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, 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, Dr. Matthews said.
To show the power of the froghoppers’ suction, Dr. 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. As froghoppers must rhythmically pull on this diaphragm to suck, the nose-like 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,” Dr. 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 head. 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 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,” Dr. Matthews said, adding that the froghoppers would still be fine even yards above the torch.