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!