Army Ants

 

One of the most maligned predators in the tropics are the "army ants" a name that is applied loosely to many species in several genera on several continents.  In some places, usually Africa, they are called "driver ants".  What they all have in common is a tendency to forage communally - in huge groups.  On our trips to Costa Rica, we have seen Army Ants in several places - Santa Rosa, Monteverde and La Selva to name 3.

Eciton burchelli parvispinum - Army Ants The Army Ants we saw in Santa Rosa were the type to forage for small invertebrates.  They were probably Eciton burchelli parvispinum, but I didn't bring back any workers to key out.

Like most other social hymenoptera, the colony is composed of individuals of different castes.  These include workers, queens and males.  Workers make up most of the colony and may exhibit further polymorphism (variety of forms).  In this colony there seemed to be three types of workers: small all-black workers, slightly larger workers with patches of reddish brown on the head and thorax, and major workers with very large jaws (below) and a light brown complexion, particularly towards the front of the body.  All of the workers are infertile females. Eciton burchelli parvispinum - Army Ants
Eciton burchelli parvispinum - Soldier

To the left is a major worker, or soldier that was accompanying the raiding column.  These workers are larger and possess huge jaws.  These are used to drive off any organism foolish enough to attack the ants, and also to help subdue larger prey items.  Native peoples apparently have used these soldiers to close wounds; if the soldiers' jaws are applied to opposite sides of a wound they will snap closed, drawing the sides of the wound together.  The body is then twisted off and the head left to hold the wound closed.

 

 

The ants aren't all-powerful.  Most vertebrates and flying insects simply move out of the way.  And, the ants have enemies.  At Santa Rosa we noticed an ant lion which had dug a pit in the middle of the trail.  One of the army ants had fallen into it and was dragged under, even though other ants tried to intervene and haul it out.  Picture and video to right.

A museum in California imported a colony of Army ants, only to have them wiped out by an infestation of small beetles!

 

Ant caught by ant lion.

The ant lion strikes back

 

 

 

 

 

Army ants don't have a permanent nest, instead they form a bivouac, a temporary nest made of the interlinked bodies of the workers.  I looked around for it a little bit, but you don't want to be poking around a raid of Army Ants casually - I never did find it, but it was somewhere near this group of ants. Eciton burchelli parvispinum - Army Ants
Eciton burchelli parvispinum - Army Ants with Scorpion On to the carnage.  The ants overwhelm any small invertebrate not quick enough to get away.  Each and grabs a leg and pulls, and then they start cutting up the prey.
Above and right:  Even formidable predators such as scorpions can be overwhelmed by the concerted effort of the ants.  Unlike most ants, new world army ants do not sting, so they have to subdue the prey by biting it. Eciton burchelli parvispinum - Army Ants with Scorpion
Eciton burchelli parvispinum - Army Ants with Long-horned Beetle This relatively large beetle was foraging near the ants.  What will happen to it?
Caught!  Like the scorpion, the beetle is dragged down by the ants.  Note the ant on the right pulling the beetle's leg out straight. Eciton burchelli parvispinum - Army Ants with Long-horned Beetle
Eciton burchelli parvispinum - Army Ant foraging columns The ants form columns thick enough to see from many feet away.  The sound of millions of ants moving through the dry leaves sounds like a gentle rain.  
The ants cover every inch of forest floor and even surge up shrubs and trees along the route.  If a worker finds something of interest she secretes chemicals to attract other ants. Eciton burchelli parvispinum - Army Ant foraging columns

 

Eciton burchelli parvispinum - Army Ant foraging columns

 The swarms can cover a considerable amount of forest floor.

 

 

 

 

 

 

 

 

 
Eciton hamatum - Army Ant At La Selva we saw two different species of army ants, Eciton hamatum and Eciton burcelli foreli.  These uniformly pale colored ants are Eciton hamatum.  Note the difference in size between the major worker and the minors.
Here majors guard one side of the column as it advances quickly.  These ants were out at about midnight and we did not see them carrying much in the way of prey. Eciton hamatum - Army Ants
Eciton hamatum - Army Ants Note the teardrop shaped insect in among the ants - what do you think will happen to it?
Another view of it. Eciton hamatum - Army Ants and myrmecophile

Eciton hamatum - Army Ants and myrmecophile

It turns out this is a myrmecophile - literally a "lover of ant houses".

A number of insect orders have species which fall into this category, including beetles and Orthoptera.  This one looks a bit beetle-like to me, but who knows?  In all cases, these insects cover themselves with chemicals so that they chemically resemble the ants.  The ants don't see well, so to them if it smells like an ant it is an ant.  The myrmecophiles also know how to get the ants to feed them - or they simply eat the larval ants.

The lack of prey in this column might be explained by the white insect being carried in this photo.  That is a pupal ant, probably belonging to the colony.  It might be that this group of ants was moving its bivouac.

 

Eciton hamatum - Army Ants
Eciton hamatum - Army Ants
 

 

Eciton hamatum - Army Ants

 

 

 

 

 

Eciton burchellii foreli - Army Ants

These are Eciton burchellii foreli from La Selva.  They were foraging off the trail on a different day.

 

 

 

 

Originally, I had these ants identified as Army Ants - not ethe label on the image.  An ant expert pointed out, however, that these are ants of the genus Pheidole, not army ants of the genus Eciton.  I looked back at my photos and to my amazement found that these ants were foraging within the Eciton raid - they were in very close proximity to the Army ants.

It's as good a time as any to point out that most predatory ants feed by overwhelming their prey with a group attack; the big difference with the army ants is the fact that the whole colony goes out foraging together and in the same direction.  These ants are overwhelming a stink bug; these insects usually have pretty good chemical defenses.

 Pheidole  sp.

 

 

 

 

 

Videos of Army Ants in Action

 

 

Army ants on log

 

 

 

Army ants on log - closer view

 

 

More army ants

 

 

 

 

 

Army ant columns

 

 

 

 

Army ants at Monteverde.  This video, taken June 5, 2007, shows columns of army ants entering the buildings at the Monteverde Butterfly Garden and carrying off various small insects.  Really big file, about 3.5 minutes long.

Video may take a few minutes to load...

 

 

This site is a good reference on Army Ants from Costa Rica

 

References:

In addition to the web references listed above, these two books are excellent!

Hölldobler B, Wilson EO.  1990.  The ants.  Harvard University Press.  732 pp.

Hölldobler B, Wilson EO.  1994.  Journey to the ants.  Harvard University Press.  224 pp.

As are  these articles:  

 

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