Heart of the swarm

Army ants (Eciton hamatum)
Army ant foraging expedition. Larger prey is carried underneath the abdomen of the ant and may sometimes require the combined efforts of a small legion. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

Army ant raiding column
Photo taken in Corcovado national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

The Altamira ranger station in La Amistad has all of two trails. One is a serious affair, a 20km, mostly uphill trail that wends its way through numerous ecotypes and terminates in isolation in the aptly named “Valle del Silencio”. One might be forgiven given my previous track record that I would jump at the opportunity to strand myself in remote jungle with a recovering hip. However, I decided to exercise the small bit of caution that I reserve for special occasions and refrained. Perhaps the incessant warnings and consternations of the family were finally being heard… or perhaps I’m just saving up for something even more daring and audacious…

This meant that in the meantime I was stuck on the only other trail: a measly 3km loop optimistically named “Gigantes del Bosque” though which constitutes mostly disturbed forest that borders a farm for the first part of the trail. The first couple days passed without much to speak of, the usual plethora of common grasshoppers, and crickets in the secondary growth and an intrusion of cockroaches underfoot, making the forest floor literally seethe and pulse with a beat all its own. I was beginning to despair when I came across an army ant column.

Now I have a fascination with the leafcutters and army ants that borders on the obsessive. Every time I see a column there is an instinctual need to get down on my hands and knees and pay homage. To observe the minute goings on and wonder what that collective brain of theirs will come up with next (Hint: It usually involves sending out small snaking trails of soldiers to flank and bite me while I’m down. Fortunately these maneuvers simply heighten my respect for these warriors since they’ve naturally read Sun Tzu somewhere down the evolutionary line and are aware of the effectiveness of surprise…but I digress). Prior to arriving in Costa Rica I had read Army ants: The biology of social predation by William Gotwald. Written in a remarkably accessible manner not only was it entertaining but it provided me with my impetus for visiting Central/South America rather than Asia/Africa. Enter the mimics. It seems like every army, even one made up of ants travels like a circus with a retinue of bewildering creatures to service every form and function. Though the army is chiefly composed of the original faction and its queen which splintered from a former colony (fission) along with its descendants, a few are not true army ants. Sure they look like them, run like them and smell like them (some physically steal the colony specific pheromone from the pleural and pygial glands of their host ants), however they don’t bite like them (perhaps the true marker of the army ant!). These charlatans are necessarily rather few and far between so as not to draw too much attention to themselves (usually 1/1000) and they range from commensal (innocuous) to parasitic in behaviour. Gotwald writes an entire chapter on these interesting interlopers but I just thought I’d mention two that I found while examining the hordes. The first is a Staphylinid beetle and the second a rove beetle.

Tachinid fly
In addition to the more insidious mimics scrounging off the hard work of the colony there is a litany of followers and commensals. One such class is a variety of flies that follow the army ant column. Seen here perched safely out of reach of a raiding column. This fly takes advantage of all the potential prey insects stirred into a frenzy which accompanies every raid. At times the fly may even make so bold as to snatch dead prey from the jaws of a worker. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

Army ants carrying an ant mimicking Staphylinid beetle larva
Since the army ant column is in a constant state of motion due to its nomadic nature can be very difficult to determine the nature of the prey captured. Hence I was very fortunate to catch this photo which I only realized afterwards was of a mimic rather than prey larva. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

In situ shot of army ant mimicking Staphylinid beetle in army ant column
Though this photo leaves a lot to be desired it highlights the difficulty of framing a moving subject at high magnification. This was the only true ‘in situ’ shot I managed of the Staphylinid beetle without extracting it from its natural environment ie. the army ant column. The beetle is deceptively small, about the same size as other minor workers. Despite the differences highlighted in the highly magnified view below, one can imagine that in a colony of millions that these insects go unnoticed notwithstanding the scrutiny of a few keen eyed biologists and those who actually go specifically looking for them. Found during a day hike in Corcovado national park, Costa Rica. Photo details: Canon mpe-65mm, MT-24EX twin flash.

