Eciton rapax, the dark giant

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Eciton rapax

The attention of most Neotropical army ant observers is focused on just two common species, Eciton hamatum and Eciton burchellii. But the genus contains several rarer species, and one of my favorites is the handsome Eciton rapax. I was delighted to stumble across a raid one afternoon at Jatun Sacha where I snapped a few photographs.

Raid pattern of Eciton rapax, from Burton & Franks (1985)

This striking black-and-yellow insect is notable for a number of traits. It has the largest workers of any Eciton. The raiding columns are longer. The sting is more painful that that of its congeners. It has a particular taste for ponerine ants, the group that contains most of the big carnivorous hunting ants.

For reasons that aren’t well understood, the raid pattern of Eciton rapax (see figure at left) is intermediate between the orderly columns of E. hamatum and the chaotic swarms of E. burchellii.

Most unusually, E. rapax is the single species without a specialized soldier caste. Remember those evil monsters with tusks? Never once found in a colony of Eciton rapax.

As with most army ants, precious little research effort has been directed their way. One little gem, however, is this 1985 paper by James Burton and Nigel Franks that quantifies the prey (they REALLY hit Odontomachus hard) and the worker size distribution (yup- no submajors or majors!).

returning home with prey
Although they are primarily ponerine predators, Eciton rapax won't shy from taking social wasps

The worker force varies in size, but small workers are rather like scaled models of large workers instead of a differently proportioned caste.

big sis, little sis

The bright yellow gasters- perhaps warning potential meddlers of a potent sting- make E. rapax an easy species to identify in the field.

military precision

Myrmecologists would do well to learn more about this species. Contrasting differences among related taxa can reveal a great deal about evolution, and E. rapax is such an unusual departure from the better studied army ants that even small efforts will be well worth their while.

on the trail

addendum- videographer Kuai Shen sends in a lovely clip of Eciton rapax- also from Ecuador- showing just how spider-like army ants appear in the field:

The.Rise.Of.Invisible.Collectives from Kuai Shen on Vimeo.

20 thoughts on “Eciton rapax, the dark giant”

    1. I looked around genbank and BOLD for Eciton rapax DNA sequences with the aim of answering that very question, James, but apparently no one has ever done genetic work on this species. My guess is that it’s a secondary loss, but that’s just a hunch.

  1. If a science fiction movie was to be about army ants taking over the universe, these should be the model species.

    The golden abdomens are stunning!

    Did these ones bite you?

    1. Yeah- this is a beautiful, beautiful ant!

      In spite of their reputation for having a painful sting, I found these ants to be rather gentle and didn’t get so much as a bite, even though I spent at least an hour laying full down in the leaf litter right next to the trail.

  2. Gorgeous! I am very envious. I have never seen these ones in the flesh. We have quadriglume in the cerrado, which is almost as big and specializes in large Ectatomma, But, it lacks this lovely coloration.

    1. They really do look like wolf spiders, but I think that’s just convergence of form for running rapidly over irregular terrain. You’d never mistake them for spiders, if only because spider don’t run in columns by the thousands.

  3. Those are the most beautiful ants I’ve ever seen!

    Trying to reverse-engineer the photos to figure out your lighting – indirect flash bounced off of white paper overhead?

    ‘Returning home with prey’ is a winner.

  4. “Eciton Rapax” , hay aqui en Argentina, en las costas de los grandes Rios Parana Guazu , Alto Parana.Su, – vivak- , siempre esta en agujeros de los troncos de los grandes Àrboles. He visto un ejercito de electrizantes e inquietas ,Eciton Rapax, matando a una culebra y todo ser a su alrededor,quedè perplejo……y con miedo. Desde Argentina saludo atentamente al staff.-

  5. I found your photography site by googling a species name that had no entry in Wikipedia. One of the carpenter ants I think.

    I’ve spent hours trolling it now. Beautiful work.

    I’ve always been interested in the social insects. My favourite is termites though. Despite that, one of the highlights of my life was finding an ant dairy farm. I watched it for an hour and got to see almost all of their behaviour; Milking, grooming and protection. One of the ants decided I was a danger and gave me the full threat display.

    I’m rather envious of you being able to work with ants this way.

    I wish you well in your future endeavours. Thanks so much for such interesting and informative photos.

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