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.
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!).
The worker force varies in size, but small workers are rather like scaled models of large workers instead of a differently proportioned caste.
The bright yellow gasters- perhaps warning potential meddlers of a potent sting- make E. rapax an easy species to identify in the field.
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.
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: