From the Field – Dodging the Bullet

    The sting of the ant Paraponera clavata is said to rival the agony of a bullet wound, giving this ant its common name: the bullet ant, known simply as ‘bala’ in the Latin American countries where it is found. Though not usually fatal, the neurotoxic venom of the sting can throb for 24 hours and is said to be the most painful of all insect stings. The Schmidt pain index poetically describes the bullet ant sting: “immediate, excruciating pain and numbness to pencil-point pressure, as well as trembling in the form of a totally uncontrollable urge to shake the affected part.”

    With this in mind, I set out to collect hundreds of bullet ants at La Selva Biological Station in Northeastern Costa Rica. The lowland wet forest around La Selva is one of the most studied forests in the world and is also renowned for its unusually high density of bullet ant nests. Many have been baptized by the fire of a bullet ant sting, an experience that is a rite of passage amongst both tropical biologists and indigenious tribes in South America.

    The author collects bullet ants by hand with forceps and vials at La Selva Biological Station, Costa Rica. Credit: Virginia Emery

    The author collects bullet ants by hand with forceps and vials at La Selva Biological Station, Costa Rica. Credit: Virginia Emery

    Bullet ants are second in size only to the dinosaur ants of South America (Dinoponera gigantea) and can easily grow to an inch long. These so-called ‘primitive’ ants can offer a unique glimpse into the lives of the earliest social insects. As a student of ant communication, I was eager to learn more about bullet ant nesting dynamics and chemical communication.

    Setting out on the hunt, I trekked through the jungle, ever mindful of the venomous snakes and golden Nephila spider webs that draped off nearby trees. Partway down an overgrown trail, I spotted a worker bullet ant foraging on a fallen log.

    I waited patiently as she carried a dead grasshopper in her formidable mandibles, moseying back to her underground home. Where do you live, little one, I wondered, following her with my gaze. She clambered over to a sapling, its trunk barely two inches wide, and disappeared into the ground at its base. My eyes lit up. It was the perfect nest. The rest of my mission would seem a fool’s errand: to dig up the nest up and capture hundreds of bullet ants alive.

    All ants use chemical nametags to communicate identity. Information such as occupation, reproductive status, and colony membership are contained in chemicals called cuticular hydrocarbons, waxy substances on the exoskeletons of most insects. Initially used as a protective barrier against desiccation and microbes, ants have evolved to express a language of hundreds of hydrocarbons.

    We can extract these hydrocarbons from insects, but only when they are collected alive and freeze-killed. The composition of these extracts is then analyzed using a technique called Gas Chromatography-Mass Spectrometry (GCMS).

    This involves baking the extract—a complex mixture of compounds—until it separates into its component parts, and then firing electrons at these components, breaking them up in predictable ways to produce patterns specific to each compound. In my case, GCMS tells me what the wax on a bullet ant is made of.

    As one of the most ancestral ant lineages, a better understanding of the chemical communication of Paraponera clavata could go a long way toward understanding ant communication in general.

    The bullet ant is thought to have behavioral castes, including foragers, guards, nurses, and queens. This division of labor is not centrally orchestrated by the queens or any other single ant, but is a symphony of constant communication between workers. No one knows whether these ‘primitive’ ants secrete chemical nametags. To learn what the wax of each caste is made of, I would need to collect the ants alive. And to get to the nurses and queens, I would need to delve into the nest’s darkest chambers.

    The excavation day dawned as any other in the rainforest. It was oppressively hot, humid, and cacophonous. I gathered my tools: a couple of buckets, shovels, and long metal forceps for grabbing squirrelly ants. I coated the inside of the buckets in fluon, a slippery substance, and tucked two syringes of epinephrine, or EpiPens, into my backpack. There is no telling how the body will react to a neurotoxic venom it has never encountered before.

    Despite the dangers, I had recruited an eager undergraduate helper named Walter. Together we trudged out to the nest site, buckets and shovels on our backs. Our anxiety was palpable as we sweated through the jungle.

    We cut the tension with nervous conversation about recent field station gossip, but always our talk circled back to things we had heard from colleagues about painful bullet ant stings. Unlike most who had been stung, we would be facing hundreds of furious ants determined to protect their home.

    Finally we reached the nest. Bullet ants usually build their nest at the base of a tree, digging deep into the roots and using the tree trunk to access the canopy where they hunt. The plan was to collect as many guard and foraging workers as possible and hopefully decrease their numbers. Then we would dig for the other members of the colony.

    We started our capture of foraging workers. One, two, three, ten. Slowly ants stopped leaving the nest. I took a long stick and poked it into the nest. A few angry guard workers scuttled out and we grabbed them. I probed again, mining for the black gold. A few more ants reacted to my poking stick, but then nothing. No matter how much I cajoled, no more would venture out. It was time to dig.

    “Are you ready, Walter?” My eager helper was white as a sheet, anxious like me to get this over with. We both took a deep breath, and on the count of three, plunged our shovels into the rich black earth.

    Instantly, the furious workers started boiling out. The provoked ants were making a huge racket: their alarm signal, a high pitched squeaking sound, comes from rubbing two striated parts of their hardened exoskeleton together.

    As our buckets began to fill, the squeaking became audible over monkey calls and parrot guffaws.

    Deeper and deeper we dug, and yet the queens remained hidden, likely further under the tree, nestled deep between the curving roots. A flash of white amidst the waves of black workers: we had just opened a brood chamber, where the immature ants were raised and protected.

    The nurses carrying worm-like white larvae and cocoon-enshrouded pupae were quickly tossed into a separate bucket. We had workers, guards, and nurses captured for chemical analysis, but the queens remained elusive.

    “Walter,” I said, eyes never leaving the wave of ants, “I’m going to pull the sapling down.”

    It was the only way to get to the queens and was the reason I had chosen a nest under such a small tree. I grabbed the tree and pulled it over, hard. Suddenly, Walter screamed. Convinced that he had been stung, I turned to see a huge bullet ant crawling up his mosquito netting right in front of his face. Staring cross-eyed at the squeaking horror, Walter froze as I swatted the stray ant off. That was close.

    Alas, the sapling’s roots reached too deep. We could not access the deepest tunnels with our small shovels. We gathered our buckets of ants, about two hundred in total, and hiked back to camp.  Deep in the jungle, we left the queens in the ruined nest, their chemical brand still a mystery. And there too we left the mystery of the sting. We had escaped unscathed, for today.

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