A Light Cahill dry tied Catskill style, is a true classic. Since the originals were tied in southern New York better than a hundred years ago, proper proportions, materials and methods of tying have been up for debate. Throw in some regional preferences and a discussion of what insect a Light Cahill actually represents and you've got a real mess. All that said, this is how I, like to tie a light Cahill.
Here I'm going to use a Dai-Riki #305 in a size 12 to better show the tying procedure but I usually like to fish with a 14 or a 16. For thread, I've chosen UTC 70 Denier in the color cream because it closely matches the dubbing and hackle I'm going to use. Start your thread on the hook shank, leaving a bit of space behind the eye, then position your thread at the 2/3's to 3/4's point on the shank. This is where the wing will be located. Although other feathers can be used there is nothing quite like good quality wood duck for the wing. Pull the bottom, shorter fibers of a feather back so most of the tips are aligned. Snip the butt end off but don't throw it away as it can also be used for dry fly wings or nymph tails and legs.
Now, pick up the feather tip and sort of fold and roll it into a bundle with the tips aligned. Measure the bundle against the hook shank. I like wings that are the length of the hook shank plus the hook eye. Using your thumbnail as reference point, move the bundle unless the edge of your nail is at the tie-in point. I need to stop here for a second. You can by all means use a pinch wrap to secure the wing, but I've found the thumbnail to be more accurate in terms of placement. Also, did you notice how the thread wanted to jump forward when I took the wrap? Here it is again in slow motion. To correct for this, give your bobbin a counter-clockwise spin. This will spin the thread and make it want to jump back toward your fingertips rather than away from them, this way you can tie the wing down exactly where you want it.
Take 3 or 4 nice tight wraps, one on top of each other to secure the wing and then carefully snip the butts off at an angle. Pull the fibers back, and take 3 or 4 more wraps immediately in front of the bundle to hold it upright. Divide the fibers into 2 equal clumps and make a couple of figure 8 cross wraps to help separate them. A single complete wrap ensures that what you've done so far won't come unraveled.
Now this part's a little tricky. You want to get a full wrap around the base of each wing. Once again, spinning the thread counter-clockwise helps here as it keeps the wraps low on the wing. Take a complete wrap or two behind the wings to save your work. A small drop of head cement at the intersection of the wings will help them to stay put.
For the tail, I like to use the long stiff fibers from a feather on the outside edge of a rooster neck. Pull back the fibers to align their tips and then strip about a dozen free from the stem and then snip off the curlys. Place the fibers against the near side of the hook shank and allow the thread torque to push them over onto the top. With only 2 light wraps, you'll be able to slide the tail back and forth to get the correct length. Lift the fibers up a little bit as you make wraps toward the hook bend. This will keep the tail fibers on top of the hook shank. I do like my tails long, about the length of the entire hook.
For the body, I'm going to use super fine dry fly dubbing in cream. It's difficult to under-dub this fly - use your dubbing very sparingly. End with your thread a short distance behind the wing.
For hackle, I'm going to use a size 12 feather from the same neck as the tail. Snip the base of the feather off where it starts to get real webby and the stem gets thick. With the dull or back side of the feather facing toward you, snip a few fibers from either side of the stem to give your thread some purchase. Then strip a few fibers off the top of the stem to ensure it orients correctly when wrapped. Place the feather on the near side of the hook and take wraps, first behind the wing and then in front, to secure it. With your fingers or hackle pliers grab the tip of the feather and begin winding it around the hook shank with the dull side facing forward. Take 2 or 3 wraps behind the wing and then pull the wing back and make adjacent wraps all the way up to your thread, just behind the eye. While holding the feather tip up, take a single wrap of tying thread over the top of the feather. Then switch hands and take a couple of jam wraps right in front of the stem. With the hackle secure, you can then snip it off close. While whip finishing the fly, do your best not to trap hackle fibers that would block the eye. Finally, you can snip or cut your tying thread free. Add a drop of head cement and your light cahill is complete.
By varying the color of the tail, body and hackle as well as the overall size of the fly, Catskill-style dry flies can be tied to represent a multitude of mayflies. Although this pattern dates back more than 100 years, it's still absolutely valid to this day.