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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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Dr. Bortrum

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