of this wetting physics and attain superhydrophobicity by both coating their bodies with oil or wax to optimize their surface chemistry 3 , 10 , 11 , and using hierarchical roughness structures to maximize the air fraction (minimize the solid-liquid contact) between the liquid and their bodies 4 . The roughness appears in many forms in insects and plants alike, including: nanopillars on drone fly 12 and dragonfly wings 13 , micropapillae on lotus leaves 14 , micropapillae with nanofolds on rose petals 15 , needle shaped setae with nanogrooves on water strider 11 , 16 and crane fly legs 17 , and arrays of hair with star-shaped cuticular projections on termite wings 18 . Insect surface chemistry and roughness helps them to stay dry, but their interactions with water brings other challenges as well.…”

Surface‐Dwelling Aquatic Insects in Low‐Energy Freshwater Environments Are Highly Impacted by Oil Spills and the Surface Washing Agent Corexit EC9580A Used in Oil Spill Response

“…Water striders live on the water's surface, using microsetae, fine hydrophobic hairs, to maintain buoyancy and allow them to stride across the water (Kaitala 1987; Mahadik et al 2020). Disruption of this mechanism may lead to immobility and death: we observed that oil directly affected the strider's ability to skate across the water's surface.…”

Counterintuitive Wetting Transitions in Doubly Reentrant Cavities as a Function of Surface Make‐Up, Hydrostatic Pressure, and Cavity Aspect Ratio

“…In this context, a new class of microtextures has been developed, comprising microcavities that broaden below the inlets such that the cross‐section of the space between adjacent cavities spanning cavity inlets and the intervening wall resembles the serif‐T shape [ 7,16–22 ] ( Figure A–C). These bio‐inspired [ 18,23 ] microtextures are known as doubly reentrant cavities (DRCs), and they can entrap air inside them on immersion in liquids due to their topography, regardless of their surface make‐up. [ 21 ] In fact, the transition of these air‐filled cavities—Cassie‐states [ 24–26 ] to the fully‐filled or the Wenzel‐state [ 27 ] depends on a number of factors, such as the compressibility of the trapped air, liquid vapor pressure, capillary condensation, the solubility of the trapped air in the liquid, and the cavity geometry.…”