Once you see one, you'll see many!

 Dumpster memory dig.  I have seen these and never knew their names somebody once told me periwinkle, but that was not right.  I know now but...
Have you seen them? Do you know what they are?

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[Link here to see what one artist decided to do with these bugs!]

Caddisfly and the above are Caddisfly larvae.  

[link here to find out more]

You can see here a mass amount of recently hatched caddis flies. If you see one you will see many cases prior to the hatch.  The caddis fly is sensitive to pollution.  If you find them around you will know the water is clean.

You can get hatch charts at fly fishing supplies.  Just before the hatch you can go to the clean streams and look for the cased larvae that has sealed itself in.  Just look as soon as the thaw comes around and keep looking and once you seen one you will always find them.