I have a theory about this which Robert Paxton thought was unlikely but it was clear that the the tests have not be done to prove or disprove it. I wonder if the conditions in the spermatheca will need to match the sperm in some way in order to nurture it for the 5 years that are necessary to span the queen's life. If that's the case it could be that exotic sperm might not last the whole 5 years. Hence as the queen gets older the workers become purer. That might explain the changing nature of a colony, which is probably something we have all seen. Even if all the different types of sperm match the spermatheca equally it could also be that long lived races have evolved long-lived sperm. I can't see any reason why there should not be a difference in this respect between sub-species.
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