Dumb question?

Ok, So I have watched most of the how to fly fish videos on Orvis and other websites.  One thing it talked about was fly selection.  It gave a tip to pick up rocks in the water you are fishing to see what kind of bugs are living there to see what the "local food" is.  Makes sense to me.  My "dumb" question is, do people ever take these bugs and put them on a hook and use them as you would a fly?  If yes or no, why or why not?

Actually, in NYS, if you are looking under rocks in a trout stream, it is a violation to use anything you find for bait.  When I last bought a PA license~30 years ago, you could gather up to 50 of any invertebrate per day for personal use for bait.  Conventional methods like running the larva through with the hook point tends to really mess up a lot of these soft critters, but I watched a guy hammer fish in the 900 on Spring Creek in Caledonia once, and when I asked what he was using, he told me leeches that he was digging out of the cress roots.  These are small, no more than 1/4" long.  He had a size 14 fly hook and he was attaching them to the hook with a drop of superglue.  If he couldn't find a leech, he glued on a scud.  But you could get a ticket in NY for it, if a warden happened by!
 
I've always got some green caddis, and some tan caddis, and some orange caddis, in my box when I'm on the SR, and they have worked magic even on kings and browns in the fall when bigger stuff was not working. Fichy is right on with that tip!
 
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dtrobe-Certain nymphs migrate to slow/shallow water or up on rocks when they are getting ready to hatch.  March Browns, stone flies, Isonychia to name a few.  When they do its a good indicator of what the fish may be feeding on.  The Dobson Fly or Hellgramite actually leaves the stream bed entirely when it molts to the adult stage.    

Yep, although they are awesome for bait, hellgramites you find under the rocks on shore they are a no no.  This fly kicks **** in a 10 or 12 and can be tied with green rubber leg  coated in clear nailpolish  and peac0ck hurl.   Quick easy (Thanks for this one Charlie)

In all my years of stream pounding trout using anything I could find under rocks I never knew it is illegal. Ive used slugs, hellgrammites, crayfish and any other insect I could slide a hook in, funny how some things we think to be harmless are no nos.

Hey John shuffle your feet to dislodge nymphs, etc. is also illegal in some states.

Ahhhh NY. lol I love their fisheries but many of their rules suck. But hey, you cant hunt on sunday in PA, but you can pick clippers and catch creek shiners.

If you dislike to the no bait from a trout stream law, you can blame Art Flick, who got the reg adopted by the legislature in the 50's in reaction to the bait dealers on the Schoharie who would throw a box spring out in the river, and come back later to retrieve, pick all the hellgrammites that had adopted the new home and leave all the caddis and mayflies to dry out in the sun.  He saw a decline in the hatch quality on the Schoharie, and the bugs returned after the mattresses were no longer out there. 
 
I know that one of the best baits for brook trout in ponds is live minnows, but I also know that nothing ruins a brook trout pond faster than  minnows becoming established and outcompeting the brookies for food. So you will never see me using a minnow in a NY pond.  The inverts are the food for the stream trout, if everybody were picking what they needed for bait, you might actually slow the growth rates on the trout you are after.
 
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