David Merrill '97 has developed a big interest in tiny creatures

By John Marchetti

Special to the Gazette

Of all the creatures great and small that roam our earth, we tend to reserve a special disdain for insects. They sting us, they infest our homes, and they ruin our picnics. Perhaps worst of all, they give a bunch of us the heebie-jeebies. In many minds, bugs are something to avoid, or better yet, exterminate.

Yet for Harvard College Research Program participant David Merrill '97, they have always been a source of fascination. "As a child I was always turning over rocks in the backyard," he says, "and watching to see what scurried away." His interest in insects has lead him to a position in a Harvard research laboratory and an exciting project on a complex relationship between ants and caterpillars.

Merrill actually started his research career at Harvard working with another type of creature that makes most squeamish: bats. As a freshman, he studied their use of sonar with Professor Cynthia Moss. Moss soon picked up on Merrill's interest in insect biology, however, and introduced him to Professor Naomi Pierce, a colleague who specializes in insect interaction. By the end of his sophomore year, Merrill had carved a niche for himself in the Pierce lab.

After a summer spent at the Smithsonian Insect Zoo in Washington to study ant colony growth, Merrill returned to Harvard and his work with Pierce. He began to examine closely the symbiotic relationship between a species of Australian butterfly and an ant species. (That would be Jalmenus evagoras and Iridomyrmex anceps for those of you in the know.) In this apparently mutualistic association, the ants protect the butterfly in its larvae

form of caterpillar from predators. In return, the ants are rewarded with nutritious secretions from the backs of the caterpillar.

"In the field and in the lab in which I work," Merrill relates, "this is a spectacularly vivid display. The ants literally swarm over the caterpillars, preening and lapping and scurrying about until all that can be seen is a seething, glittering mass of industrious ants." Merrill has found evidence, however, that the benefits the ants reap from the association are not as clear-cut. He has discovered that the caterpillars seem able to minimize what they provide the

ants while still enjoying their full protection.

For starters, caterpillars clump themselves in tight "aggregates" in order to share the burden of keeping the ants happy. "Providing the secretion has a draining effect on the caterpillars," Merrill explains, "so if they group together, the strain on each individual is less." Feeding ants spread themselves over the aggregate rather than swarming a single caterpillar.

More importantly, the caterpillars seem able to "fool" the ants into tending them at the expense of their own brood. Merrill happened upon this observation accidentally. In an earlier, unrelated experiment, the size of ant colony he was nurturing shrunk through attrition. To his surprise, the remaining ants left their young behind to tend the caterpillars, in effect sacrificing their own reproduction for the sake of the butterflies'. The end result? Fewer ants.

Merrill's observation throws new light on the relationship between the two insects. Conventional wisdom had long considered the relationship a mutualistic one, where both ant and butterfly benefit equally. However, his findings suggest it is actually a parasitic one, where the butterfly prospers at the expense of the ant.

So what could possibly motivate the ant to ignore its own instinctual urge to reproduce? While his study is still very much in progress, Merrill thinks that the answer may lie in the ant's heavy reliance on chemical communication. "Professor E.O. Wilson has done a lot of work in this area," says Merrill. "He showed, for instance, that fire ants will lead each other to food by dragging their stingers in the dirt and leaving small traces of poison. This seems to explain why we see long trails of ants moving toward food."

It is possible that caterpillars have succeeded in "cracking the code" of the ants' chemical communication, Merrill believes. The caterpillars may use their secretions to skillfully fool the ants into tending them with lesser reward. Pierce, Merrill's HCRP mentor, points out that there are other instances of this phenomenon. "Lycaenid caterpillars can 'break the code' of ants so they are favorably recognized and tended rather than attacked," she says. "In

some species, they are actually carried into the ant nest where, like wolves in sheep's clothing, they devour the ant brood."

Merrill will test his hypothesis that the caterpillar uses aggregation and secretion volume mediation to deceive its attendant ants in a series of experiments. He will present the results in his senior thesis for the Biology Department. In the coming months, Merrill will also launch a project with the venerable Professor Wilson, whose years of work with insects recently landed him a Pulitzer Prize.

After graduation, Merrill says he'd like to take a year off to "travel and chill out for a while." Nevertheless, he is currently applying to medical schools. (He had toyed with the idea of pursuing a Ph.D.) Merrill is considering a specialty in medical entomology, or the study of the relevance that insects have in human health issues. Medical entomologists, for instance, have tried to engineer a mosquito that did not spread malaria. "It might be a perfect pairing

of my interests," he says.