Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
There are about 5,000 species of aquatic beetles, and I won't be including them all on this site. I've also only included certain aquatic beetles and terrestrial families which I've found frequently on trout streams.
The common terrestrial forms are especially important during the late summer when they get knocked into the stream in any number of ways. As far as I know there are no specific life cycle habits which toss these insects into the water in great numbers; they just become important because there are so many of them around incidentally.
This is an iridescent green terrestrial beetle. They're common in northern Wisconsin's forests during the middle of summer, and I found them on the water's surface more than any other terrestrial insect. I saw the trout take a few and caught some trout on an imitation.