I just felt like starting a discussion on the topic. Let's see if it goes anywhere, shall we?
Formians. The centaur ants, as they're sometimes called, have been presumed to be from Arcadia for the majority of 2e. Sure, they've made the occasional foray into Mechanus. However, come along 3e and 3.5e, and suddenly the lawful insects are originating in Mechanus and bringing layers of Arcadia to themselves (something that in 2e was attributed to the Harmonium's efforts for finding the common good, fat lot of it that did them). Of course, the entire thing was a ploy by WotC to replace the goofy-seeming Modrons with the ferocious law aligned Formians as the exemplars of Mechanus. We, the fine members of Planewalker, will not stand for such blatant disregard for canon (I hope)! There's nothing wrong with change, as long as it happens logically and, for the most part, gradually. That's what keep cohesion, thereby believability, in a setting. So, seeing as the continuation of the Planescape setting has, at least for now, fallen into our hands - what shall we do with the Formians?
Personally, I prefer to keep them originated in Arcadia. The reason for Menausus slipping into Mechanus should also stay as the Harmonium's 'training camps'. However, to promote some change (even the lawful planes aren't meant to be stale) the Formians of Menausus that were taken along for the ride might have succumbed to their lawful natures, completely abandoning the "good" aspect from their existence. Thus, they are at war with Modrons for territory (although, as seems to be the current consensus, the Modrons have closed in on themselves in order to evolve, leaving the Formians to overrun many parts of Mechanus without much bloodshed). The "good" Formians from Arcadia are repulsed by the new breed - they see them as more insectile and savage. That leaves us with the new 'breed' of Formian described in Fiend Folio (3e). Are they bred for defense by the significantly reduced Arcadian Formians or for war by the expanding Mechanus hives? Or, are they a natural evolution from the significant changes in the planes?
I'd really like to hear some more opinions on the subject. My little monologue above is just a starting point - feel free to critique or add to it. Just try to base your arguments on the setting as set up in 2e books, rather than WotC's retake of it in Manual of the Planes. While good, that book isn't Planescape canon in any way besides, perhaps, d20 numbers for planar qualities.
My formians page.