Hmmm. Crane fly larvae will not enter a worm bin as there is no food source there, and I'm not sure what you mean by "black beetles". I can tell you that the organisms that inhabit a worm bin or compost pile are either decomposers feeding on the OM, or predators feeding on the decomposers that feed on the OM. You will not find critters that feed on living plants inhabiting your bin in large numbers (though a few slugs may use the bin as a hangout during the heat of the day, returning to the garden at night to eat your hostas and lay eggs).

If you can post a photo we might be better able to actually identify the visitor, but a reliably identification from a cursory description just isn't possible.

I'm sorry I couldn't be more helpful!

Kelly S