To Drive Away Wasps, Caterpillars Recruit A Phalanx of Ants
By WILLIAM K. STEVENS
Published: August 6, 1991
MAYBE it was inevitable that singing caterpillars would be discovered by someone whose heroes are Charles Darwin and Charlie Parker.
The music made by the butterfly larvae would never pass for Parker-style bebop. But Dr. Philip J. DeVries, former jazz guitarist and present-day lepidopterist extraordinaire, has found that they do make rhythmic calls to rally their bodyguards: ants that protect the caterpillars from wasps that would otherwise kill and literally butcher them.
This relationship among caterpillars, ants and wasps is typical of the elaborate interdependencies that have evolved among species of the tropical forest. The interrelationships are more marked than those of species in temperate zones, for reasons that are still not fully understood. It may well be a combination of the forests' wet warmth, their antiquity and the year-round stability of the tropical climate that has allowed evolution to spin such intricate ecological webs.
Whatever the reason, biologists are accelerating their efforts to explore this irreplaceable treasurehouse of evolution in the face of widespread destruction of the forests.
Dr. DeVries, an independent scientist who was awarded a MacArthur Foundation grant in 1988, has found that caterpillars of many species in a certain family of tropical butterflies, called Riodinidae, are equipped with minute sound-producing organs. Two of the organs, just behind the head, are tiny grooved rods that look much like guiros, Latin American percussion instruments made of gourds. The others, jutting up from the head in serried ranks, look like nothing so much as guitar picks. Beating a Tattoo
While the caterpillar moves its head in and out, the two rods, called vibratory papillae, beat a tattoo on the guitar picks. This, Dr. DeVries found by recording and analyzing the result, produces 23 pulses of sound per second at an average frequency of 1,877 hertz.
"It's like running a comb over the edge of a table," Dr. DeVries said in a recent interview at the American Museum of Natural History in New York, where he is a research associate.
Humans could hear the sounds if they were transmitted very far through the air. But they are not. Instead, they are borne along by the leaves and stems of the plant where the caterpillar has taken up residence, and from there to the ants.
The mechanism comes into play as part of an elaborate symbiotic relationship in which caterpillars, ants and plants are linked in an evolutionary pact of mutual aid and sustenance. The caterpillars feed on newly sprouted leaves of many species of plants and also drink nectar exuded by the leaves. In turn, the caterpillars dispense life-sustaining amino acids from two tiny protuberances near their tails. The ants feed on this.
"They do a little drum paradiddle on the back of the caterpillar and it produces the amino acids," said Dr. DeVries. Like a Doberman
In return, the ants protect the caterpillar from wasps that walk up and down the plant, looking for caterpillars to kill and cut up for food. "The ants are very territorial about food resources and also extremely good visually," said Dr. DeVries. "They will run out and bite the wasp on the leg. The wasp doesn't want to bother with it, and goes away. It is like having a German shepherd or a Doberman around to protect your kids."
The caterpillar also secretes a chemical, in little puffs, that puts the ants on instant alert. "The ants snap their mandibles open and freeze," Dr. DeVries said. "At a visual stimulus, they will bite."
The caterpillars' sound-producing organs, he believes, are a means of keeping the protective bargain permanently in force. "I view them as tools that caterpillars have evolved to maintain ant associations at all times," Dr. DeVries said. "It does little good if you're associated only loosely with ants in the face of wasps. It is the combination of factors that keeps ants around them all the time."
These sorts of symbiotic relationships are common in tropical forests, ecologists are coming to believe, because of greater sunlight, higher rainfall and a broadly more stable climate than exist in temperate zones. The combination allows "a fuller expression of evolution and the buildup of extremely complex biological communities," says Dr. E. O. Wilson of Harvard University, an authority on ants and an advocate of saving the rain forests.
Tropical species, he said, tend to be packed tightly together in highly concentrated habitats that in many instances are extremely circumscribed -- a single mountainside, for instance, or the headwaters of a single river. During ice ages, when life in temperate zones is largely wiped out, many tropical species abandon the tropical lowlands, which turn from forest to grassland or savanna, and retreat to wet refuge forests in the mountains. This affords them time and continuity of existence, thus aiding the evolution of intricate interrelationships.
Even in warmer eras, like the present, species in the temperate zones are not as tightly packed together as in the tropics, and there is less opportunity for elaborate symbiotic relationships to develop.
Singing caterpillars are just one illustration of "extreme forms" of symbiosis in the tropics, said Dr. Wilson. "I could recite a volume of examples" like them, he said.