White foam that looks a bit like frothy spit is a familiar sight to gardeners. It is sometimes called cuckoo spit because, like the call of the cuckoo, it is a sign of spring. Inside is the nymphal form of a familiar leaping insect, the frog hopper.
The nymphs, called spittlebugs, make the foam, although it isn’t actually spittle. The bugs create the mass of bubbles by emitting air from their abdomen into their copious and watery urine, mixed with some sticky fluid to aid in bubble formation.
“They’re really more piddlebugs than spittlebugs,” Philip G. D. Matthews, a researcher in the zoology department at the University of British Columbia, who just published a report on how spittlebugs manage to breathe in that nest of bubbles.
Dr. Matthews’s specialty is not strange things insects do with their urine, but the various ways insects get oxygen from their environment. For example, there are diving insects that take a bubble of air with them underwater. Its membrane makes it easier to draw oxygen out of the water.
It was seeing the foam nests that prompted Dr. Matthews to study the insects. “It got me wondering exactly how would a spittlebug be able to breathe if it was submerged” in a mass of bubbles,” he said. One possibility was that it drew oxygen from the bubbles, like a diving insect. But that didn’t turn out to be the case, except in extreme situations, as he reported in The Journal of Experimental Biology.
Dr. Matthews and two students, Kephra I.S. Beckett and Anne B. Robertson, captured easily found spittlebugs in areas around the university and took them back to the lab. The spittlebugs showed no signs of distress in captivity and continued to go about their usual business, sucking watery sap out of the plants and producing an incredible amount of urine, 150 to 280 times their own body weight every day. For a 150-pound human that would be about 2,700 gallons a day.
They could watch the bugs, observe them under a microscope and record their oxygen use and production of carbon dioxide. Insects don’t have lungs. They breathe through tiny tunnels from exterior holes called spiracles that allow the air to flow through tunnels into their bodies. The spiracles are gathered in a groove that runs to the tip of the abdomen.
Under a microscope, the researchers could see and record the spittlebugs breaking the surface of the foam with the tip of their abdomens, apparently using it like a snorkel. When the insects were doing this snorkeling, carbon dioxide increased in the container they were placed in. That meant they were breathing. The researchers also measured oxygen in the foam itself. Same result.
When the spittlebugs were scared, they retreated deeper into the safety of the foam and stopped breathing. “When you startle them they pull their abdomen into the foam,” Dr. Matthews said, “and you can see that the rate of carbon dioxide drops to zero, because obviously they’re now hiding within the foam and the gas exchange has stopped with the atmosphere.”
They weren’t breathing. “They don’t stay in there for very long, it’s barely a minute,” he said.
When the researchers forcibly kept the spittlebugs under the surface of the foam, the insects were able to break the bubbles to get oxygen. But it was a method of last resort.
Another purpose of the foam may be for protection from other insects and birds that want to eat the spittlebugs. It keeps them hidden and it has a bitter taste. Spittlebugs themselves can transmit diseases to plants, but Dr. Matthews said they are not a major agricultural pest.
Eventually the spittlebugs retreat into one large bubble and undergo a transformation to emerge as froghoppers. During the transformation, oxygen comes from within the big bubble and the air outside as the foam dries and becomes less of a barrier. By the time the froghopper is ready to leap away to a brief life that involves no eating and no urination, the spittle is so dry and powdery.
Dr. Matthews’s team came up with an additional insight. Scientists thought the sap in plants would be very hard to suck out, requiring a great deal of energy and, consequently, oxygen use. It should be under strong negative pressure to keep it from leaking out easily. For unknown reasons, they found, the sap is not that hard to suck out and the spittlebug metabolism and oxygen use don’t increase that much while they are feeding.
Dr. Matthews did develop an appreciation for his subjects during the course of the experiment. “They’re very sweet,” he said. “They have these big bulbous noses and they’re just these slow little guys that wander around on plant stems.”