The Klikiss were an insectoid race that was thought to have become become extinct 10,000 years before the beginning of the series. Their only remains are numerous ruins (some more or less intact, whilst others are wrecked beyond repair) scattered amongst worlds across the Spiral Arm. It is later discovered that some of the Klikiss' technology is still salvagable, as proven by the use of the Klikiss Torch and the inter-planetary portals found in a large number of their cities.
Aside from this, the only other legacy left behind by the Klikiss race is the large number of robots (apparently designed in their likeness) that were reactivated half a millenia ago by the Ildirans. The robots themselves are very intricate in terms of design, with largely unknown inner workings. Though they all look identical, they each consider themselves unique in some respect, as they assign names to each member of their race. They have blunt speech mannerisms, and often do not reveal too much about themselves or their motives.
Although they do not expose them very often, the Klikiss robots are equipped with a variety of built-in weapons systems and advanced wings that allow them to fly. It is worth noting that their infrastructures are practically indestructible, with nothing seemingly able to deal them damage; three robots were hit by a monsoon and came back with no damage, and later a large number of them could exist at the core of a gas giant with no trouble whatsoever.
The cause for the Klikiss' disappearance is a mystery to the rest of the galaxy, and the intelligent robots claim to have no knowledge of the events. Thus the dead race provides a very complex mystery, one which the human race has tried to unravel by studying the Klikiss ruins.
In reality, they were wiped out by the Hydrogues, their robots (which turned on them) and the Ildirans, who did so to avoid being wiped out by the Hydrogues. The surviving robots are free to roam around the galaxy, but their purpose is dubious. The Ildirans do not have many dealings with them, although the Klikiss robots acted as negotiators between them and the Hydrogues, and some humans are wary of them. Although it appears that only a few of them survived, the 'awakened' ones are reviving their dormant comrades. They are allied with the Hydrogues, and are at war against Humanity in order to free their servant robots ("compys") - a reason why the Ildirans do not use similar technology.
Some small number of Klikiss evidently survived the genocide, as they reappear at the end of Of Fire and Night, ready to reclaim their old worlds, now colonised by the Hansa.