Terrestrials (ants, beetles, grasshoppers, etc.)
Terrestrial insects are those that live primarily on the land as opposed to aquatic insects who live primarily in the water. One of the primary definitions of terrestrial is "of or relating to land as distinct from water". When terrestrials become fish food, it is because they either fall into the water, fly into the water or are blown into the water...in other words, it is accidental.
As I think I have said somewhere on this site, I have fished the Henry's Fork for 40 years. I have been asked many times what my favorite fly on that river is...if I have to answer that question without qualifying my answer it would be a black beetle in size 16. I cannot tell you how many times that fly has saved my cute little butt...and it works on most other streams as well.
I can tell you that I have used it very successfully during the peak of heavy PMD hatches when I could not get a feeding fish to take my PMD imitation...or any of a dozen PMD imitations...even when going down to 6-7 X tippet. I do not know what it is about a black beetle that makes trout eat it when they seem to refuse everything else. I just know it works very often and under difficult conditions. I have seen rainbows...big rainbows move 2-3 feet to eat a beetle when other bugs were thick on the water and the fish had not moved 6" out of its feeding lane...After these many years, I have decided that when a trout sees a black beetle, it's little brain shows him an image of a banana split.
I find terrestrials most effective on windy days but I fish them hard and often until it gets too cold for bugs to move around.