Water boatmen and backswimmers usually provide best fishing during early
spring and fall mating and migration flights. I once had a good day during
such a flight and what a day. A water boarman flight followed by little
caenis mayfies and then by midges. On that day the trout wanted the boatman
right on the top - I can't remeber the exact name of the pattern but it used
a closed cell foam body cut to shape, the back was painted and marked with
pantone then coated with aquaseal. Legs were from superfloss.

Here's a good site:

http://www.flycraftangling.com/entomology_boa.html

Phil Rowly who hosts the site (which has good info on all stillwater
hatches)is a well knwon fly tyer and is a business associate of Brian Chan -
mentioned in posts above. Chan, a biologist by profession, is also a senior
executive in Fisheries BC the Public Body that manages British Columbias
frshwater sport fishing opportunities.