Nymphs in tank

I just found 2 nymphs in my tank
One is about 2-3cm, the other about 1-2cm.

I am afraid they are the ones that are lowering the population of my cherry shrimp.

How do I check for more of these from the tank?
Do they burrow into the gravel ?

No picture because I squished them against the fish tank glass and cleaned them out.