EnvironmentScience

This insect drinks your milkshake

The insect Philaenus spumarius, which is about the size of a Tic Tac, has a butt catapult that flicks its globules of liquid waste into the air and away from its body on a regular basis.

UPDATED: July 15, 2021, 01:55 PM IST

A meadow froghopper urinates so much that it could drown itself. Luckily, the insect Philaenus spumarius, which is approximately the size of a Tic Tac, has a butt catapult that regularly flicks its globules of liquid waste into the air and safely away from its body.

“At this tiny, tiny scale, ballistics become really complicated,” said Philip Matthews, an associate professor of comparative physiology in the zoology department at the University of British Columbia. “But they can flick it away pretty far,” he said, clarifying that “pretty far” here means 2 to 4 inches.

Among entomologists, the froghoppers’ urinary powers are well understood. But the insects’ suction abilities, which long confounded scientists, have turned out to be much more impressive, according to a paper on the meadow froghoppers’ feeding mechanisms published Wednesday in Proceedings of the Royal Society B.

Froghoppers, which are common in Europe and North America, are known to spread certain bacterial diseases among plants. They urinate almost constantly because the insects feed on pure xylem sap, a liquid that is so bereft of nutrients one must sip and sip and sip, sometimes up to 24 hours straight.

Most sap-drinking insects drink phloem, a sugary liquid in plant vessels that is easy to get because it is driven by positive pressure, meaning it gushes forth from a plant stem once pierced by mouthparts. In contrast, the xylem is driven by negative pressure — its vessels actually pull inward — which makes the watery liquid excruciatingly difficult to suck out. Such negative pressures exist inside the unbroken columns of xylem vessels where water is pulled up from the roots into the leaves to evaporate into the atmosphere, Matthews said.

To show the power of the froghoppers’ suction, Matthews, Elisabeth Bergman, a master’s student he advised, and Emma Green, an undergraduate volunteer examined the insects’ morphology and tested their metabolic abilities in 2019. Their test subjects hailed from the weeds near their lab.

The researchers took Micro-CT scans of the heads of adult froghoppers and analyzed the morphology of their cibarial pump, a structure in their head that allows them to pull the xylem sap into their face. Like a plunger inside a syringe, a diaphragm is pulled by muscles to increase the volume of the chamber and draw in xylem sap. Because froghoppers must rhythmically pull on this diaphragm to suck, the noselike structure between their eyes, called a post-clypeus, is terrifically strong to accommodate all of that muscle.

“It’s like a huge bicep on their head,” Matthews said.

Using the dimensions of the froghoppers’ cibarial pumps, the researchers calculated how much negative pressure the insects might be able to generate inside their heads. Their calculations suggested froghoppers might be able to generate up to 1.6 megapascals, a pressure greater than the tension inside many xylem vessels.

This showed that the froghoppers were capable of sucking much more than anyone previously believed. If the insects “were on the top of the Statue of Liberty’s torch, they could have a straw going all the way down to the ground going into a glass of water, and they could be quite happily sucking it up,” Matthews said, adding that the froghoppers would still be fine even yards above the torch.

After calculating the insects’ powerful suction, the researchers wanted to confirm the action didn’t use more energy than it gained. To test this, they placed froghoppers and a length of pea plant in airtight acrylic chambers to measure how much carbon dioxide the insect produced after 30 minutes of slurping sap.

Although the insects appeared still to the human eye, magnified videos of the froghoppers’ faces revealed just how much their face muscles move during feeding.

“All of a sudden, a bug sitting there doing nothing looks like its nose is jiggling around like crazy,” Matthews said, referring to the froghopper’s post-clypeus.

The pea plant was grown hydroponically, bare roots dangling into a solution of nutrients. This made it easy to swap out the solution for polyethene glycol, a fluid with even stronger negative pressure than the nutrient solution. Matthews compared drinking the polyethene glycol to a cyclist biking up a hill instead of on the flat ground. The researchers reasoned the froghoppers would slow down when faced with the even more resistant fluid. But the froghoppers managed to keep up their same sucking velocity, albeit with a rocketing metabolic rate.

Alberto Fereres, an entomologist in Madrid, said the study helped to explain how Philaenus spumarius could feed on plants with “very negative tensions,” such as rain-fed olives and grapevines.

The metabolic measurements demonstrated the insects could gain more energy than they expended even while sucking xylem sap at full throttle. “That’s their existence,” Matthews said. “Drinking and filtering and peeing and pumping.”

Although this process is extreme on the side of a froghopper, a single sucking bug would most likely be imperceptible to any plant. Unless, of course, there is an infestation, in which case its copious gobs of butt-flung liquid waste can even resemble rain.

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