One of the most important things to a choosy trout is the one thing my close-up pictures can't show: motion. Fly tyers imitate motion by using lively materials, and fly anglers imitate it with subtle rod-work, but both need to understand how the real critters move.
Most of these were shot long ago with an ancient digital camera, but I'll be adding higher-quality material this year.
Larvae: Many classes of aquatic insects, such as caddisflies, midges, craneflies, dobsonflies, alderflies, and many more, are known as "larvae" rather than "nymphs" in their juvenile stages. They have mostly soft bodies rather than hard exoskeletons. These insects also advance through a "pupa" stage before reaching adulthood.
Nymphs: The juvenile, underwater stages of mayflies, stoneflies, dragonflies, and damselflies and other aquatic insects whose juvenile stages are covered by hard exoskeletons. The word can also refer to the fishing flies which imitate these creatures, in which case it is used as a blanket term for flies imitating any underwater stage of an invertebrate (except for crayfish and leeches).