Impostor in the nest: a beetle disguised as an army ant eludes capture by ants as well as entomologists

Natural History, June, 2003 by Robert Dunn

When most people think about the explorers and adventurers of the past, figures such as Captain James Cook or Sir Edmund Hillary come to mind: heroic individuals who explored the world's greatest oceans or climbed the world's highest mountains. My own heroes were another group of explorers, who set out with more modest conquests in mind. They were the natural historians who headed for the hills to chase a new species of beetle, or snare a new bird, or climb a hollow tree to capture a new snake. As an entomologist working in the tropics, I see these collectors as my sometimes humbling, sometimes fumbling predecessors. When I kneel in the forest and turn over rocks, I feel some of the awe my predecessors must have felt.

Unfortunately, though, the days are now few when I get into the field as a biologist, with no more tangled purpose than to find and observe rare species. Darwin had a ship that carried him to biologically unexplored terrain. My colleagues and I are preoccupied with committee meetings, student cheaters, and asbestos abatement.

So when Carl W. Rettenmeyer, a biologist and an emeritus professor at the University of Connecticut, bumped into me in the hallway in the fall of 2002 and asked whether I would join him on an expedition to the cloud forests of Costa Rica, I started packing. Our mission: To look for a mysterious army ant and a rarely seen but look-alike beetle that lives in its midst.

The team that left for Costa Rica included Carl and his wife Marian; Charlene and Adam Fuller, photographers, collectors, general natural historians, and veterans of the Rettenmeyer army-ant expeditions; and David Lubertazzi and me, graduate student volunteers and all-around grunts. As our plane veered south from Hartford Airport, we left behind the frozen forests of New England for forests where insects, particularly ants, run the show year-round. For mammal watchers, the tropics can bring disappointment; large vertebrates are as scarce there as anywhere else. But the bugs, oh the bugs! Insects overflow in the tropics, both in number and kind. To an entomologist, the tropical forests are more than rich: they are overwhelming. Turn over a log, and one of the hundred or so small animals that scurry away is likely to be a new species of insect.

Many of the first explorers in the New World wrote home about army ants, as have more contemporary writers, natural historians, and the like. Army ants, particularly the species that raid above ground, are dramatic, abundant, and hard to miss. Some of the early or popular accounts of army ants are accurate, but most of them owe more to fantasizing than to observation. The army ants of myth eat everything in their path--children, tapirs, entire villages. They are monsters, to be sure, but predictable ones, scary and inexorable automatons conjured by our collective imaginations. In Carl Stephenson's short story "Leiningen versus the Ants," a Brazilian official says of army ants: "They're not creatures you can fight--they're an elemental--an 'act of God'! Ten miles long, two miles wide--ants, nothing but ants!"

Real army ants are both more interesting and more complex than those of story or myth. Real army ants don't kill people; most of them don't even forage above ground. A typical army ant species lives in nests underground that are built out of the living bodies of its workers. It migrates en masse from place to place as it feeds on the soft brood of other social insects.

The army ant we were looking for, Neivamyrmex sumichrasti, was first documented by the French naturalist Francois Sumichrast, working at the time in Mexico. Sumichrast wrote of the ant that would later bear his name:

   All the researches that I have made up to
   this time to discover the formicarium [nest]
   ... have been fruitless, and I cannot obtain
   any information from the natives
   where these insects are common.

He observed and collected N. sumichrasti from Mexico, but the species ranges throughout the highlands of Mexico and Central America.

Sumichrast's sketchy text was one of the only published accounts of the ant, until Carl Rettenmeyer and his student Roger D. Akre found the ant again in Monteverde, Costa Rica, in 1963. Rettenmeyer studied the species long enough to become fascinated by the odd tagalong guests that live with it. In the years that followed, he often thought about returning to Costa Rica to study N. sumichrasti and its guests more completely. Last winter, almost forty years after that initial encounter, he finally got the chance.

Most ants cohabitate with guests, animals that live in or around the colony and depend on the ants for food, shelter, protection, transport, or some combination thereof. Some guests are welcome, others are not, but most are neutral: just there. Many such interlopers are so well adapted to life in the ant's special world that they can survive nowhere else. A single colony of army ants might host dozens of species of beetles, tens of species of mites, and a variety of silverfish and flies. That diversity and beauty has fueled Rettenmeyer's lifelong passion for army ant guests. One of his favorite guests, and the focal point of our mission, was a beetle that's been collected so far by only one scientific expedition (Rettenmeyer and Akre's 1963 trip to Costa Rica)--a little creature named Ecitosius robustus, which, roughly translated, means "the robust army ant beetle."