The Space Marines had failed to keep the Cy-Bugs contained on that unknown planet, one by one they perished at the teeth and claws of the Cy-Bugs, until even Sergeant Calhoun had fallen.
They had devoured some of their victims alive, so some of the Cy-Bugs took on the partial appearance of the humans, along with gaining their memories and knowledge.
Once everything had been devoured on the planet and it was left an even more desolate place than when the scientists had arrived, the Cy-Bugs managed to get the Space Marines' ships going and had made their way back to Earth.
The Queen was the commander and the humanoid Cy-Bugs became her generals and advisers, though, through the Hivemind, she had gained their knowledge as well.
The spacecraft had landed in a mountainous area and the Cy-Bugs had quickly abandoned it, leaving it for the caves where they could hide and breed.
Some humans had seen it land and went to investigate, but found it empty and devoid of life. There were some questions and investigations, but nobody connected it to the missing Space Marines, as their mission had been top secret, since it concerned military matters.
The Cy-Bugs quickly multiplied, laying hundreds of thousands of eggs in a matter of weeks.
The little ones grew rapidly and their hunger could not be contained.
Soon, there was nothing left in the wilderness to devour, so they started snacking on humans.
By the time people realized that there was a full-scale invasion going on, it was already too late. The Cy-Bugs' hunger was endless and they multiplied extraordinarily quickly.
Even worse, very few weapons could penetrate their metallic shells, only the higher grade weapons and missiles could even hope to do damage.
It didn't take long before the first country was decimated as the robotic parasites marched on the world.
Even if they completely ate a human and took on their appearance, they still were more Cy-Bug than human, the feral instincts all but wiping out what remained of their minds.
There was no escaping from the devastating horde and one by one, all the countries fell under their control.
Now, there were a few survivors scattered throughout the world, somehow managing to eke out a meager existence, forever listening to the tell-tale sound of buzzing wings and marching legs.