It took awhile for the story to develop but in the end it became known that the Menoptra were the original inhabitants, while the Zarbi were simply goosed into being intelligent by a force known as the Animus which had landed on the planet years before. The Animus used the Zarbi to take over the planet forcing the Menoptra to flee the planet.
With the aid of the Doctor and Companions all was set right, the Animus was vanquished and the Menoptra were enabled to return to Vortis.
An interesting feature of the story was the weird hand movements and speech style of the Menoptra. It took awhile to grok this, but it conveyed some kind of bug-like-ness to their behavior.
One thing I found shocking is the ease with which the Zarbi were able to cart off the TARDIS. Supposedly the TARDIS is impregnable etc, but I suppose it's just a box and if you have enough man-power (er.. ant-power) it can be carried away.