With all the recent attention devoted to Pheidole‘s apparently latent ability to produce supermajor workers at the drop of a hormone, now is an opportune time to mention Pheidole fimbriata.
Pheidole fimbriata is, according to Corrie Moreau’s research, the single sister species to the remaining 1000+ in the genus. That is, the first thing to happen when Pheidole first head-butted itself into the world some 60 million years ago was that the fimbriata lineage split from the others. Oddly, while the main lineage branched into a diversity insane enough to give taxonomists headaches, the fimbriata lineage contains but a single giant and rather strange Neotropical species*.
This lumbering ant is important. Pheidole fimbriata provides an independent window backwards through time in which to infer how supermajors emerged. Rajakumar et al did not include P. fimbriata in their study, as the logistics of South American research can be difficult, but surely this species is now worth a look.
On the surface, P. fimbriata is a typical Pheidole in that it has, like most of its congeners, just two size classes of worker. It is dimorphic, not trimorphic, and no one thought to include this ant among the supermajor-producing species.
But just look at it. The relative head size is massive compared to most Pheidole. It’s much more like the supermajor of Pheidole rhea than a typical major.
If P. fimbriata turns out with further developmental work to really be a supermajor, I can see a pair of interesting possibilities. First, perhaps the original Pheidole caste structure was dimorphic supermajor/minor. Under this scenario, the major caste as we know it may actually be the more recent innovation. Second, perhaps the Ur-Pheidole was like P. rhea, trimophic, but the fimbriata lineage somehow lost the major caste, leaving it with just supermajors and minors.
This is all just speculation, of course. But someone really ought to start looking at the biology of P. fimbriata. It’s an amazing ant.
*Incidentally, P. fimbriata was one of the more common Pheidole where I was living in Paraguay. They appeared most frequently at night, running in dense columns through my lawn not unlike army ants.