Well... that depends on what you mean by wets...
Nymphs as you are aware are imitative patterns that represent the larval stages of sedges, upwings and stoneflies.
wet flies can be broken down into several chunks..
"Traditional" wets with paired wing slips can imitate several things, sedges being the most obvious, some patterns such as butchers and alexandra are said to imitate pin fry.
North country, soft hackled wets are more likely to imitate spent/crippled insects on the surface film of the water.
then you have all the bumbles and crunchers and french partridge type wets...
The main difference is the way in which you fish the patterns. up stream, down stream, dead drift, pulled..
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