Defensive Upchucking: A Key to Caterpillar Survival
July 16, 2007—
Vomiting is not as straightforward an activity as one might think. Humans generally throw up to empty ourselves of whatever nasty things we may have ingested. Caterpillars, however, employ this tactic to avoid being eaten.
Jacqualine Grant is fascinated by caterpillars, and thus she chose their regurgitation practices as a subject for her PhD dissertation on the gut morphology of Lepidoptera larvae.
An assistant professor in Michigan Tech's School of Forest Resources and Environmental Sciences, Grant discovered that not all caterpillar barfing is the same, contrary to popular belief among entomologists, who often ruminate on such matters. Caterpillars have a variety of defensive weapons in their arsenal, and the more potent the weaponry, the less they rely on vomiting to gross out predators.
“They have lots of defenses,” Grant explains. “There’s a Brazilian caterpillar that, if you brush against it, you die.” Others just do their best to look inedible. “There’s a caterpillar that looks like a bird dropping, and it’s so good at it, it doesn’t need to throw up.”
Grant looked at 36 species of caterpillars (though not the Brazilian one, whose victims perish from massive internal hemorrhaging) and found that all butterfly and moth larvae are not created equal. “If you rely primarily on regurgitation for your defense, one part of your digestive tract, the crop, is enlarged, so you can be a more efficient puker,” she says. “The best pukers have the biggest crops.”
The panic moth caterpillar, for instance, is extremely well adapted for puking. “They throw up all this nasty stuff on spiders, and the spiders go away,” Grant says. Monarch butterflies, however, don’t need this anatomical adaptation because predators learn quickly to keep a safe distance. “They have a horrible flavor,” says Grant.
Her team also discovered why the larvae of cabbage butterflies, which make their living devouring vegetables around the world, are so wildly successful. They extract a compound called pinoresinol from their favorite foods (cabbage, broccoli, cauliflower, etc.) and secrete it on tiny hairs covering their bodies. Pinoresinol, in combination with other chemicals, repels ants and, presumably, most other creatures interested in cabbage worm for dinner, allowing the larvae to forage through gardens unmolested.
Grant herself seems immune to the caterpillar ick factor. “I just like them,” she says of her research subjects. “I think they are fascinating and beautiful. Even the spines are elegant and symmetrical.”
Grant has revealed the fascinating and beautiful in another oft-disparaged group of creatures: young salamanders and tadpoles. She was commissioned by the US Geological Survey to paint portraits of all the larval salamander and pollywog species found in Great Smoky Mountains National Park. Her 36 watercolors appear on a USGS poster, which is displayed in the atrium of the Noblet Building.
She has been painting amphibians and reptiles for seven years, since she attended a workshop as a graduate student at Cornell University. One of Grant’s early efforts, a toad atop a toadstool, was auctioned by the Society for the Study of Amphibians and Reptiles and raised more money than any other item, she reports, still surprised at its success.
Grant studies amphibians as well as paints them, and is beginning two studies on frogs, one on the genetics of chorus frogs in South Dakota’s Black Hills, the other on the effects of road salt on native frog species. Other projects will address earthworm population genetics in the Huron Mountains and the effect of deer scat on hemlock groves.
Grant’s interests have made her a popular speaker, particularly with children. For three classes of second graders in Ithaca, N.Y., she once brought in a salamander, a toad, a pair of sand boas and a corn snake. “They loved it,” she remembers. “I fed the salamander crickets. The students were all lined up to see the feeding take place, and they got so excited they collapsed in a huge heap.”
Another public event ended on a more serene note. “I took about 100 people up to a local preserve to hear frogs chorusing in the spring,” Grant says. “Just as the sun set, the green frogs were finishing and the peepers started. As the sun went down, the gray tree frogs started. And then, in the background, the pickerel frogs started to sing.
“It was a lovely evening.”Grafr
Michigan Technological University (www.mtu.edu) is a leading public research university developing new technologies and preparing students to create the future for a prosperous and sustainable world. Michigan Tech offers more than 130 undergraduate and graduate degree programs in engineering; forest resources; computing; technology; business; economics; natural, physical and environmental sciences; arts; humanities; and social sciences.