Speed and acceleration are vital strategies for predators to catch their prey and escape with their lives. However, the greatest acceleration capacities recorded in nature are part of more comforting and less traumatic contexts: reproduction and waste excretion.
The ballocory of ‘Pilobolus’
Because resources are limited, to avoid competition between parents and offspring, plants and fungi have developed highly elaborate strategies to disperse their fruits, seeds, and spores.
Among them stands out the ballocoria (a type of ballistic dispersion) based on the self-generation of internal pressures, as does the devil’s pickle. However, the ability of this shooting machine is small compared to that of the coprophilous “cannon mushrooms” of the genus Pilobolus. These hold the world record for acceleration in the remote launch of propagules, which are self-propelled by means of an explosive mechanism comparable to firing a firearm.
The life cycle of a Pilobolus it begins with a sporangium laden with thousands of spores that lands on the grass. When a herbivore grazes, it consumes the sporangium. This resists the attack of gastric acids, survives through the gastrointestinal tract and, without germinating, emerges on the nutritious feces.
Upon germination, each spore generates a mycelium that feeds by secreting digestive enzymes on the droppings. When nutrients dwindle, Pilobolus reproduces by asexual spores formed inside a sporangium, whose structure is unique. It is made up of a transparent hyphae, the sporangiophore, a filament barely a couple of millimeters long, which ends in a more or less spherical vesicle into which the fungus inflates turgidity due to osmotic pressure.
A single blackish lentiform sporangium develops at the end of the vesicle. When the pressure inside the vesicle reaches a sufficient level (about 7 atmospheres), the vesicle bursts and the sporangium shoots out at a speed of 90 km/h. This means acceleration from 0 to 20 km/h in just two nanoseconds, more than 20,000 times the gravitational acceleration. It would be equivalent to a human being being thrown at one hundred times the speed of sound.
The superdrive of the leafhoppers
Shooters (family Cicadellidae) are the earliest known examples of “superdrive” in nature.
They are sucking insects that introduce their mouth stylets into the xylem of the plants through which the crude sap ascends. As they feed on raw sap devoid of sugars, their diet is very poor: 95% water and traces of inorganic nutrients. Therefore, they are forced to constantly absorb sap and urinate up to 300 times their body weight daily. By comparison, humans urinate about one fortieth of their body weight each day.
Kinematics of the catapult launch of drops
A propulsion system consists of a mechanical assembly and a propeller that contains an energy that will be transformed into a force. In biology, propulsion systems use muscles as motors and limbs such as wings, fins, and legs as means of propulsion.
Overdrive is a physical phenomenon that until now was only known in inanimate objects. It is that the kinetic energy transfer experienced by any elastic sphere propelled by a catapult whose launch surface is superhydrophobic can be increased by up to 250% compared to propelling rigid objects.
When the exit velocities of the urine drops from the distal end of a leafhopper and those reached by the released drops were measured, it was found to be a clear example of superpropulsion.
Evolution has placed at the distal end of the leafhopper’s abdomen a spring-and-lever mechanism like a simple catapult: the anal style.
When a leafhopper prepares to urinate, the anal style sags back from its initial resting position to make room as the insect compresses the urine into a droplet that gradually swells.
By compressing the droplets just before release, the insect stores potential energy in them in the form of surface tension, causing them to act like elastic membranes. As the charged blob approaches the optimum diameter for takeoff, the stylus flexes back about fifteen degrees and then, like the arm of a catapult, launches the blob at speeds of up to 115 km/h with forty times acceleration. greater than gravitational.
The ejection of super-propelled droplets is also a strategy for leafhoppers to conserve energy through their feeding and excretion cycle. Drip urination is the most efficient form of excretion from an energy point of view: the transformation of potential energy into kinetic energy means that drip urination requires four to eight times less energy than if it were propelled by jets.
Knowing how these insects use superdrive can provide bioinspired solutions on how to design electronic microfabrication systems that overcome stickiness and viscosity with less energy. Applying physics research to a biological process with possible applications in other fields reinforces the idea that curiosity-driven basic science is both potentially valuable and truly fascinating.