Army ant mimicking-Staphylinid beetle
Finally able to isolate a mimic after several hours of observation the Staphylinid beetle though having some of the morphological traits of an army ant (enlarged gaster (posterior of the abdomen), long legs and small head) the differences become even more apparent upon closer observation. Therefore how does this obvious intruder maintain its facade? It appears that the deception is mostly chemical. Since most ants, and army ants in particular have poor vision they rely chiefly on tactile and chemical cues known as pheromones to communicate with one another. With respect to the former, it has been posited that the sensitive antennae with which ants palpate one another in greeting recognizes distinct morphological traits, building up a ‘mental picture’ to distinguish ‘ant’ from ‘non-ant’. This image is further refined by pheromones. Since each ant colony produces a colony-specific pheromone, these commensals are obliged to live, grow and breed within the colony lest they be perceived as foreigners. Their exclusion from the colony would almost certainly result in death without the protection and abundant source of food provided by the colony.  Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

Army ant mimicking- Staphylinid beetle
Dorsal view illustrates a more ant-like form, though the large compound eyes and segmented abdomen as well as the elytra are a giveaway. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

Army ant mimicking rove beetle
Another shot that could be improved upon, however, once again the constant motion made it difficult to adequately capture. Once again the differences seem to outweigh the similarities between the rove beetle and its model- the army ant. Interestingly though the beetle will raise the posterior of the abdomen to maximize the apparent size. Apparently in an effort to appear more like the gaster of its model. Taken during a day hike in La Amistad national park, Costa Rica. Canon 5D mark II, MPE-65mm, MT-24EX twin flash.

Of course the finds didn’t come easy. I pored over the flowing river of ants for hours while swatting at the swarm of mosquitoes eagerly hovering overhead. We, the mosquitoes and I, moved in tandem, as I leaned closer to examine any irregularity in the tide of ants, I found every exposed speck of flesh targeted. Their favourite being the small of the back where my shirt rode up over my pants… seems like I was just another piece of ass to them! After an hour and enough mosquito bites to keep me scratching for days to come I thought I had it! About the same size as a small worker, the form was just a little off. Excitedly I snapped away, my adrenaline rendering me temporarily oblivious to the party going down on my ass. When I glanced up at the screen playback, however, all I encountered were out of focus and slightly blurry snaps. Not entirely surprising given that army ants are almost constantly on the move when in-between camps and rarely stop unless they are busy tearing something apart or are helping their peers with some form of labour. Of course inserting a probing finger into a column of army ants ins’t exactly well advised and so the mimic made good its escape, successfully delving into the leaf litter and away from my lens. Though frustrating, I also found this heartening, the proof of existence had been right there in front of me, capturing it on the other hand might prove a little difficult! And so I hunkered down, pulled my pants up and my shirt down and determined to weather the biting while formulating a new plan. I had a microscope case with me that would act well as a specimen jar. It wasn’t ideal since I wouldn’t be able to photograph it in its natural environment running and hiding in plain sight, though it seemed like the only suitable alternative. Another hour passed and I was beginning to despair when I saw a form similar to the first mimic. I positioned the jar and flicked it into the jar. At least I thought I had. The tenacious bugger was holding onto the stick and had reversed directions. Other ants began to break formation and mill about, steadily approaching my prostrate form. Either clueing into my scent or picking up on cues from their comrades others began to break ranks. For a moment I was afraid that I had triggered a full scale assault! Still keeping my eye on the mimic though I made a daring exfiltration from the heart of the swarm. I had only a moment to eye and confirm that I had in fact succeeded before I beat a hasty retreat and tap danced off the last of the ants that had managed to gain purchase. “Success”! I beamed inwardly while absently scratching my behind on the long, itchy trail back to camp.

Army ant soldier (Eciton hamatum)                                                                                                                                                         Brandishing its large mandibles. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

Army ant soldier having its mandibles cleaned by a minor worker. While standing guard for the column the soldier is treated every once and a while to a cleaning and once over by the smaller minor workers. These smaller ants clamber over the larger ant offering a spot inspection. This is presumably to clear it of any parasites. Photo taken in La Amistad national park, Costa Rica. Photo details: Canon 5D mark III, Mpe-65mm, MT-24EX twin flash.

One Response to Heart of the swarm

  1. Pingback: Bugs before birds |

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