I am doing investigation into association of sediment size with the movement of mayfly nymphs in a choice chamber, sampling the organisms from a stream. I will be doing a chi square test to determine statistical association. I have three species of mayfly to choose from - swimming, burrowing, and flattened and I will investigate two. My question is this: biologically speaking, either from your knowledge or from the literature etc., where and why would you expect there the different mayfly nymphs to migrate towards, in terms of size of rock/sediment (e.g. categorically speaking, small, medium and large which I will categorise in the field). I'm really looking for for a biological reason for why there might be an association - any at all!
Thank you in advance.
UPDATE: I'm now also looking for sources of secondary data/published papers in journals/literature which might talk about this topic/have similar data/anything at all that I might be able to use. A google search of 'sediment size distribution swimming mayfly nymph' for example hasn't yielded very much so far, but I'm hoping an expert might be able to find such research more quickly.