Big head workers in Pachycondyla luteipes

There is sometimes things that you see all the time but never realized. That is why I have appreciated the following article because it made me realize those points (and at the same time gave an exception).

In the article: Small queens and big-headed workers in a monomorphic ponerine ant, by Kikuchi et al., published in the early preview of Naturwissenschaften, they found the first example of workers bigger than the queens in Ponerinae. At least for some part of the body, including the head.

I have never thought about it before, but as they said in the article, for the Poneromorph species and the Myrmicinae, the workers are always monomorphic. Is that true?
I remembered reading an article on the African Pachycondyla (Megaponera) analis by Longhurst (1979) in which the author referred to two distinct subcastes of workers (major and minor)and referred to it as a dimorphic species with a bimodal size distribution, and a intermediate caste.
If it is true (or even if there is a very strong pattern), why is that?

Secondly, I am more and more interested by the Poneromorph group and it seems to me that many assumptions were made on this group of ants which was quickly classified as "primitive" and so maybe understudied on some aspects of behavior. However, through my readings it seems to me that this subfamily is impressive by the large array of ecological, morphological and behavioral diversity.
The proportion of some life traits is definitively different between the Poneroid and the Formicoid clades, but however are not totally absent from the first one.

I am glad to see an article showing a new stereotype broken by an exception (maybe some more in the future) in the Kikuchi article.

Here is the link to the article: http://www.springerlink.com/content/h844405u27736g43/fulltext.pdf