Due to the oxygen content of the blood. same reason why blood is bright red when you get a cut.
Amphibians, tunicates, lancelets
I think the answer is commensalism because the organism that does the digging is unaffected in relation to the other organism who actually benefits by having an already made home (the hole) for protection & shelter.
They no probably no longer eat the same things, or one of the populations has adapted to its environment and the other flies continue to be normal.