Oddly enough, I have it in mind to tie and fish some spiders.
I only fish the Duchy, and fish it most days of the week, although sea trout has been the order of the last few weeks.
I would echo what the others have said above, Black Gnat, Blue Upright.... I know Richard may think it bonkers

but even a Devon Half Stone will pull a fish out along with a March Brown at this time of year. GRHE Nymph (Dark with tungsten bead) works well, and for Bellever and Hexworthy areas I have found Lunns / orange spinners or Pheasant tail's are a must as spinners fill the air
I am coming around to the oipinion that many moorland brownies are fairly opportunist and will take most things, many turn to eating fry and most of the larger trout seem to be hooked at night while ST fishing - I hooked a thing trhat was 7 inches long the other day that had attacked my 1 1/4 inch tube!
Size and colour are the
most important factors - I keep on saying it, so I shall say it once more (

*): You never hear anyone coming away from the Duchy waters saying ' I wish I had used a larger fly'. 14 through to 20 and as Pete T says, a couple of casts and move on, dont flog it, the good fish will surprise you after 1 or 2 casts
All the very best,
DN
PS - If you would like to pm me some details I would (time permitting) gladly meet up and point you in the right direction for access / egress points and places of interest / eating / drinking etc.