The Plurality of Worlds. Brian Stableford

In an alternate history of 1572, five intrepid British adventurers venture into the ether to seek out new life and new civilizations. They discover a galaxy-wide invertebrate empire, including the insect-like inhabitants of the moon. But the empire is under attack from enemies both without and within–and so is the planet Earth!–and the five men must find a way to thwart the Selenites and save our world from an insect tyranny. Grand space opera in the unique Stablefordian style!