The true March Brown is a much localised ephemeropteran. Fortunately for us fly-fishers the difference is not that obvious between a few of these very large Brown species, nor dare I say, is it to the trout! Of the various big brown jobs, the common and quite widespread Large Brook Dun is the fly most commonly confused with the genuine March Brown. So my pattern admirably covers both, also both species are of very similar sizes within a millimetre or two! Again, as a pattern approaching Greendrake Mayfly size, it lends itself to the Mohican extended body style, giving it a very predictable correct sit, and the soft poly wing collapsing like the natural on the trouts chomp. Ive used the pattern in this form for several years, and though our stretch of the Wharfe does not have the true MB, this fly still scores well, since we have the Large Brook Duns.