Nature up close: Dragonflies


A blue darner dragonfly. These extreme predators – tiny compared to their prehistoric ancestors - are also examples of the relationship between oxygen levels and body size.

Sherri O'Brien

By "Sunday Morning" contributing videographer Judy Lehmberg.

Nature: Dragonflies

Dragonflies are in the Order Odonata. Odon is the Greek word for tooth. Dragonflies don't have true teeth but they do have extremely large, strong mandibles with sharply-pointed tooth-like serrations. I've, unfortunately, experienced their mandibles first hand. A very well-meaning student brought me a dragonfly he found under a large plate-glass window presuming it had hit the window and died. I took the dragonfly from him as several other students gathered around to admire it. Suddenly it came to life and bit me. (It wasn't dead, it was just stunned.) That dragonfly drew blood! My student was really apologetic as we all learned how strong dragonfly mandibles are. Their strength is one of the reasons they are such successful predators. If they were a little larger -- more on that in a moment -- we would be rightfully scared to death of them.

A dragonfly nymph preparing to shed its exoskeleton to become an adult. Its bucket-shaped jaws are tucked in under its head.  Verne Lehmberg

Dragonflies spend most of their life in freshwater ponds and lakes. Their life starts as an egg laid in freshwater. When the egg hatches they go though a series of larval stages that can last up to two years. Dragonfly larva (nymphs) are aggressive predators who eat anything they can catch, from mosquito larva to tadpoles, and even small fish. Their jaws function much differently than an adult dragonfly's. To understand dragonfly nymph jaws, think of a bulldozer that can totally retract the front bucket on hinged joints. The bucket can reach way out to grab prey, then quickly retract to swallow their catch. When they are ready to change to adults, the nymphs crawl out of the water, their exoskeleton cracks open on the dorsal side, and they gradually emerge. After several hours their wings stiffen up and they are ready to fly.

Dragonfly adults catch their prey while flying. They are one of the most efficient predators in existence as they catch about 90% of the insects they chase. They owe their predatory skills to fast flying (they can reach speeds of 45 mph), superb maneuverability, and eyes which cover almost all of their head.

Each dragonfly eye has about 30,000 facets. Verne Lehmberg

Dragonflies are also some of the oldest insects. They evolved about 350 million years ago when Earth was a much different place than now. Modern dragonflies have a wingspan of between two and five inches, but we know from fossils that some of the first dragonflies had wingspans of about two-and-a-half feet. Recently scientists carried out some revealing experiments to discover why dragonflies were so much larger then.

Researchers have known that Earth's atmosphere contained more oxygen 300 million years ago than it does now. Our atmosphere is currently about 21% oxygen, but it was around 30% during the Paleozoic era. For years it was believed early dragonflies were larger because they benefitted from the increased oxygen levels. Knowing most organisms are dependent on oxygen, that higher percentage sounds great, but now scientists realize it isn't so good. Although oxygen is essential for most living organisms, it is extremely reactive. Think about what happens when a fire suddenly gets more oxygen. We take it for granted that oxygen is "good" because we are so dependent on it, but because it is so reactive, it can do major damage to living tissue. Premature babies are a good example. Because they are born too early, their lungs are not fully developed and therefore less capable of absorbing oxygen. Preterm babies are normally placed in an incubator with a higher-than-normal oxygen concentration, which is essential for their survival, but can cause damage to developing retina cells, delicate lung cells and other tissues.

So how did the increased oxygen levels affect ancient dragonflies? It may seem counterintuitive but potentially toxic oxygen levels did make dragonflies larger. The reason is intriguing and could give us some insight as to yet another reason why global warming will cause even more harm than we already know about.

To understand the relationship between oxygen levels and body size it is essential to understand the relationship between an animal's surface area to the volume of tissue inside. Smaller animals have a larger surface area-to-volume ratio, while it is exactly opposite in larger animals. One of the major implications of this relationship is smaller animals must exert more energy to maintain their normal body temperature in cold environments. Think of the animals that live in the Arctic, such as polar bears and musk ox. They are large animals, have a smaller surface area-to-volume ratio, and therefore can maintain their body temperature during Arctic winters. Smaller Arctic animals, such as the Arctic fox, have shorter snouts and legs to reduce their surface area as well as arteries in their legs which transfer heat to the leg veins, rather than losing it to their environment (better than their more Southerly cousins).

A resting dragonfly.  Verne Lehmberg

Now consider dragonflies. Dragonfly nymphs absorb oxygen through their exoskeleton and anal gills. The oxygen is absorbed by simple diffusion, which the nymphs can't control. When atmospheric oxygen levels were higher 300 million years ago, they would have also been higher in the water where the nymphs lived. When the oxygen levels in their cells increased, the result would have been cell damage, perhaps cell damage so severe the nymphs died. As some dragonflies grew larger, their surface area-to-volume ratio decreased, along with the total amount of oxygen they absorbed. The lower oxygen levels meant less tissue damage, so larger dragonflies survived while smaller ones did not. This would also explain why larger dragonflies didn't survive as oxygen levels decreased later in our planet's evolution. Their performance levels would have decreased and they would become sluggish in lower oxygen levels. A sluggish dragonfly is not a good predator, so they died.

As the Earth warms, so does its water. Many animals, including dragonflies, are ectothermic (or cold-blooded). That means they can't control their body temperature. Their temperature is similar to that of their environment. As ectothermic animals warm, their metabolism speeds up, as does their need for more oxygen. Terrestrial insects have trachea in their abdomen. They can increase their oxygen levels by enlarging the trachea openings (the spiracles). That doesn't apply to aquatic insects or other aquatic species, however. Fish, clams, aquatic worms and many other aquatic species can't survive in low oxygen situations, so as our world warms, aquatic animals may suffocate from lack of oxygen. 

Judy Lehmberg is a former college biology teacher who now shoots nature videos.

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