This stonefly was 10.5mm, but you can't tell that from above. You need to know where the abdomen ends in order to realize the size. From the head to the tip of the wing is 13.5mm.

First stonefly I've seen this year. On a metal railing at the park. Nice to see something other than dwarf spiders and bark lice.

On what I believe is called the prosternum, there are some very conspicuous white spiky areas.