A first contact story between humanity and "moties" - a strange alien race with a biologically-dimorphic caste system.
The moties have many subtypes that vary both in body plan and in psychology.
The Motie Civilization is at least tens of thousands of years old, with many layers of debris from fallen cities everywhere, many layers of craters from atomic wars, and abandoned asteroid colonies abandoned for tens of thousands of years. The Moties breed quickly and periodically fall into major overpopulation wars.
The Moties themselves reproduce at an alarming rate, which causes their periodic population collapses.
The moties are discovered after sending a probe with a solar sail into a human star system.