Abdomens: The rear and usually the longest (ten-segmented in mayflies) portion 
of an insect's body, to which the tails are attached.

Duns: 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.

Emergence: The transformation of a nymph or pupa into the adult winged stage of 
an insect. The term may refer to the emergence of an individual, or the daily or 
yearly event in which all individuals of a species emerge.

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.

Molt: When aquatic insects with hard exoskeletons (like mayfly and stonefly 
nymphs) grow bigger, their exoskeleton does not grow with them. Instead they 
grow a new, larger one underneath and shed the old one when it's too small. This 
process is called molting.

Multibrooded: Producing more than one generation in a single year. Baetis 
mayflies are a classic example. Insects which produce a single generation with 
two distinct peaks (like the June and September hatches of Isonychia bicolor 
mayflies) are not multibrooded, because the fall insects are offspring from the 
previous fall instead of the current year's spring.

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).

Oviposit: To lay eggs.

Setae: Little hairs on insects.

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.

Synonyms: A former name of a taxon, usually a species. Entomologists frequently 
discover that two insects originally described as different species are one in 
the same, and they drop one of the names. The dropped name is said to be a 
synonym of the remaining name. These changes take a while to trickle into the 
common knowledge of anglers; for example, Baetis vagans is now a synonym of 
Baetis tricaudatus.


*This information is from Wikipedia