Consider how these formidable formians became the dominant predator on their planet- think of it in terms of evolution and what has made them successful. A few things that I can envisage:
Immediate chemical communication
Hive insects use chemicals to communicate within their colonies. You might retain this literally or you might decide to use another form of communication- a natural radio system or similar. In any case you could play it so that the knowledge of one is shared by all the others in the area, as long as they are linked by these lines of communication they effectively share in the thoughts of the whole system. If this is chemical the range is relatively limited and messengers may propagate information between any isolated groups or along their established pathways ( perhaps even their pathways are designed for this purpose, a kind of natural telegraphy ) effectively they work together as a single entity.
If they are using a radio or similar, you might find that it is interacting with the players communications in the presence of the creatures and this would both make their lives more difficult in their company and potentially give them clues as to how the formians could be interrupted.
Different sizes and roles
If you think of a termite colony, there are many different sizes and roles of termites- soldiers, workers, nursemaids and so on, which work together to create the colony. You could do something similar with your creatures with much greater range- maybe some are as huge ( even carrying others like walking busses ) and others are tiny. Combine this with the shared senses described previously and even a tiny ordinary looking ant could be a spy, sharing information with the whole colony.
Intelligent yet mindless
The colony may behave as though it is smart- perhaps devoloping in a planned and regular fashion, maybe allowing resources to regrow by raiding on a schedule that spreads out over years ( loads of interesting plot hooks around this ) but when you deal with the individuals of the colony, they are almost mindless. They are dedicated to a single task and don't really take interest in anything that isn't either part of that task or impeding it. So the players could be watching them stream past from a few feet away and there is no problem, but if they got into the way, soldiers would soon turn up to deal with them. Each role of insect is only interested in its own job, however, so many will try to ignore the players unless they create an immediate threat or the insects they are dealing with are hunting at the time.
Consider that the colony is really the life form here, so attempting to interact with an individual member is a little like trying to interact with an individual cell on a mammal.
The mindless yet relentless enemy even without a directing intelligence ( used well in The Year Of Our War ) is a very effective enemy. If the intelligence behind it is purely dedicated to the survival and growth of the colony then you have something that seems to me plausible and sufficiently alien to pose an interesting challenge for your players.