Thursday, October 3, 2013

September's Wild and Woolly Days

Woolly Bear caterpillar (c) John Ashley
Lateral (side) and ventral (belly) views of a Woolly Bear caterpillar
Our lazy August suddenly turned into a wild and woolly September. For the last few weeks I've had to dodge and weave through dozens of bears (exactly 12 dozen on my most recent foray!) just to reach our remote little post office. And not just any old bears, as these guys have more than a dozen claws on 10 of their 16 feet.
It's not just happening here in the semi-wild west, either. Newspaper and television reports of bears by the hundreds are also filtering in from far scarier places, like Ohio and North Carolina.

I hope you're ready, because Woolly Bears are on the move.

If there's one caterpillar that's familiar to everyone, it's gotta' be the black-on-both-ends with orange-in-the-middle (most of them, anyway) Woolly Bear. It's the larval stage of the seldom-seen Isabella Tiger Moth (Pyrrharctia isabella). Every fall they suddenly emerge from the vegetation all across North America, from southern Alaska, over to Nova Scotia and down to central Mexico. These caterpillars are looking for rocky places to hunker down for the winter.

For such a familiar animal, there are a number of odd things about Woolly Bears. The first quirk always gets me - we have two different common names for one animal, depending on whether we see it as a caterpillar or a moth. (Actually three names, if you include the larval, "Woolly Worm," of southern U.S. parlance.)

While the multiple moniker thing is mildly annoying, the Woolly Bear's other oddities are actually quite intriguing. As a moth, they mimic the sounds made by their relatives to deceive hungry bats. As a caterpillar, their method of locomotion is receiving lots of scrutiny from robot designers. And then there's also that whole weather-predicting reputation thing to look into.

Adult Isabella Tiger Moths are tasty morsels - or so I've read. But a number of related tiger moth species are unpalatable, especially to bats, which are the primary predator of moths. Most species of tiger moths have modified regions of the thorax, called "tymbals," that produce high-frequency clicks which are used as mating signals. But as a tasty species among toxic cousins, Isabella Tiger moths have become "auditory mimics," able to reproduce the sounds of toxic moth species to fool foraging bats. There's also evidence that they use bursts of clicks to "jam" the sonar of moth-eating bats.

Woolly Bear proleg and crochets (c) John Ashley
One Woolly Bear proleg showing many sharp cachets
Female Isabella Tiger Moths that don't get eaten will mate and lay many eggs that hatch during the summer. These eggs hatch into either Woolly Bears or Woolly Worms, depending on where you grew up. But to kids everywhere, the caterpillars' rhythmic "running" motion is simply irresistible.

Turns out, it's also irresistible to robotics engineers. "Hyper-redundant robots" are experimental designs meant to mimic the "travelling wave gait" of caterpillars. That is, these robots have many possible degrees of movement.

Caterpillar crawling is distinct from other soft-bodied animals, like worms and clams. It isn't just muscle expansion/contraction or the forward progression of a peristaltic wave through its body. It also includes some kinematics that are similar to the walking and running mechanics of animals with a bone structure. But how this all works with respect to caterpillars' elastic tissues "remains to be discovered" by the big-brained engineers.

What we do know is that caterpillar locomotion is 4.5 times less efficient than vertebrate motion. The cost per stride does not differ much, but the caterpillar's distance per stride is only 25% to 30% that of comparable animals.

Woolly Bear Caterpillars motivate on eight pairs of legs. There are three sets of black, jointed legs on the forward end (bottom photo), four sets of fleshy prolegs on the middle segments (top photo), and one more pair of prolegs near the back end. Each proleg has more than a dozen toenails, called "crochets" (middle photo). Small muscles in each proleg do not push the caterpillar forward, but merely lift the leg from ground contact as body motion moves through those segments. Even smaller muscles serve to press or remove the crochets, effectively tightening or loosening the animal's grip.

It's more enchanting than efficient, but it works well for an animal that is essentially a furry bag of fluid. But while Woolly Bear races mesmerize more and more people each year, it's the reputation for forecasting the weather that first made these bi-colored animals famous.

According to legend, the more orange segments in the middle of the Woolly Bear caterpillar, the milder winter will be. Starting in 1948, the curator of insects at the Museum of Natural History in New York, Dr. C. H. Curran, tested this legend by collecting and measuring as many Woolly Bears as he could find during one day each fall, over a period of eight years. His relatively high, orange segment averages (5.3 to 5.6 out of 13 segments) did seem to correspond with relatively mild winters.

More-rigorous research that followed showed that there really is a link between amount of orange on a Woolly Bear and the severity of winter - but it links back to the previous winter. The amount of orange corresponds to the caterpillar's age, or how early last winter ended and the previous generation of Woolly Bears resumed eating. The sooner Ma and Pa caterpillar start growing in spring, the earlier they will pupate into moths, and the earlier the female moths will lay the hidden eggs that hatch and suddenly appear as September's speedy caterpillars.

This weather information has done nothing to diminish the amount of furry fun that will take place later this week. The 41st annual Woolly Bear Festival is being held in Vermilion, Ohio. The festivities sound almost as wild and woolly as my daily foray to the post office.

Below the bear: the ventral view photo (top) was made from below a sheet of glass that the Woolly Bears were racing across. All images made with a 105mm micro lens and ring flash. No caterpillars were harmed in the making of these photos.
Woolly Bear caterpillar legs (c) John Ashley
Three sets of jointed legs on the forward end of a Woolly Bear caterpillar