What is it?

Hi Everyone!

Can anybody help me?  I was innocently inspecting my rue plant in my garden bed, hoping that I just might start finding some Black Swallowtail Butterfly caterpillars on it sometime soon--when I discovered this big thing.  I am suspecting it may be a Praying Mantis egg case, but do not know for sure. . . does anybody know?
If it isn't Praying Mantis, what is it?  And if it IS a Praying Mantis egg case, how long until it hatches out and what do they eat???  I really really really don't want anything eating MY butterfly caterpillars. . .
On the one hand anything from nature is potential good home school science material.  But NOT if it is something that is going to eat my butterfly caterpillars!!!

Whatever it is, does anybody know if it would be a good thing to bring inside in a container and watch for science class?  And how small would the critters be when they hatch--exactly how small do the air holes on my containers have to be to prevent escapees in the house?

Thanks for any help that you can give!

~Susan