The Thorax Slate Olive is a good trout fly for when you catch a green mayfly in your hat. The slate-colored top post is stealthy as all can be, but maintains enough visibility to help the angler keep an eye on it. Which is important as this trout fly continues to trigger bite after bite during mayfly hatches in the spring and summer. During the season, catch mayflies in your hat or check spiderwebs to identify the color. If they are a shade of green, tie on one of your Thorax Slate Olive flies.