Complex sentence: Even in flight the ants will soon realise that they have chosen the wrong place to start their mating flight, if left alone the colony will nearly always choose an external wall to use as their official take off.
Split into simpler sentences: Even in flight, the ants will soon realise that they have chosen the wrong place to start their mating flight. If left alone, the colony will nearly always choose an external wall to use as their official take off.