Animal Phylum Arthropoda (Arthropods)

Dun: Mayflies have two adult stages. They first emerge from the water as duns (scientifically known as the subimago stage). They then molt into the spinner (imago) stage, in which they mate and die. Sometimes the word "dun" is confusingly used to refer to a brownish gray color in fly tying materials.

Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.

Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.

Spinner: There are two winged stages of adult mayflies. They emerge from the water as duns, molt on land (usually) into their fully mature stage, spinners. As spinners, they mate, lay eggs, and die.

Stillborn: In fly fishing, a stillborn insect is one which got stuck in its nymphal or pupal shuck during emergence and floats helplessly on the surface instead of flying away. It is a specific class of cripple, although it is sometimes used interchangeably with that term.