A meadow cicada urinates so much that it could drown itself. Fortunately, the insect Philaenus spumarius, roughly the size of a tic tac, has a rump catapult that regularly flings its globules of liquid waste into the air and safely removes them from its body.
“Ballistics gets really tricky at this tiny, tiny scale,” said Philip GD Matthews, associate professor of comparative physiology at the University of British Columbia’s Department of Zoology. “But you can push it quite a distance,” he said, making it clear that “quite far” means two to four inches here.
The urinary powers of the frog leafhopper are well known among entomologists. But the sucking abilities of insects, which long puzzled scientists, have proven to be far more impressive, as shown by a paper on meadow leafhopper feeding mechanisms published Wednesday in Proceedings of the Royal Society B.
Frog leafhoppers are widespread in Europe and North America and are known to spread certain bacterial diseases among plants. They urinate almost constantly because the insects feed on pure xylem juice, a liquid so low in nutrients that you have to sip and sip and sip, sometimes for up to 24 hours at a time.
Most juice-drinking insects drink phloem, a sugary liquid in plant vessels that is easy to get because it’s powered by positive pressure, which means it gushes out of a plant stem that has once been pierced by mouthparts. In contrast, Xylem is powered by negative pressure – its vessels actually pull inward – which makes sucking out the aqueous liquid terrifyingly difficult. Such negative pressures exist in the uninterrupted columns of the xylem vessels, where water is drawn from the roots into the leaves to evaporate into the atmosphere, said Dr. Matthews.
To demonstrate the suction power of the cicadas, Dr. Matthews, Elisabeth Bergman, a Masters student he advised, and Emma Green, a student volunteer, studied the morphology of insects and tested their metabolic abilities in 2019. Their test subjects came from the weeds near their laboratory.
The researchers made micro-CT scans of the heads of adult leafhoppers and analyzed the morphology of their cibial pump, a structure in their head that allows them to draw the xylem sap into their face. Like a plunger in a syringe, a diaphragm is pulled by muscles to increase the volume of the chamber and draw in xylem juice. Because cicadas have to pull rhythmically on this diaphragm to suckle, the nasal structure between their eyes, known as the post-clypeus, is tremendously strong to accommodate all of this muscle.
“It’s like a huge bicep on your head,” said Dr. Matthews.
Using the dimensions of the frog horrors’ cibial pumps, the researchers calculated how much negative pressure the insects could create in their heads. Their calculations suggested that frog leafhoppers might be able to generate up to 1.6 megapascals, a pressure greater than the tension in many xylem vessels.
This showed that the frog leafhoppers were able to suckle much more than anyone had previously believed. If the insects were “on top of the torch of the Statue of Liberty, a straw could go all the way to the bottom of a glass of water and they could suck it up quite happily,” said Dr. Matthews, adding that the Froghoppers would be fine even meters above the torch.
After calculating the strong pull of the insects, the researchers wanted to confirm that the action consumed no more energy than it generated. To test this, they placed frog leafhoppers and a piece of pea plant in airtight acrylic chambers to measure how much carbon dioxide the insect was producing after 30 minutes of sipping its juice.
Although the insects appeared silent to the human eye, enlarged videos of the frog leafhoppers’ faces showed how much their facial muscles move during feeding.
“All of a sudden a bug sitting there and doing nothing looks like its nose is wiggling like crazy,” said Dr. Matthews, referring to the post-clypeus of the froghopper.
The pea plant was grown hydroponically with the bare roots dangling in a nutrient solution. As a result, the solution could easily be exchanged for polyethylene glycol, a liquid with an even stronger negative pressure than the nutrient solution. Dr. Matthews compared drinking polyethylene glycol to a cyclist riding up a hill instead of on level ground. The researchers argued that the froghoppers would slow down when confronted with the even more resilient liquid. But the frog leafhoppers managed to maintain their suckling speed, albeit at a rapid metabolic rate.
Alberto Fereres, an entomologist in Madrid, said the study helped explain how P. spumarius might feed on plants with “very negative tensions” like rain-fed olives and grapevines.
The metabolic measurements showed that the insects were able to gain more energy than they used, even when they sucked xylem juice at full throttle. “That is their existence,” said Dr. Matthews. “Drink and filter and pee and pump.”
Although this process is extreme on the side of a frog leafhopper, a single sucking bug would most likely be imperceptible to any plant. Unless there is an infestation, then its copious clumps of liquid waste can even resemble rain.