One may catch trout knowing nothing about insect and their life. So why is learning some basic of entomology important?
To be able to identify the family of a bug and its stage will increase the odds of success and will give you the full context about the art of fly fishing

Mayflies, sedges and stoneflies spend most of their lives underwater, they undergo metamorphosis before emerging as winged insects. Specific terms identifies each stage.

Larva: More generally, this term describes the insect during its underwater phase of life. Although “nymph” is commonly used in angling, “larva” is the broader biological term.

Nymph: In fly fishing terminology, the aquatic, immature stage of mayflies, sedges (caddisflies), and stoneflies is collectively referred to as the nymph stage. These insects spend the majority of their lives underwater in this form.

Pupa: Specific to sedges (caddisflies), this is the transitional stage in which the insect undergoes metamorphosis before emerging. During this phase, it prepares to leave the water and become a winged adult.

Dun (Sub-imago): Unique to mayflies, this is an intermediate winged stage. Duns have muted, opaque coloration and are not yet sexually mature.

Spinner (Mature Imago): Winged, fully mature, ready to mate. Typically within 12 to 36 hours the mayfly from a dun it becomes a spinner. Spinners display brighter, more saturated colors.

Spent: After mating and laying eggs on the water’s surface, the insect dies. In this final stage, it is referred to as spent.