Leptothorax paroxenus is a workerless inquiline parasite of L. canadensis that my students and I discovered in Mississauga, Ontario, in 1975. The place where we first found the new species was in a group of very old white pine stumps situated under a row of hawthorn trees at the edge of a field. The stumps probably dated to the early nineteenth century when the field was cleared. The field was on the east side of Erin Mills Parkway where the interchange with Highway 403 is now located. Since then, we have found L. paroxenus at several sites in Oakville, Milton, and Mississauga; and others have found it near Montreal.
As is often the case with inquilines, L. paroxenus occurs in places that seem less than ideal for its host. L. canadensis is a northern species that mainly lives in the boreal forest of the pre-Cambrian shield. However, L. paroxenus has always been found very near the extreme southern edge of L. canadensis' range. Another interesting thing about L. paroxenus is that electrophoresis reveals it to be extremely similar to L. species A, the other common, free-living Leptothorax found in the boreal forest of the pre- Cambrian shield. Both L. paroxenus and L. species A share the unusual trait of producing both normal winged queens and wingless female reproductives (called "ergotogynes") in the same colonies. These facts suggest that L. paroxenus is a very recent evolutionary offshoot of L. species A.
Colonies of L. canadensis usually contain several queens. Shortly after adopting an L. paroxenus parasite queen, the host workers and the parasite kill some, but not all, of the host queens. This photo shows a newly adopted L. paroxenus ergotogyne just left of centre, biting an L. canadensis queen's left antenna, while several host workers bite other parts of the host queen. Note that part of the host queen's left hind leg has already been bitten off.
After a host colony adopts an L. paroxenus parasite, the surviving host queens become socially subordinate to the parasite. In this picture, a black L. canadensis queen regurgitates food to a brown L. paroxenus queen.