08/09/2006
Nomadic Killers
Last year, when on a Panama Canal cruise, I didn’t realize that we had sailed near Barro Colorado, an island in the canal and home to as many as 50 colonies of Eciton burchelii. E. burchelii is a much studied and species of the army ant. Recently, a good friend in Hawaii and a longtime fan of this Web site suggested recently that I should write more about ants. When I learned last week that Dan has a bad cold and sore throat, the least I can do is follow his suggestion, especially since I’ve just read a couple of fascinating articles on ants in the August National Geographic.
One of the articles was actually a one-page essay on ants, “The Civilized Insect” by famed Harvard biologist Edward O. Wilson. The other article, “Army Ants. Inside the Ranks”, was written and photographed by Mark W. Moffett, whose photos are superb. I also found an article by Rene Ebersole in the Aug/Sep 2005 issue of National Wildlife posted on the National Wildlife Federation’s Web site. The article discusses the works of Carl Rettenmeyer, T. C. Schneiria and Scott Powell on army ants on Barro Colorado spanning a period of seven decades.
An E. burchelii colony has five different ranks of army ant. At the top is the queen, which measures 24 millimeters (mm) in length. Next comes the major, or soldier, which is less than half as long at 11 mm but has fierce 4-mm long pincers that curve back as does a fishing hook. The soldier does the heavy duty fighting to protect the queen and the colony. It’s also sometimes the equivalent of a suicide bomber in that those curved pincers are hard to extract from an enemy and the soldier has a good chance of dying in the battle. The other three types of workers are the submajor (10 mm, with significantly smaller pincers), the media (8 mm) and the minor (5 mm). (I measured “actual size” photos of the different ants in the Geographic article.) The latter three worker classes carry out the jobs of killing and carrying prey and feeding the queen and the larvae.
The queen really has the best of it and may live several years. The others only last a few months. The queen’s domain is huge, with anywhere from 300,000 to 700,000 subjects. Where do all these ants live? The ants I know live in the ground, but not E. burchelii. Because of the fact they devastate the areas covered by their “swarm raids”, they periodically move to a new location and a new source of food. They’re nomads and don’t have time to build fancy digs. Instead, they simply gather together, interlocking toe to toe, to form large nests with their bodies. These living nests are called bivouacs and are generally found hanging under a log or overhanging branch.
In the bivouac, the queen labors mightily, laying somewhere between 50,000 and 300,000 eggs! The eggs hatch into larvae, which then develop into pupae, from which emerge the worker ants. This reproductive cycle plays a key role in determining how long the bivouac stays in one place, typically about 20 days. When the eggs hatch into larvae, there are a huge number of hungry mouths to feed and that’s when the bivouac breaks up and the ants strike out for new territory, ravaging and pillaging as they go. The migration to the new site takes about two weeks and they cover about a hundred yards a day (or night; some appear to travel at night). While traveling, they carry the pupae from the previous generation with them. If they travel by day they form a new bivouac every night.
After two weeks, the ants form a more lasting bivouac at their new site and begin a strange pattern of swarm raiding. The first day’s raid fans out in a certain direction out about a hundred yards. (Most army ants raid in columns; E. burchelii columns fan out as they advance.) The next day, the ants head out in a direction roughly at a 120-degree angle counterclockwise to the first raid. The third day, it’s about 120 degrees counterclockwise to the second raid. On the fourth day, they’re back to where they started only now they shift enough counterclockwise to the first raid so they don’t cover the same area twice. Continuing this pattern, after 20 days they’ve covered virtually all the territory surrounding the bivouac.
This seems to me quite clever. If I were an insect or other prey near one of the raids and saw what was going on, chances are I’d scurry out of that area. However, if nothing happened the next two days, I’d probably assume all was back to normal and move back in, only to be scooped up on in a raid on the third day! In 20 days, the ants have ravished the area surrounding the bivouac, new larvae have arrived, and it’s time to move on.
A rare event occurs when an E. burchelii colony splits into two separate colonies. When this happens, it begins with not one, but two swarms starting out from the bivouac in opposite directions. Soon, a handful or so of newly minted queens, and the queen mother herself set out dashing in the two opposite directions. The race goes to the fleetest of foot under nonideal conditions. Each of the queens has an entourage that periodically gathers around her, covering and slowing her down. Eventually, one queen in each direction forges ahead and when the site of the next semi- permanent bivouac is reached she becomes queen of that colony. The other queens are left behind to die or fall victim to predators.
In their swarm raids the ants typically dine on wasps, scorpions, cockroaches, spiders and the like as well as on other ants. They tear up the larger bodies into smaller pieces, which they carry underneath them, making it possible for fellow ants to share the load on longer specimens. Some birds, such as the antbird, rely on the swarms to stir up and expose insects or animals on which the birds dine. A swarm “hums”, thanks to the buzzing of parasitic flies flying ahead of the swarm front.
In his brief essay, Wilson estimates the total number of ants on Earth as about 10 quadrillion (10,000 trillion) and the combined weight of all those ants is roughly about the same as the weight of us 6 billion or so humans. Ants have been around at least 100 to 200 million years, watching the dinosaurs go extinct and chances are they’ll be around watching us go extinct, which could happen sooner than we think if we don’t start behaving better!
Speaking of insects, I was shocked one day this week when my wife and I returned to our car on the fourth parking level at our local mall after our early morning walk. I looked down to see a large green insect walking towards the mall entrance doors. It was a praying mantis! Could it have been attracted by the ads for one of the perpetual sales at Macy’s or was it interested in something more expensive at Nordstrom’s or Tiffany’s (it is an upscale mall)? If it doesn’t get run over by a car and ends up in the mall, will it become a tasty tidbit for the sparrow that has made its home inside the mall? In the unlikely event that I find the answers to these questions, I’ll be sure to let you know.
Dan, I hope you’re feeling better.
Allen F. Bortrum
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