It seems that the male sexual member has a corkscrew twist to it. So too does the female duct, except it's the OPPOSITE THREAD.
They are able to breed if and only if the female gets really, really relaxed.
Now, think about it ~ corkscrew structure, opposite threads, and still one species.
It's beginning to look like we know less and less about more and more and still no resolution of what breeding constraints help establish a separate species!
I’ve heard about that. They call it “lock and key” sexual selection and it does complicate species identification.