Every spring hordes of spiky, red-spotted gypsy moth caterpillars attack oaks, sweet gums, and maples, scooping semicircles from the edges of leaves as they plod along. And every spring curious though anguished yard owners watch the caterpillars creep along at a stately pace of less than three feet per minute and wonder: Why, if they’re so small and they move so slowly, must the pests eat so much?
Ounce for ounce, most terrestrial pedestrians use about the same amount of energy--derived from food and oxygen--per foot of travel, a measure called cost of transport. Whether you’re a very small mouse or a very large beetle, if you have similar body mass, you’re going to have a similar cost of transport, says Rutgers University ecological physiologist Tim Casey.
But caterpillars aren’t built quite like vertebrates and adult insects, which walk using muscles attached to hard body parts. Caterpillar bodies, with no internal hard parts, are more like water balloons with legs. Their three pairs of small, jointed front legs and five pairs of soft, sticky rear prolegs--fleshy appendages that won’t appear in the adult moth--attach only to the body wall. As Casey has learned, this body design makes gypsy moth caterpillars the Cadillacs of the animal world, expending more energy per mile than any other terrestrial animal, including sporty models like rabbits and jaguars. They are extraordinarily uneconomical, says Casey. For the amount of oxygen they use, they don’t get a lot of distance.
Casey tested caterpillar mileage with the latest thing in insect fitness, a custom-made, motorized treadmill. The caterpillars run on a two- inch-wide rubber band stretched between two rollers that are turned by a slow motor. Casey encased the treadmill in a clear plastic box and measured the oxygen used by gypsy moth caterpillars jogging at several different speeds, including their preferred, three-foot-per-minute gait. He also videotaped the action, thus revealing the sequence of steps that make up the caterpillar walk.
A caterpillar first lifts its hindmost prolegs and pulls them forward a little. Once planted, they become an anchor to push against. A single wave of muscular contraction travels from back to front, squeezing the body and lifting each pair of prolegs up and forward in turn.
Although 80 caterpillars tried out for the team, only 9 ran for the four minutes it took Casey to measure their oxygen consumption at different speeds. It was very difficult to get caterpillars to run on the treadmill at all, says Casey. They would sort of roll up in a ball or try to climb the walls. When all was said and done, however, the finalists consumed, on average, 4.5 times more oxygen than calculations based on their body mass alone predicted they should.
Casey suspects that two factors account for this terrible fuel economy: short stride and no elastic rebound. Each caterpillar step spans only about a quarter the distance stepped off by other animals of equal mass, such as cockroaches. A caterpillar loses efficiency like a car outfitted with small wheels--the car would get the same number of wheel revolutions per gallon of gasoline as would a car having larger wheels but would travel fewer miles. Furthermore, each time a caterpillar takes a step, the pair of prolegs comes to a complete standstill, storing no energy; at any one time most of the prolegs stand stock still. A racing rabbit, in contrast, stores energy in elastic tendons each time it lands, and regains some of that energy as it springs away.
What’s the caterpillar’s evolutionary gain from such costly locomotion? Perhaps the ability to grow quickly, Casey speculates. The caterpillar’s simple sacklike body serves it well as it stores food and grows from only a few milligrams at hatching to nearly a gram in three weeks.
The discovery of the gypsy moth caterpillar’s low-mileage figures won’t have any immediate benefit for imperiled trees, Casey admits, but it can enhance computer models of the life histories of gypsy moths. And from the standpoint of pest control, he says, somebody who has a better understanding of gypsy moth life history is in a better position to nail them before they do too much damage.