To take advantage of an unlikely source of nutrition, insects small enough to sit on an eraser have to suck harder than any known creature.
Grasshoppers Philaenus spumarius pierce plants with their mouthparts to feed solely on xylem sap, a fluid made primarily of water that moves through the internal plumbing of plants. The substance is not only largely nutrient-free, but is also under negative pressures, similar to vacuum. Sucking sap requires a suction power equivalent to a person drinking water from a 100-meter-long straw.
Such a feat seemed so unlikely to the little insects that some scientists questioned whether the sap of the xylem could really be under such negative pressures. But biomechanical and metabolic tests suggest that grasshoppers may produce negative pressures in excess of one megapascal, researchers reported on July 14 in Proceedings of the Royal Society B.
"It's incredibly impressive. (Scientists) used a number of techniques to address a long-standing problem," says Jake Socha, a biomechanist at Virginia Tech in Blacksburg who did not participate in the work. “These insects are very well adapted to generate” extreme negative pressures.
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The problem is long-standing because measuring negative pressures is complicated. Inside the xylem, the sap is pulled like a string, trapped in a strip, and loosened between spongy soil and aerated leaves. Drilling the plant with pressure probes can easily break that internal stress, so scientists typically use a more indirect method. By cutting part of a plant and sticking the leafy end in a pressure chamber with the stem coming out, researchers can increase the pressure exerted on the outside of the plant until it only exceeds the internal pressure of the plant and the sap of the xylem summarizes the stem. . This strategy suggests that negative pressures from the xylem sap may exceed one megapascal.
That tiny grasshoppers and other insects feed on xylem sap has sparked skepticism about these measures, says Philip Matthews, a comparative physiologist at the University of British Columbia in Vancouver. Elephants, for example, only generate 0.02 megapascals of negative pressure when they suck large amounts of water through their trunks (SN: 6/3/21), scarce compared to grasshoppers.
Some scientists think that “it’s too expensive to energetically extract these things, which (the pressures of the xylem) can’t be so negative,” he says. "It has to be easy to figure out if (the grasshoppers) are going to survive on something so diluted."
Skeptics of the skeptics, Matthews and colleagues tried to measure grasshopper suction skills using two approaches, one biomechanical and one metabolic. Grasshoppers produce suction power with a drum-like structure on the head, where muscles pull on a membrane to generate negative, piston-like pressures. Using four-insect micro-CT, the researchers measured the length and strength of these structures and then calculated the suction potential of the insects using the simple physical pressure formula equal to the force divided by area. In principle, the team found that grasshoppers can produce negative pressures of 1.06 to 1.57 megapascals.
“Obviously they can generate these stresses, so they have to feed on xylem stresses around this level,” Matthews says. "I wouldn't evolve such a massive capacity unless I was using it."
The team validated this more abstract estimate by calculating the amount of energy grasshoppers spend while sucking bean, pea, or alfalfa plants. That energy should be proportional to the pressures that insects have to overcome in plants. By placing grasshoppers in chambers that measure the emitted carbon dioxide, researchers could calculate the metabolic rate of insects. The team also used cameras to track the amount of liquid that the bugs removed.
Once grasshoppers began sucking, their metabolic rate increased by 50 to 85 percent from the resting rate, and insects excreted more than when they were at rest, the researchers found. The effort is “like running a marathon,” Matthews says. "They move a huge amount of fluid …. If an error were of human size, they would be stepping on 4 gallons of liquid per minute."
Although xylem sap is primarily water, there are enough nutrients to enhance the oversized capacity of grasshoppers, the researchers estimate. “They get a net energy gain,” says study co-author Elisabeth Bergman, a comparative physiologist also at the University of British Columbia.
Bergman and colleagues suspect that the sucking power of grasshoppers and other xylem sap specialists may be unmatched among animals. There are simply no other contexts in which food is locked up under such high negative pressures, Bergman says. "These little mistakes are fantastic machines for sucking."