These land-borne critters are often the key to mid-summer and fall trout fishing success!
Terrestrials (anthropods) are the largest of the animal phylum and include insects, spiders, ants, grasshoppers and crustaceans; just about everything crawling on the river bank. From a trout perspective, they're a helpless, protein-rich entree without much of a prayer of escape once they're blown onto the surface or into the water.
There's probably no more popular terrestrial than the grasshopper. It's easy to see, easy to use, very effective, and has a legitimate place in every fly rodder's fly box. As each summer progresses, the weather warms, the grasslands dry, hoppers get larger and in their search for food and moisture move closer to the stream banks, lake shore, and farm ponds.
Fly fisherman should always be prepared with at least a modest selection of terrestrial imitations (in a few sizes.) The proper choice certainly depends on the time of the season, but also might be determined by the weather; how rough the stream or smooth the stillwater might be, how strong the wind and how sophisticated or wary the trout are at the moment.