Terrestrials are land insects that end up in the water and wish they hadn’t.
Consider fishing terrestrials in the wind, or when the water rises. Terrestrial drift can provide a great deal of food for a fish in a day. Terrestrial drift is largely a surface phenomenon but it can also trigger subsurface feeding.
Grasshoppers and crickets provide lots of nutrition in areas of high grass. Willow Beetles, Japanese Beetles, Jassids and Inchworms find themselves in the water and become trout food.
I helped Fran Gough with his insect study in the Little Lehigh. Fran put nets in a riffle and checked them every hour on the hour periodically for 4 months. We noticed substantial numbers of ants in the drift during the early morning hours.