Mysteries of mayfly molting

Scientific experiments are usually meticulously planned out – every detail accounted for in order to avoid the influence of uncontrolled factors that could skew results. Still, even with the best-laid plans, surprising outcomes sometimes occur.

“It was truly an exciting and unexpected find! The first time we observed a molt, we could not figure out what was happening or why the data output was looking so different,” Allison Camp, a third-year PhD student studying environmental toxicology and aquatic insects at North Carolina State University and the lead author of a recent paper published in Freshwater Science, said in an email.

Camp and her co-authors had intended to study the affects of increasing temperature on mayfly larvae oxygen consumption. Mayflies and other insects are a fundamental part of aquatic ecosystems – among other functions, they provide a major food source for fish and other predators.

When Camp and her co-authors began monitoring how much oxygen mayfly larvae consume, they noticed something odd. Most of the mayflies were taking in more oxygen as the temperature increased, but some were exhibiting a completely different pattern – a steep drop in oxygen consumption, followed by an abrupt peak. It was as if some of the larvae were holding their breath, then gasping to make up for lost time.

As Camp said of one of the mayflies that displayed the unusual breathing pattern, “it wasn’t until we opened our respirometry chambers at the end of the experiment that we realized there was a molted exoskeleton in the chamber.”

Molting is an intense process during which a larva sheds its entire exoskeleton, including the lining of its respiratory system. Insects don’t circulate oxygen throughout their bodies via blood vessels like humans do – instead, they have an extensive network of respiratory tubes, called tracheoles, which deliver oxygen directly to different tissues.

The shed exoskeleton of a mayfly larva. The branched, feathery structure at the base of the head is part of the lining of the respiratory system. 

(Image by Allison Camp)

As Camp and her colleagues found, molting larvae show a distinctive respiration pattern – for three to four hours before they shed their skin, their respiration rate increases, then it plummets as the molt begins, when their breathing is restricted for 45-60 minutes as they shed their outer skin (and along with it, the lining of their tracheoles). Fifteen to 30 minutes after the lowest level of oxygen consumption, the rate spikes to roughly twice that of larvae that aren’t molting, followed by a gradual reduction and then recovery, until the breathing rates of the molting and nonmolting larvae converge. Camp says this process can take several hours. 

As the researchers point out, molting is a risky process for mayflies – it’s an energetically costly endeavor, and it’s associated with an increased risk of dying. They write, “molting is highly disruptive to respiratory physiology and is a far more challenging process than we had previously imagined.”

Moving forward, Camp plans to continue studying molting and the affects of temperature on mayfly development. 

“We are looking at growth rates and growth efficiencies across different rearing temperatures right now, and molting is an important piece of that puzzle because it requires so much energy,” she said in an email. Perhaps another unexpected result will help Camp and her colleagues further unravel the mysteries of mayfly molting.

Cloeon dipterum, the mayfly Camp and her colleagues studied, is a member of the baetidae family, a group commonly known as the small minnow mayflies. 

(Image by Allison Camp)