Living the right-sized life
by Eric Sorensen | © Washington State University
I want to walk on water, climb walls, and dance on the ceiling. If insects can do it, it’s only fair that I should, too.
But this thing called physics has decreed otherwise. Carol Anelli, a WSU entomologist, can tell you why, having a lifelong fascination with ways insects can at times make us seem relatively slow, earthbound, and weak.
Anelli first came upon the wonders of insects as a child among the woods and fields of a suburbanizing central Connecticut. She would pull caterpillars from her father’s garden, raise them on carrot tops and milkweed in her mother’s Mason jars, and watch their metamorphosis into monarch and swallowtail butterflies that would fly around the kitchen. She still has the homemade medallion of the Junior Insect Collectors of Moths and Butterflies, an unsanctioned club of neighborhood preteens.
Even after she studied biology at Southern Connecticut State College in New Haven, she thought she was going to be a cashier. Then she found work in a Yale immunology lab where advanced scientific study was more a given than a fantasy. Four years later, she left to get a University of Illinois doctorate in insect physiology. Her studies were grounded in chemistry and physics, imbuing in her a sense that the forces of evolution and the physical laws constraining it “are really the story of insect success on the planet.”
Key to that success is an elegant, fundamental feature: their size.
“To understand insects is to understand that they’re small, relatively small,” she says. “There are physical constraints on them, just like on us, but they’re different ones than we deal with. The effects are way different on a small organism than on a large one like us.”
Both insects and humans are under the same, constant gravitational force. But we weigh more, which is to say we are more prone to gravity’s persuasions. So when a water strider sets foot on a water surface, its weight is not enough to overcome the surface tension caused by cohering water molecules. It might as well be stepping on a trampoline. Water striders also trap bubbles in tiny, water-repellent foot hairs, giving them flotation. A human best get a boat.
In some cases, our extra weight helps, increasing the friction underfoot when we walk. But some forego friction for adhesion, the attraction of dissimilar surfaces. They take advantage of this with an occasional sticky secretion, or a combination of goop and pads, claws, and hairs that let them grip the microscopic nooks and crannies of, say, a wall, a ceiling, or, in the case of an annoying fly, your face.
To take a big-picture view of such small-scale innovations, consider that the sizes of living things cover 21 or so orders of magnitude. A mycoplasma will weigh less than a picogram, or 10-12 grams. A blue whale will weigh more than 100,000 kilograms, or 109 grams.
Straddling the middle of the mass scale—100 milligrams, or 10-1 grams—a bee-sized insect can jump from any height with impunity. Tipping the heavier end of the scale, humans pay a price by falling harder.
“If an insect falls from a height many times its length, nothing happens,” says Anelli. “If you fall from a height many times your height, you’re gone.”
Insects also have the benefit of wearing their skeletons on the outside. Insect armor has a waxy layer to keep them hydrated in spite of a high ratio of surface area to volume.
Larger animals tend to lack armor. Not that it wouldn’t come in handy, as soldiers have learned over the last few millennia. It’s just that it is heavy, and an insect’s small size gives it the gift of relatively greater strength. That’s because a muscle’s strength is a function of its cross section. If you have a lot of muscle cross section and little mass, you can, like an ant, move more than your weight. But larger animals like us have had our mass increase on the cube while our muscle strength increased on the square. Our muscles couldn’t keep up.
In spite of its size, and because of it, the insect is one of the planet’s greatest living success stories. Its immunity to gravity, its exoskeleton, its right-sized musculature, and other physics-related advantages help explain why four out of five species on the planet are insects. Ants alone account for 15 percent of the earth’s terrestrial animal biomass. They may be small, but there’s strength in those numbers.
Carol Anelli detailed the superhuman feats of insects and the physics behind them as part of this year’s Common Reading Program. The program has 3,000 freshmen reading UC-Berkeley Professor Richard A. Muller’s Physics for Future Presidents: The Science Behind the Headlines, bringing science to the forefront of many campus discussions. Topics like global warming, nanotechnology, quantum physics, atomic-age comics, sustainability, and space travel have all been broached by dozens of WSU faculty and visiting speakers.
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