[_5659]

Few observations:

Cicada groups split off while they occupied glaciated territory. The Appalachia mountain range is famously, formed by glaciers. The Valley and Ridge part of Appalachia, was a major highway of immigration and colonization. It is more likely that territory borders respect the geography and that the brood respects geography, rather than to each other.

A rough inspection of the brood distribution and the continental divides shows approximate respect to the St. Lawrence and Eastern Continental Divide.

There also seems to be some respect to the division between Mississipi Flyway and Atlantic flyway, which are bird migration routes.

It is my understanding that an organism highly dependent on deciduous forests will respect those regions and this includes most of the South.

Turns out, humans like deciduous forests too. They're useful for timber, charcoal and potash (fertilizer). The borders of this territory lines up pretty squarely with the border to Canada. That makes civilization a predator of the cicada through deforestation. The Onondaga brood is named for the Onondaga people.

As I understand, the Brood distribution effort is done by crowdsourcing.

Given the longevity of the brood cycle and the inconsistency in historical record, it could very well just be the case that the recency of this effort has not synced with an emergence.

That's my guess why cicadas and humans line up.

More practically, cicada cycles provided a necessary evolutionary flush of nutrients, bacteria and fungi across the distributions. They breed in such overwhelming numbers that the entire ecosystem of predators is likely to prosper from their emergence. So perhaps humans settle where there is game and timber, and where there is game and timber it is likely that the area prospered from a cicada cycle.