Pterourus, Scopoli, 1777

Pavulaan, Harry, 2024, Determination of a new spring-flying species of the Pterourus glaucus complex (Papilionidae) in southern New England, The Taxonomic Report of the International Lepidoptera Survey 12 (1), pp. 1-26 : 18-19

Pterourus late-flight hybrid(?) species

“Mid-Summer Tiger Swallowtail”

Status: The flight period in Massachusetts ( Fig. 3 ) indicates a strong bimodal flight, and the summer peak in that state is well-defined, though total numbers are inflated by glaucus in summer. In Rhode Island ( Fig. 4 ), the bimodal pattern is evident but masked by greater numbers of glaucus . In Connecticut ( Fig. 5 ), the bimodal pattern is inflated by the presence of both the “Mid-Summer Tiger Swallowtail” and glaucus . Ording (2008) studied the “early flight” ( canadensis ) and “late flight” hybrid in Vermont and concluded the late flight (Mid-Summer Tiger Swallowtail) may be considered an incipient species, with morphological characteristics “virtually identical to P. appalachiensis ”. Ording also suggested: “The LF is now either geographically isolated or temporally isolated from both of the parental populations”. My observations have demonstrated that this concept falls apart in southern New England, where four taxa occur.

Habitat: Primarily a resident of deciduous forest habitats, though adults are frequently found in urban and suburban habitats.

Larval Hosts: The hostplant has not yet been confirmed in southern New England. These require documentation based on rearing. First instar larvae of intermediate characters (between glaucus and canadensis ) have been documented on P. serotina in July.

Habits: This butterfly appears to be primarily a forest canopy dweller, much as is P. appalachiensis of the Appalachian Mountain region. Adults are mainly observed in rapid flight in forested habitat, coming down to nectar at numerous nectar sources such as Buddleia sp. (Butterfly Bush) in gardens. The species is not common, and generally only seen as single individuals or in very small numbers.

Broods: Apparently one brood. Obligate univoltine. Sympatric flight of glaucus in late summer confuses matters and requires extensive rearing to determine if hybridism influences voltinism. Confirmed early dates: July 5 (Connecticut); July 10 (Massachusetts); July 6 (Rhode Island). Confirmed late dates: September 9 (Connecticut); August 21 (Massachusetts); September 12 (Rhode Island).

Distinguishing features and variation: Adults are large, matching glaucus in size. Wingspan is given as 5.5” (139 mm) maximum in Connecticut ( O’Donnell et al., 2007). Ording (2008) measured the forewings of Vermont specimens and determined the wings to average 50.60- 51.42 mm in length. The females of the “Mid-Summer Tiger Swallowtail” are most distinct [thus it is recommended that the selection of a female specimen should serve as a holotype]. Dorsally, the female individuals are readily differentiated from glaucus by the greatly reduced row of blue submarginal chevrons on the dorsal hindwing. In many specimens, the blue chevrons are nearly absent, except always appearing in cells CuA2 and CuA1. The interior of the dorsal hindwing is clear yellow and is never covered by the subtle blue wash of scales found in most glaucus females. Also, the female black form has not been documented. In all other respects (i.e.: wing shape, extent of bands, shape of marginal crescents) the dorsal side is nearly identical to glaucus females.

Ventrally, the diagnostic submarginal yellow band on the forewing of the “Mid-Summer Tiger Swallowtail” is solid, though the anterior portion of the band may be broken in many individuals. The postmedian boundary between interior yellow ground color and the submarginal row of blue chevrons forms a slightly scalloped but nearly straight black ribbon. The hindwing marginal lunules are shaped more like crescents as in glaucus , though the shape is variable. Also, the black band along the inner (anal) margin is narrow as in glaucus .

The dorsum of “Mid-Summer Tiger Swallowtail” males ( Figs. 31 , 39 ) is nearly identical to glaucus males and can easily be misidentified.

Wang (2017) illustrates a first instar “hybrid” larva from southern Ontario that is intermediate between first instar larvae of glaucus and canadensis , in which the white saddle mark of both (parent) species is present but the posterior and anterior bands typical of canadensis first instar larvae are reduced. First instar larvae collected in Massachusetts in July possessed two saddle marks: the large middle saddle mark and a reduced anterior saddle. Future studies would help document that this is consistent for the Mid-Summer Tiger Swallowtail in New England.

Comments: This taxon is currently under study in Canada ( Wang, 2017; Schmidt, 2020; Ontario Butterfly Atlas, 2020). It is theorized that the “Mid-Summer Tiger Swallowtail” could be undergoing early stages of speciation in sympatry with glaucus in the historic “hybrid zone”. One possibility is that secondary contact since the last glacial maxima, over a broad area, may indicate a previous, incomplete early stage toward speciation. While it is apparently more stable as a species-level taxon in Ontario ( Schmidt, 2020), in New England it appears that there may be rampant hybridization with fully sympatric glaucus as indicated by a great many intermediate summer-brood individuals that cannot be reliably assigned to either.

Distributional County Records: Confirmed photo or specimen records for this study. Connecticut: Fairfield, Hartford, New London. Massachusetts: Bristol, Essex, Hampshire, Middlesex, Worcester. Rhode Island: Providence, Washington.

Figures

Fig. 3. Combined flight pattern in Massachusetts with bimodal pattern evident (1991-2019).

Fig. 4. Combined flight pattern in Rhode Island (1946-2023).

Fig. 5. Combined flight pattern in Connecticut (1980-2020).

Fig. 31. Pterourus (sp.) “Mid-Summer Tiger Swallowtail”, male. July 12, 2008, Newbury, Essex Co., MA. Leg. Matt Arey. Dorsal

Fig. 39. Pterourus (sp.) “Mid-Summer Tiger Swallowtail” (male). July 12, 2008, Newbury, Essex Co., MA. Dorsal (left), ventral (right).