A century ago, William Morton Wheeler inked this iconic illustration of the striking polymorphism displayed among members of an ant colony. You may have seen it; Andrew Bourke and Nigel Franks used it as the cover for their 1995 text Social Evolution in Ants.
I always assumed Wheeler’s figure depicted some exotic tropical marauder ant, a voracious jungle species with massive soldiers for slicing up hapless prey. I don’t read captions carefully enough, I guess, because I learned recently that this charismatic creature is actually a local harvester ant, Pheidole tepicana. Not only that, but the lab downstairs from mine keeps several captive colonies for research on caste development. Obligingly, last week they let me stop in with my camera to take some photos.
The largest workers have absolutely massive heads:
A salient fact about Pheidole tepicana- and presumably the reason why Wheeler singled it out- is the extensive variation in shapes and sizes of workers. Most ant colonies are monomorphic; that is, all their workers are roughly the same build. Most of the thousand or so species of the genus Pheidole are impressively dimorphic, with a mix of small normally-proportioned minor workers and a few large-headed major workers. A small subset of Pheidole species take this a step further and have become trimorphic, usually by adding a grotesquely front-heavy supermajor. Here we can see all three morphs:
A major worker with two supermajors:
A minor with a supermajor:
A male, with majors and minors:
Hello Mr. Photographer!