To be clearer, there can be two definitions of 'reversing'
- reverse mounting a single lens using a mount-to-filterthread adapter
- reverse mounting two lenses in tandem, one is convensionally mounted to the body and the second is reversed and mounted using a filterthread-to-filterthread adapter, to hold the second lens reverse-mounted on the end of the first lens
In the case of #1, there is no diaphram mechanism operated by the body, so the lens itself must have an aperture selecting ring and also a auto/manual switch (open/closed switch) to permit easier focus wide open and then stepped-down shooting.
In the case of #2, the lens mounted conventionally on the body provides the aperture control and the reversed lens is simply an alternative to using the 'close-up filter'; the second lens' diaphram is pointless.