Originally Posted by
Philly
It depends on what type of insect you want to imitate. It's fairly simple for a mayfly, if the body is 10 mm that would be 12 according to Dick Pobst "Trout Stream Insects" An Orvis Streamside Guide. There is a chart in the front of the book that equates bug body size in mm with a hook size. Now it's get tricky with Caddis Flies. If you measure a Caddis fly and base your fly hook on length including the wings, you'll end up with a fly a lot larger than the natural really is. I learned that the hard way, tying up some Little Black Caddis on the size 16 dry fly hook whose shank length is 7 mm as recommended in the book. If you look at a caddis fly, you find that the body is about 60% the overall length of the critter. So for a caddis whose total length is 7 mm, the body is about 4 to 5 mm long. To get a close match to the actual size you need to tie the fly on a hook whose shank is 3 or 4 mm long. Otherwise with the overlapping wing, so common in caddis dries, you might end up with a fly whose overall length is maybe 12 mm. You need to take this in consideration for Stoneflies also.
Here's a quick summary of the chart in the book.
Hook Natural
8 2XL= 20 mm
10 2XL=13-15 mm
12 2XL=13-15 mm
12= 10-11 mm
14= 9 mm
16= 8 mm
18= 6 mm
24= 3 mm
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