Caterpillar bodies not built for speed.

Caterpillar bodies not built for speed

A caterpillar's soft, squishy body doesn't just make a juicy meal for birds; it also limits the caterpillar's crawling to an inefficient creep.

That's the conclusion of ecological physiologist Timothy M. Casey, who measured the oxygen consumption of gypsy moth caterpillars as they "sprinted" to keep up with a tiny treadmill. From these data, he calculated how efficiently the caterpillars convert energy into motion -- an analysis similar to figuring the fuel economy of a car. The results: Caterpillars are gas guzzlers, and remarkably slow ones at that.

In the April 5 SCIENCE< Casey reports that these caterpillars need about 4.5 times more energy to travel a given distance than do animals with solid skeletons. At their speediest, they wriggle along at about 100 yards an hour.

Casey, of Cook College at Rutgers University in New Brunswick, N.J., says his findings clarify an observation made in a number of previous energy-efficiency studies: Regardless of body shape or size, creatures as varied as humans, rats and cockroaches use similar amounts of energy to move a given weight a given distance. Those animals differ from caterpillars in one important respect, however: They all possess a solid internal or external skeleton, whereas caterpillars rely entirely on 'luid-driven locomotion.

The inefficiency of this "hydraulic skeleton" may explain the caterpillar's exceptionally uneconomical movement, says Casey, who describes the system as "a tube within a tube." When an outer muscle layer contracts, the fluid between the two tubes forces each body segment forward.

Studies of how much energy gypsy moth caterpillars spend on different activities may help researchers assess the insects' "overall energy expenditure," he adds. This provides an estimate of how much food (i.e., leaves) the caterpillar needs to eat, which in turn helps scientists gauge the best way to control the proliferation of these voracious pests.