Saucer bugs in the family Naucoridae are like scuba divers, using air-trapping hairs
Learning to Swim
Michael A. Wall, Ph.D.
Like many children, my son Connor did not embrace his first toddler swim lessons with a lot of enthusiasm. In fact, anything that vaguely resembled getting wet above the shoulders was met with steadfast disapproval. Okay... tantrums. But really, who could blame him?
Humans are terrestrial organisms from a long historical lineage of terrestrial organisms. We are not built for aquatic living. Even the greatest Olympic swimmer looks gangly and uncoordinated relative to the grace and speed of a shark or barracuda. Yet, we are drawn to water as an essential component of life, a provider of food, a creator of energy, and as a system of transportation. And we are not alone.
Long before humans toddled into the evolutionary landscape, insects and their relatives emerged as the first land animals. Derived from aquatic relatives, these terrestrial organisms became highly specialized to life in the mean streets of a dry world. Yet eventually, some groups of insects returned to the water. It is in these groups that we find interesting parallels of how insects and humans have tried to shed their terrestrial roots to take a stab at life in the water.
Despite my best bubbling motorboat impression, it was curiosity that got Connor. Curiosity and a 99¢ snorkel conquered the aquatic environment’s greatest challenge to us terrestrials—the lack of air. Yet this umbilical to the atmosphere is old news in the world of insects. Over 200 million years ago, terrestrial relatives of water scorpions (insects that look like aquatic preying mantids; see photo) saw an opportunity for more food in the water. Initially these insects probably just dipped their upper bodies into the water to hunt, leaving the air-breathing spiracles of their abdomen exposed to the atmosphere. Water scorpions that could go deeper in the water would have greater access to food, so eventually natural selection led to the development of the insect snorkel.
In the case of water scorpions, all air-breathing is accomplished by two spiracles that are set at the ends of long tubes. The tubes stay in contact with the atmosphere and our six-legged friends can breathe easily while underwater.
Shackled to the surface by a snorkel, Connor realized he was missing out on something more in the underwater world (like the quarter his dad temptingly threw into the deeper water). Physics actually constrains how long a snorkel can be, so the snorkel solution is of limited value. Ultimately Connor sucked it up (literally) and took the plunge (ditto), taking a small hunk of the atmosphere packed away in his pint-sized lungs. Twenty-five cents and 250 million years later, Connor caught up with the saucer bugs, a group of predacious aquatic insects.
Saucer bugs, in the family Naucoridae, occur in slow-moving water and ponds and have no need to stay attached to a snorkel, nor do they have gills like a fish. Instead, saucer bugs periodically visit the surface to grab some air. Unlike humans who suck that air into their lungs, insects have no lungs and must keep the air on the outside of their body so that it may passively float into their spiracles as needed. This is no small feat as air bubbles are bound by the laws of physics to rise in water. So how does the modest saucer bug keep a grip on something as elusive as air? Covered in a complex network of fine hairs that repel water, saucer bugs use these hairs to trap air at the water’s surface. The air remains trapped in this prison of hydrophobic hairs until the saucer bug absorbs all the oxygen and must return to the surface.
My son now swims like a predacious diving beetle (which is pretty great, if you have seen a predaceous diving beetle swim), doing cannonballs, dives, flips, and twists, always emerging in time to breathe deeply. This is the irony of water for humans.
We must have access to water (for more serious reasons than a dip in the pool) but too much is smothering and too little is deadly. From the simple snorkel to miles of canals and strategically placed dams, we humans in the last several hundred years have created solutions to our problems with water and have dramatically changed the waterscape worldwide with little regard for the evolutionarily engineered living aquatic marvels that already had it figured out. Does our human creativity rival that of the evolutionary marvels that exist in the insect world around us? I doubt it. The real test for us, as is explored in the Museum’s newest exhibition Water: H2O=Life, opening July 19, will be to see if we can manage to use water to preserve ecosystems, so that all life—water scorpions, saucer bugs, and kids named Connor—an survive and flourish.
Dr. Michael Wall is Curator of Entomology at the San Diego Natural History Museum and proud father of two little swimmers.
Water scorpions in the insect family Nepidae are underwater predators with arms like a preying mantis, but the elongated tubes on their rear are used like a snorkel to breathe air. Photo by Michael Wall.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, July 2008