Let's Dance, Said the Caterpillar to the Ant
AT this late summer season in North America, many places resound with insect music. Crickets, locusts, katydids, and others call incessantly in aid of love and war - or courtship and defense, to use the biologist's less emotive terms. But what of insect talk we do not hear? The August din recalls a story that surfaced in June in a research paper in the journal Science. You could call it the case of the tap-dancing caterpillars.
Briefly put, entomologist Philip J. DeVries of the University of Texas at Austin has found that caterpillars of a number of butterfly species communicate with ants by tapping on leaves and twigs. The sound travels through the leafy twiggy substrate rather than through the air. Its purpose seems to be to recruit an ant bodyguard for the caterpillar.
It's one of those mutually beneficial arrangements that ecologists call symbiosis. The caterpillars produce amino acids, sugar secretions, and other goodies that the ants collect. The ants, in turn, protect the caterpillars. In fact, Dr. DeVries says that, if insect predators find one of these caterpillars without the ant bodyguard, the caterpillar has ``no chance of survival.''
If that's all there were to it, the caterpillars would get the worst of the bargain for the ants would tend simply to collect the food and run off to their nests. Thus, as DeVries notes, ``There is a premium for any caterpillar species involved in symbioses with ants to maintain a constant cadre of ant guards.'' Ants communicate by vibration. The caterpillars have ``learned'' the ants' vibration language. They tap out calls that ``persuade'' ants to linger and defend them.
To put it in less anthropomorphic terms, here is a finding that points to a prevously unknown aspect of insect evolution. DeVries explains that ``our current understanding of insect communication suggests that acoustical signals evolved in response to courtship and rivalry, mate recognition, short-distant communication between colony members of social insects, or as defense.''
Now, he adds, ``This study points to the possibility that, under selection for symbiotic associations, the calls of one insect species have evolved to attract other, distantly related insect species.''
The caterpillars involved belong to the Riodinidae and Lycaenidae butterfly families - the only butterfly families in which some members are known to form symbioses with ants. DeVries notes that, until now, entomologists didn't know butterfly caterpillars made sounds or that sound was involved in symbiotic relationships with ants.
But, as he researched the subject, DeVries found that the ability of butterfly caterpillars to tap out calls has evolved independently at least three times - twice among riodinid species and once among the lycaenids. Moreover, it has always evolved as part of a relationship with ants. His survey covers butterfly species from Australia; South, Central, and North America; Europe; and Thailand. He also carried out experiments in the field in Panama and in the laboratory.
Some of the caterpillars tap away with appendages, called vibratory papillae, whose function was unknown before. Other species use mechanisms not yet discovered.
The sound itself, which DeVries compares to a snare drum played with brushes, is a series of pulses. It has an average pulse rate of 16.5 pulses per second and an average dominant frequency of 896 hertz - with average high and low frequencies of 1480 and 370 hertz, respectively. Try that on a music synthesizer.
When caterpillars are resting, they don't appear to call to ants. But, when active, they walk along wagging their heads and tapping out their seductive messages. DeVries's microphone has detected these calls up to distances of five centimeters from the dancing insect as the vibrations traveled through leaves, stems, and bark.
Speculation as to whether insects can be said to think or be conscious is controversial and is dismissed by many biologists. But there's no doubt that there are many subtle modes of insect communication. Every chirp, buzz, and tap has a meaning.