Here's a fascinating and squicky insight into the marvellous symbiosis between carrion beetles and the mites that live on them, courtesy of Greg Laden:
From the carrion beetle's perspective, there is a strong possibility that there are already fly larva (maggots) on the carcass eating it when it arrives to lay its eggs, or soon after. This situation is probably made worse by the fact that house flies are quick and seem to be able to travel long distances, while beetles are slower and travel less far than flies in a given period of time. The only way carrion beetles will ever be able to raise their young in this scenario is to do a better job of accessing or using the mouse carcass than the fly does, so any variation that arises in carrion beetles that facilitates this will be strongly selected for.
This is known as the life/lunch dichotomy. In the competition for the use of the meaty carcass of a dead mouse, if the fly loses out it gives up the equivalent of lunch ... there are still other opportunities, in this case, poop, for it's young to eat. The carrion beetle, however, may be giving up its life (or the life of its offspring, really) because mouse carcasses are very rare, so if the one carcass it manages to locate is eaten up by fly maggots, it's offspring will not survive.
This is where the mites come in.
The carrion beetle pays a huge cost carrying the mites around wherever it goes, because they are heavy and affect its ability to move and fly. But otherwise, the mites do nothing .... they just hang on for the ride, waiting for the beetle to locate a dead mouse. Then, when the beetle does located a dead mouse, the mites do not eat it. Rather, they eat the maggots, the fly eggs, and larva of anything that is not a carrion beetle. They clean the carcass of the potential competitors of the carrion beetle's larva.
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