This is caddisfly larva and this particular specimen, I believe, is in the family Hydropsychidae. They are net spinners as opposed to case builders. You can go crazy adding details to imitate these but over time I've found that the simpler patterns actually seem to work better. Although green is my go-to color, rusty brown also works very well.
To tie the fly, I'm going to use a size 14, #125 Dai-Riki Emerger hook. I'm using the size 14 for this video but I really like to fish it in a 16. This fly benefits from weight and here's how I like to add it. Squeeze out a small drop of super glue, in this case, Zap-a-Gap. With your bodkin, pick up just the smallest amount and brush it on the middle of the hook shank. Take 6 or 7 wraps of 15/1000's wire, here, the lead-free stuff, right in that little skim of glue. Once wrapped, it's on the hook for good and you can use a thumbnail to trim off the excess. As you can see, it's not going anywhere.
For thread I'm going to use UTC 70 Denier in brown. Attach your thread to the hook shank immediately behind the weight. For the rib, I'm using fluorescent chartreuse ultra wire in the brassie size. Cut about a 6 inch length of the wire, this can be used to make multiple flies. Secure the wire to the hook shank and begin taking wraps with your tying thread down the bend of the hook to give the pattern a nice curved shape.
For dubbing, I really like Australian Possum for this fly. It's texture and bugginess are just right. The color here is light olive. As you apply the dubbing to the thread, be careful not to put it on too thick, as you don't want the body of the fly to look plump. Start wrapping the dubbing on the hook, making sure it covers your last thread wrap, then work your way up the shank of the hook. Ideally, you should end right where the weight ends.
Get a hold of the ultra wire and begin making open wraps up the hook . I like to use quite a bit of tension on the wire to enhance the segmented look. When you get to the bare hook shank, tie the wire off really well and then give it a couple of turns to break it. Then use your tying thread to cover up the end.
For the darker thorax of the fly, I'm again using Australian Possum, this time in dark brown. I like to start dubbing at the eye and work back, over the green, and then finish back at the eye.
Whip finish and then cut the thread off and the flies almost ready to go. I don't use head cement on these but it certainly couldn't hurt. Do brush the dubbing out to give the appearance of legs and gills. You can trim any overly large or errant fibers if you like. No, it's not the most detailed imitation, but I absolutely believe it's close enough. Again, try them in rusty brown as well as green.