This could be interesting,I did a search,I found one that looked like this,I have four of these in the yard,I have three I ordered of the web,they do not look the same,if they are,I am going to have a ton of Mantis in a short time!
I have one like that too, I'm not sure if the egg sack is incomplete or some have hatched.
I am leaving mine, as I have noticed they like to lay near the old sacks.
I have watched one hatching, there is no mistaking them ,miniature adults flat out running, once they out.
I couldn't believe my eyes one am..was outside enjoying my garden and saw what appeared to be yellow foam from a distance, dropping from the underside of a fence board. Up close, this is what I found. There were literally 100's of them hatching from the 2 egg cases. They were about 1/4" or less and perfectly formed-baby praying mantises (sp?)
FYI - just read yesterday on a garden page somewhere? one is mantis, more is mantid. Kinda different but that's what they said! What a cool picture of the babies - something I'll never see here. Apparently z4 is too cold for them most winters. Master Gardener guy said if we order them from a catalog, a few will make it over the winter but by the 2nd winter they'll be all gone. Bummer! We do have walking sticks, tho. Had one on the dining room window screen last year. They're kinda neat too!
Count yourself lucky, as they are clearly a beneficial, and sounds like you'll soon have a good population of them. . . Even with all the egg cases, they won't all make it.. cannabalistic as bootandall indicated. They eat their siblings when they first hatch out, as their first meal. Sounds gruesome!! There's probably alot more around, as they are experts at camoflauge.