ASU Research E-Magazine: Test Flying an Idea

In a southern California city called Zyzzyx, not far from the Boulevard of Dreams, ASU biologist Jon Harrison stalks dragonflies. The town, known for its unpronounceable name and tongue-in-cheek street signs, is also home to the Desert Studies Consortium Research Station.

At the side of a nearby dirt road, a ramada topped with palm fronds acts as a satellite research outpost. In the shade of the ramada, a picnic table is set with carbon dioxide and oxygen analyzers, tanks of compressed gas, Plexiglas flight chambers, and a laptop computer. An artificial pond is nearby.

Near the pond, tall grasses and cattails streak the Mohave Desert with improbable green. The pond attracted Harrison because it attracts pond hawks—a type of dragonfly.

When an unsuspecting pond hawk alights on a reed, swish! Harrison swoops it into his bug net. With help from his collaborator, fellow insect physiologist John Lighton, the ASU scientist pops the dragonfly into a flight chamber. Coaxed by a gentle thump on the chamber walls, the dragonfly takes to the air. Harrison and Lighton watch their subject fly.

The air inside the flight chamber has the same proportions of oxygen, nitrogen, and carbon dioxide as the air in the outside environment: 21 percent oxygen, 0.3 percent carbon dioxide, and 79 percent nitrogen.

The research question: How would this medium-sized dragonfly's performance change if the air in the chamber more closely matched that of the atmosphere present on Earth 300 million years ago?

Harrison and Lighton control the composition of the air in the chamber by changing the amounts of pure oxygen and nitrogen that flow from two tanks. This system allows them to put their bugs in an atmosphere that mimics the conditions experienced by the giant insects of the Paleozoic Era.

cross-section illustration of a large bug’s spiracles and tracheae
Large insects may require high concentrations of oxygen to allow it to reach into their bigger bodies.

The researchers turn a few knobs, bumping the oxygen concentration up to 40 percent and the nitrogen down to 60 percent. The dragonfly's flight becomes more vigorous. Measurement of its respiration rate tells the investigators that it is also breathing faster.

What if the amount of oxygen were changed again, this time to a level lower than that to which the insect is adapted?

Harrison flushes the chamber with air containing 10 percent oxygen and 90 percent nitrogen, and again entices the insect to fly. Though it pumps its wings less energetically, this dragonfly meets its aerobic challenge. A safety margin in its oxygen supply seems to have prevented it from failing in the oxygen-poor conditions.

However, when Harrison cuts its oxygen by half once more, the animal reaches its limit. Despite the laboring of its wings, its body won't budge.

Harrison determines how much oxygen the dragonfly consumed during the experiment by analyzing samples of air from inside the flight chamber. When the dragonfly breathes—for that matter, when any animal breathes—it takes in oxygen and gives off carbon dioxide. So just by breathing, the insect has a direct effect on the composition of the air around it.

Intense activity, like flying, steps up this process and makes the concentrations of these gases change even more. The same thing happens in humans when we exercise: hard work makes us burn more oxygen and produce more carbon dioxide.

Harrison collects post-flight air samples from what he calls a "flow through" system. When the chamber is flushed with fresh air, the "used" air is pumped into carbon dioxide and oxygen analyzers. These instruments give exact percent values for each gas in the air. The scientists use the data they generate to determine each animal's metabolic rate.

Harrison performs similar field experiments on several other dragonfly species. By taking advantage of the natural diversity in body size among different species, he can look at how size affects oxygen sensitivity. More members of larger species, he predicts, will fail the flight test at higher oxygen levels than will members of smaller species.

Respiratory patterns should also differ with body size.

When Harrison fills the test chamber with an extra puff of oxygen, removing the upper limit on respiration, bigger dragonflies ought to take advantage of the chance to breathe more. But smaller species, whose respiratory needs are fully satisfied in low oxygen, are not expected to breathe any faster.

Back in the lab, Harrison also tests his ideas by looking at size variation within a single species. He uses grasshoppers for this work.

A dragonfly begins life as worm-like larva. It then undergoes a metamorphosis into a flying adult. Grasshoppers skip the larva stage.

Baby grasshoppers are born looking like miniature versions of their parents. As they age and grow, they shed their confining outer skeletons and replace them with roomier new ones. This process, called molting, happens several times before the grasshopper reaches adult size.

Each time a grasshopper molts to a larger size, its oxygen requirements increase. Harrison is now planning experiments to see if the minimal amount of oxygen required for hopping increases at progressively larger stages. If it does, these insects may help modern biologists determine whether something as simple as breathing could explain the extravagant size of their long-extinct relatives.—Danika Painter