Me personally when I think of Megrachnids
I think of a off shoot or relative of the (Now adays(40k))
Tyranids when I was listeniing to The Horus rising audio as well as reading
about them,The descriptions closely resemble them athough they could be a off branch or relative of the Tyranids.If I was doing them I would
build em that way...specially since they have the same crests on their heads.