Swim Lessons

Editor’s Note: This article originally appeared in Entomology Today, a project of the Entomological Society of America with the goal of reporting interesting discoveries in the world of insect science and news from various entomological societies. To learn more, visit www.entomologytoday.org.

It may come as a surprise to many people, but some of those black ants encountered in forest and field — and sometimes as house guests — can swim. A study published last year in the Annals of the Entomological Society of America examines how two ant species swim and concludes bigger is indeed better when it comes to which of them swims fastest.

Longer — and, perhaps, more muscled — forelegs do the trick, according to the study, which compares swimming performance by worker ants of two North American species, the black carpenter ant (Camponotus pennsylvanicus) and Formica subsericea, sometimes known as the silky field ant. During the experiments, black carpenter ants performed like Michael Phelps while the silky field ants swam more like the rest of us.

“The association between greater foreleg lengths and faster swimming velocities for C. pennsylvanicus is first presented in our study,” says lead author Evan M. Gora, Ph.D., of the University of Louisville’s biology department. The fact that the carpenter ant is twice as heavy but just 20 percent longer than the field ant suggests that great- er muscle mass also increases propulsion while swimming, he says.

While some ants, such as the notorious fire ants, can float, most cannot swim. A few — such as the diving ant Camponotus schmitizi of Borneo, which dives into water pooled in carnivorous pitcher plants to steal insects trapped there — excel aquatically. The carpenter and silky field ant are among those species that are not associated with water but swim well enough to make it across water hazards or avoid drowning if they fall into a pool.

Even ants that can swim lack special adaptations for aquatic locomotion, such as the fuzz on the feet of the water strider that traps air and serves as a float, but swimming ants use the same legs that carry them about on land, modifying their movements for water. The diving ant uses its forelegs and mid legs for propulsion, while the black carpenter ant relies mostly on its forelegs. That said, performance of an ant in water is not always a matter of different strokes for different folks, but it also can depend, says the study, on subtle differences in the morphology of the ant’s legs.

The black carpenter ants, Gora and his colleagues found, doubled the swimming velocity, maximum velocity and maximum acceleration of their smaller counterparts. The researchers anticipated that the superiority stemmed from a difference in leg motions, but it turned out both ants generally used legs in the same way. The researchers determined which legs do what by removing various pairs of legs on individual ants dropped into a container of water and filming. The forelegs propel while hind legs serve as rudders, with the mid legs performing both functions.

Given similar swimming techniques used by both ants and the fact that their morphologies are superficially similar, the researchers looked for more subtle reasons behind the difference in swimming performance and found it in the foreleg, which provides the most propulsive power. Their measurements of leg lengths revealed that the black carpenter ant foreleg was relatively longer than that of the silky field ant.

Both of the ants studied were collected from two Louisville parks. Although they may share the same general habitat, the greater swimming performance of the carpenter ant seems linked to a somewhat different ecology than that of the field ant. Carpenter ants are wont to forage in vegetation above ground, while field ants stay put down below. Field ants therefore are likely to have a choice of how to traverse a body of water, either by swimming or marching around it. Carpenter ants, on the other hand, risk inadvertently falling into a pool or stream, raising the possibility, although not tested, that they need to swim quickly to get to shore in an emergency situation.

During the research, a departure from normal swimming behavior among the field ants revealed itself. Some of them could walk on water, their bodies elevated above the surface by their legs. Like water striders, they did not break the surface tension. Seeking why some could accomplish this feat, the researchers discovered that the walkers tended to be slightly smaller than ants that swam partly submerged.

The paper suggests that the research described could provide a basis for further study. “Expanding this work to a broader range of taxa would enable comparative tests of these proposed mechanisms and provide insight into the evolutionary history of terrestrial invertebrate swimming,” it states.

Ed Ricciuti is a journalist, author and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs and the New Urban Jungle (Countryman Press, June 2014). He specializes in nature, science, conservation issues and law enforcement.