The Australian island of Tasmania has two species of millipedes, but what is interesting is that they only ever meet on a 300 foot wide patch of land. As to why this is, we have no idea. It’s a mystery.
Two similar species often can occupy territories that meet but don’t overlap, or as is the case here, only overlap for a short amount of space. This is called parapatry and it usually happens because of a geographic feature that limits the movement of the two species like a lake, a long ridge, or a region of extremely heavy rainfall, etc. But there are no features here to demarcate the boundary between these two species of millipedes; at least nothing that we can see.
It’s as if they worked out a treaty long ago and just go about their business abiding by the terms.
A fascinating facet to this mystery is that this particular boundary line is far longer than any other known parapatric boundary. To give you an idea, it’s 50% longer than the boundary between England and Scotland. One wonders what would happen if some of them started crossing this boundary. Would it be chaos or would border agents come to get these trespassers?