BE GLAD INSECTS DO NOT PREY ON HUMANS

by Whit Gibbons

October 18, 2009

I recently wrote about a terrifying spectacle--an extinct eagle large enough to carry off a person. Scary to be sure, but what if dragonflies got as large as an eagle with a 12-foot wingspan? How awesome would that be?

Anyone who has seen a dragonfly snare a horsefly for a meal knows what effective predators they are, and like other insects, they are incredibly strong for their size. Giant dragonflies would definitely mean fewer stray dogs and cats in the neighborhood, and people would have to be careful going from the car to the house.

Dragonflies are among the most effective of aerial predators. If they reached a weight of 40 pounds, people would have some survival issues to worry about. Dragonflies spend their first months in water as larvae called naiads. Naiads, which look like something on the set of a horror film, are as dangerous to small aquatic animals as the adults are to walking and flying creatures. Dragonfly naiads, which can be more than an inch in length, have enormous jaws with pincers on the front of the mouth. They eat a variety of small fish, tadpoles, and larval salamanders.

What might we expect the naiad of a 40-pound adult dragonfly to look like: at least 30 pounds of nastiness that would be able to overcome the largest bass, an adult beaver, maybe even a person swimming in a lake? Inland waters filled with naiads would get as much media attention as a shark-infested ocean beach. People would soon decide that swimming in lakes, rivers, and reservoirs with naiads was no longer an enjoyable pastime. Water skiing would become much more of an adventure and inevitably result in some sensational, if gory, news stories.

As exciting as it is to consider a predator that could macerate us in the water and then metamorphose into something that could cart us away through the air, research on the impacts of true-size dragonfly naiads on tadpoles has revealed results that are of special interest in themselves. Predators use numerous tactics to capture their prey, which in turn develop defensive traits to avoid capture.

Each individual in nature interacts with and is affected by other species, and every species on earth today is a kaleidoscopic reflection of its evolutionary past. A response to dragonfly naiads as predators on their prey was observed by researchers at the University of Michigan in studies that led to a better understanding of the adaptive response of prey to the presence of predators. The ability of dragonfly naiads in a pond to alter morphological development of prey was observed with frogs. The finding was documented by raising tadpoles in waters with dragonfly larvae and without them. When dragonfly predators were present, the shape of the tadpoles changed significantly.

Body proportions of individual tadpoles were measured as an indicator of morphological change. An image of a tadpole's tail fin, body, and tail muscle was placed on a computer screen so that linear measurements could be made. The image analysis approach allowed comparison of a large number of tadpoles from predator-free situations with those from experimental ponds harboring naiads. The rate of growth and development of tadpoles as they metamorphosed into frogs was not affected, regardless of whether dragonfly predators were present. For example, bullfrog tadpoles reached the same general sizes regardless of whether naiads were there to eat them. However, tadpoles reared with naiads showed significant morphological differences from those raised without the presence of predators.

Among the changes was that the would-be frogs developed deeper tail fins when predators were present. Tadpoles use their tails for propelling themselves through the water, so bigger tail fins would increase swimming speed. Being faster when a voracious dragonfly predator is in the vicinity could be an effective antipredator mechanism. It is remarkable that merely the presence of naiads in the water would cause the tadpole's body to react in such a protective manner.

We should appreciate the intricate species relationships and delicate networks that comprise natural communities and make our world such a marvelous place. We should also be thankful that dragonfly naiads do not reach the size of alligators.

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