I have the same problem - it started late last year , and they are back. The grass and plants are alive with them, literally hundreds. They are about 1.5 cm, and very dark brown/grey/black, not many markings as far as I can see, which is tricky because they are super fast and obviously don't want to be around people!
My lawn is full of cracks, which I suspect might be something that's encouraging them. Whatever it is, any help on keeping the numbers down appreciated! I know they don't hurt, but they are just taking over !!