Question: What's the Reason for "Colorful" Nymphs?

                                                (Perlodid stonefly: genus Yugus)

                                       (Spiny Crawler mayfly: genus Ephemerella)

                                         (Perlodid stonefly: genus Helopicus)

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Three examples will suffice.  Question: why have some mayfly and stonefly nymphs evolved with beautiful, colorful, patterns and colors?  We're used to seeing this in insects, birds, and mammals when they're ready to mate -- especially in males.  This has nothing to do with mating.  In fact, I believe I'm right in saying that mayfly and stonefly nymphs lack sexual organs in this stage of development: genitalia only appear with the transformation into terrestrial adults.  And as adults, many of these "beautiful" nymphs are drab in appearance.  If anything, being colorful and richly patterned in the nymph stage would seem to be a bit risky: it makes them more obvious to predators -- mainly fish.  And trust me they're "obvious": I know where I've found them, and this has nothing to do with "camouflage".

Can anyone out there explain this?