Why doesn’t a caterpillar grow short and fat, like a cylindrical balloon?
The answer is that a caterpillar is really more like an accordion.
There is no mystery about why a caterpillar would want to be long and skinny. That’s how it’s able to move around easily by the simplest of muscle contractions, and to fool hungry birds into thinking it’s a twig or a leaf vein. The mystery, says cell biologist David Carter of the University of Western Ontario, is how caterpillars manage to be long and skinny, when by all rights they should be short and fat, even spherical. Caterpillars have no skeleton, internal or external, to sustain their shape; they are tiny balloons inflated by their own blood. Yet a cylindrical balloon--in fact, any hollow cylinder under uniform internal pressure--grows faster in diameter than in length, because the stress acting on the cylinder’s wall, and stretching it, is twice as great around its circumference as along its axis.
Somehow a growing caterpillar appears to evade this law of mechanics. But it has never been known how--until Carter and his supervisor, Michael Locke, began Krazy Gluing caterpillars to slides. The two researchers were testing a new microscope, looking at a caterpillar, Calpodes ethlius, that has a transparent cuticle, or outer skin layer. They were trying to do so without distorting the insect’s shape. I thought it would be a neat idea to mount the caterpillar on a glass slide and look right through into the living insect, says Carter. I tried various ways of clamping the thing down and the only thing that worked was Krazy Glue. When I tried looking at the caterpillar like that under the microscope, presto--all these folds appeared.
The caterpillar looked like a small, cylindrical accordion--a concertina--with pleats running around its circumference and spaced evenly along its axis. The pleats extended from just under the surface of the cuticle all the way into the top of the epidermis, the layer of living skin cells that secrete the cuticle. Carter and Locke realized that these pleats must help the length of a growing caterpillar keep up with its circumference.
They proved their hunch by looking at caterpillars during the last of the bugs’ five growth stages. Early in the fifth stage, each caterpillar had a pleat every 10 to 15 microns along its axis. But by the time the caterpillar had finished growing, the pleats were as much as 60 microns apart. In some full-grown caterpillars, the researchers found, the pleats were missing altogether--stretched out, apparently, by the insects’ growth.
Based on those and other observations, Locke and Carter now believe that a caterpillar, any caterpillar, begins each growth stage with lots of deeply folded pleats. As its blood supply expands, the caterpillar’s internal pressure increases, and it begins to grow--and it can grow much more easily along its long axis, by simply unfolding the pleats, than around its circumference. Once the pleats are gone, the caterpillar continues to grow by actually stretching its cuticle; because of the stress difference, it then grows twice as fast in circumference as in length. But the head start given to the axis by the unfolding of the pleats balances the stress difference and keeps length and circumference in the same proportion. At the end of each growth stage, the stretched cuticle relaxes and contracts somewhat, and the pleats are reformed.
Why do pleats form in the first place, and why don’t any run parallel to the caterpillar’s axis? Locke and Carter found the answer in the epidermis. The cuticle is not really living, but the epidermal cells are, says Carter. They’re the ones that must organize the structure. The organizing is done by stiff fibers called microtubules.
Usually microtubules crisscross a cell and help give it its shape. In a caterpillar’s epidermis, however, many of the fibers run along the top of each cell, just under the cuticle, in a circumferential direction. These fiber bundles, says Carter, act as hoops. When the cuticle contracts, the hoops don’t, and so the cuticle drapes itself over them, forming the pleats. no pleats form parallel to the caterpillar’s axis because the hoops are too stiff to bend.
Why had nobody seen before that a caterpillar is like a concertina? Caterpillar cuticle is extremely slushy--I’ve heard it compared to snot, says Carter. So to look at it we used to cut off the caterpillar’s end and inflate it in fixative. Of course, that stretched it all out, so you wouldn’t see any folds. We’d never have seen them if it weren’t for Krazy Glue.