When an insect gets too big for its exoskeleton, it sheds it. This process—known as molting—might sound matter-of-fact, but it’s not. Insects stop eating, many lie still, and they become more vulnerable to predators. Now, a study of mayfly larvae has revealed another difficulty: While molting, insects can’t breathe. Alarmingly, the respiratory impairment grows more severe with higher temperatures, suggesting that climate change and other stressors could make molting an even greater challenge.
The discovery comes from the lab of David Buchwalter, a toxicologist at North Carolina State University in Raleigh. Working with entomologist David Funk of the Stroud Water Research Center in Avondale, Pennsylvania, he studies how temperature and contaminants affect aquatic insects. Mayflies, for example, are a common indicator of the ecological health of streams. Using tiny sealed chambers, Buchwalter measures the amount of oxygen consumed by mayfly larvae (Cloeon dipterum).
Aquatic insects breathe with gills. After oxygen diffuses from the water, it passes into a branching network of ever-smaller airways, called tracheoles, which deliver the gas directly to clumps of cells. Larvae can also absorb some oxygen through their soft exoskeleton.
Molting takes their breath away. When larvae slip out of their exoskeleton, the lining of the tracheoles comes with it. “It’s like having your lungs ripped out,” says Joseph Bernardo, an ecologist at Texas A&M University, College Station, who was not involved in the research. Although it was fairly common knowledge among entomologists that the tracheal linings come out—and likely block the trachea in the process—the impact on respiration hadn’t been measured.
Buchwalter became interested after one of his experiments showed an erratic pattern of oxygen consumption. His graduate student Allison Camp noticed that the mayfly larvae in the chamber had molted. That observation was intriguing, because in their lab work, insects die most often during a molt. Could oxygen deprivation be a factor? The researchers increased the number of larvae they used in their experiments, keeping an eye out for molting. Eventually, they had 16 instances—and a pattern of wild swings in oxygen consumption, which they describe in the September issue of Freshwater Science.
Here’s what happens: In the 3 to 4 hours before molting, larvae consume 41% more oxygen than normal. Then they stop breathing for 45 minutes to an hour, while their exoskeletons slide off. Once the tracheoles are cleared, oxygen consumption spikes. “Just like if you held your breath for as long as you could, and then breathed in a huge gasp,” Buchwalter says. For about 2 hours, the insects use oxygen at a high rate. Then, apparently spent, the larvae slow their breathing for another few hours. “No one had any idea that molting so strongly affected breathing,” Buchwalter says.
Despite the drama, it’s not clear whether the oxygen deprivation actually causes damage. Buchwalter has not been able to look for any signs of tissue damage or other harm in the millimeters-long larvae. Kendra Greenlee, a developmental physiologist at North Dakota State University in Fargo who was not involved in the research, says that insects have surely evolved to deal with this stressful part of their life history, and many insects can tolerate low-oxygen.
But rising temperatures are a worry. When conditions warm by just a few degrees, insects molt more frequently. They grow faster but become smaller adults that have fewer offspring. “Really small changes in temperature can have big changes in physiology,” Greenlee says. In other experiments, Buchwalter’s team showed that the plummeting and spiking of oxygen consumption become more extreme at higher temperatures, when oxygen demands increase. “The temporary deprivation is likely more difficult to recover from at higher temperatures,” he says.
“This might be quite worrying,” says Wilco Verberk, an aquatic ecologist at Radboud University Nijmegen in the Netherlands, who was not involved in the research. “You might see climate change affect these aquatic stages more severely.” Water temperatures can also rise when trees and vegetation are removed from stream banks. Another stress is eutrophication, in which algae growth sucks oxygen out of lakes, which would make oxygen deprivation during molting even more challenging, Verberk notes.