Unknown Creature in tank, Please ID

Hi everyone. I need some help on this insect/larva i spotted in my tank yesterday. I fear they may harm my fish. I Inspect my tank closely everyday, usually just watching the fish but have not seen this till now. I saw one yesterday, then another today, a few hours later another.

I have attatched a picture. There are 3 of them, ~ 2cm long. I'm concerned because this is a guppy tank, with guppies ~3cm. They are stationary most of the time and are found between my front glass and the UGF grill. The UGF grill is (obviously) below the gravel, and i have not seen them in the open water. They move along the length of the grill but i've not seen them doing the travel. They sometimes respond to knocks on the glass by changing postion. I haven't added anything to the tank for the past month except filtered water.

Please help me identify them and ask me for any further details to aid identification. Please click to enlarge pic for a detailed look.

Wow, Thanks for the many quick replies and identification. Again, I want to confirm that they are not a threat to my guppies, because they are almost as big as the guppies. I have heard that dragon & damsel fly nymphs are predatory and eat small fish. EDIT: Yes i have a few plants in my tank. And as an addition there is a bunch of transparent eggs ~1mm diameter stuck on one plant, mid depth of the tank. Wonder what those are.

Thank you all, Problem Solved, I will do the needful.