fly size

[Ruby River]

Why do they get smaller as the summer progresses?

[flyfish99]

With a few notable exceptions, Isonychia for example, the aquatic insects hatching in summer tend to be of smaller size, so the imitations are smaller. However, terrestrials make up a good part of a trout's diet in summer. Ants, inchworms, grasshoppers, crickets, and bumblebee imitations will all work in summer, especially during the middle of the day, when there is little hatching activity. So you are not really limited to sub size 20 flies.