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There are four taxa in this species-group in Alberta, consisting of three subspecies of the Old World Swallowtail (P. machaon) and the Anise Swallowtail (P. zelicaon). The Anise Swallowtail occurs together with P. machaon throughout most of Alberta, and the following characters will distinguish these two species in most cases: the black pupil of the hindwing eyespot is connected to the wing margin or positioned below the red scales in P. machaon, compared to a centered, unconnected pupil in P. zelicaon. Hybrid individuals displaying characters intermediate between hudsonianus and P. zelicaon can be quite common (Sperling 1987, 1990, Sperling & Harrison 1994). The subspecies of P. machaon are best distinguished by location and habitat. The Hudsonian Old World Swallowtail is a boreal forest species generally associated with open bogs, not eroding prairie valleys and hillsides (P. m. pikei and P. m. dodi). P. m. hudsonianus (and P. m. pikei) also has an extensive yellow suffusion over the basal area of the forewing underside. In dodi, this area is generally black.

The immature stages of subspecies hudsonianus are incompletely known, although they will likely be similar to P. m. dodi and P. m. pikei. The mature larva has yellow spots (based on one known individual) (Sperling 1987). Adult males hilltop to await females.

The larval hostplant is unconfirmed, although there is evidence that coltsfoot (Petasites spp.) may be a host (Sperling 1987, Layberry et al. 1998). Adults nectar at Labrador Tea (Ledum groenlandicum) (F. Sperling, unpublished data).