Flotation in Turbulent Waters

two insects with long lands stand on water surface seen from above

Biological Strategy

Insects

AskNature Team

Image: Hao Wang / Unsplash / Free non-commercial use

Modify Surface Tension

Water molecules are attracted to each other, a phenomenon known as cohesion. The molecules at the surface of water, such as in a pond, do not have other water molecules on all sides of them. Therefore they cohere more strongly to molecules next to and below them. This strong horizontal cohesion creates a “tension” on the surface of water that is more difficult to break through, allowing various items and organisms such as insects to perch on top of without sinking through.

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Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Water insects maintain flotation in turbulent waters thanks to an undersurface coating of water-repellent hairs.

“Among the most superficially aquatic and numerous insects of freshwater areas are various Heteropterid bugs, such as pond-skaters, water-measurers and water-crickets. lnsects like these seldom even get their feet wet since they are able to stride or run over the water’s surface by using its surface tension, in which they scarcely make a dent. Since all of the bugs’ life is spent in this way, they need no special adaptations for an aquatic habitat, apart from an undersurface of water-repellent hairs which prevents immersion in turbulent conditions.” (Wootton 1984:132)

“Recent studies showed that the most crucial criterion [for achieving the ‘Lotus Effect’] mainly relies on roughening the surface into multiple length scales of roughness so that liquid droplets can be retained in the Cassie-Baxter state, where air pockets are trapped underneath the liquid, reducing the solid-liquid interface. These hierarchically structured surfaces have been fabricated through various routes and demonstrated to have superhydrophobic properties as well. This amazing water-repellent property is also found in other biological systems comprising a plurality of flexible hairs, and some of them have been recognized for over 100 years. Fuzzy leaves, such as the Lady’s Mantle, cause water droplets to form perfect spheres and allow them to roll off easily as a result of being lifted and suspended by coming into contact with the hairs. In the animal kingdom, this piliferous exterior plays a more crucial role for numerous living creatures not only to effectively protect their bodies from getting wet but also to provide various functions for their living activity. These hairs protrude several micrometers from their cuticles, typically inclined at certain angles, with diameters in the micrometer to submicrometer range. These structures can resist the impact of raindrops, allow locomotion on the surface of water, or even trap a layer of air for respiration when submerged. Some arthropods have been shown to have contact angles above 150^o, which allows them to walk on water.” (Hsu and Sigmund 2010:1504)

Last Updated August 18, 2016

References

Book

Insects of the World

01/01/1970 | Walter Linsenmaier

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Reference

Journal article

Artificial Hairy Surfaces with a Nearly Perfect Hydrophobic Response

Langmuir | 07/01/2010 | Shu-Hau Hsu, Wolfgang M. Sigmund

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Reference

Web page

Nature's Raincoats

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