Aedes (Ochlerotatus) impiger (Walker)

Harbach, Ralph E. & Wilkerson, Richard C., 2023, The insupportable validity of mosquito subspecies (Diptera: Culicidae) and their exclusion from culicid classification, Zootaxa 5303 (1), pp. 1-184 : 36-37

Aedes (Ochlerotatus) impiger (Walker)

subspecies daisetsuzanus Tanaka, Mizusawa & Saugstad, 1979 —original combination: Aedes (Ochlerotatus) impiger daisetsuzanus . Distribution: Hokkaido Japan (Hokkaido Island) ( Tanaka et al. 1979).

subspecies impiger ( Walker, 1848) —original combination: Culex impiger . Distribution: Canada, Finland, Norway, Russia (Northwestern Region), Sweden, United States (Alaska, continental) ( Wilkerson et al. 2021, incorrectly listed from Japan), tundra of Russia ( Gutsevich 1971, 1974).

The nominotypical subspecies was described from “Martin’s Falls”, Ontario, Canada, located at 51.30 N, 86.20 W. This locality was listed for many of Walker’s species as St. Martin’s Falls, Albany River, Hudson’s Bay, but was corrected to “Martin’s Falls” by Handfield & Handfield (2020). The exact type locality is therefore unknown since the place name only represents the shipping origin. Belkin (1968) found “Two ♀ … apparently part of type series” in the collection of the Natural History Museum, London, one of which he designated as the lectotype. Other entries in Wilkerson et al. (2021) with the anomalous “St. Martin’s Falls” include Culex implacabilis Walker, 1848 , synonym of Aedes (Ochlerotatus) punctor ( Kirby, 1837) , and Culiseta (Culiseta) impatiens ( Walker, 1848) .

The adult female, male genitalia and larva of the nominotypical subspecies were described in detail by Carpenter & LaCasse (1955), Gutsevich et al. (1971, 1974) and Becker et al. (2020). Ecologically, it is a univoltine early season species, which “is found in the treeless arctic regions of Alaska, Canada, Scandanavia [sic] and Siberia. Its range is known to extend southward to Utah and Colorado at the higher elevations.... The larvae... are found in clear pools of water formed by melting snow at high elevations in mountains where alpine arctic conditions prevail” ( Carpenter & LaCasse 1955). In Russia, Gutsevich et al. (1971, 1974) recorded it from “...the Kola Peninsula, in the Arkhangelsk Region, Nenets National District, Taimyr, mouth of the Kolyma, and the Novosibirskie Islands. It usually occurs together with A. nigripes . A. impiger predominates in Noril’sk…”.

Distinguishing characters (after Becker et al. 2020).

Adult female. Small species with brownish gray integument, scutum with conspicuous long black setae. Proboscis and maxillary palpus black-scaled; proboscis distinctly longer than forefemur. Vertex with spot of dark-brown scales; occiput covered with erect black setae and white scales. Antenna black, pedicel with some scattered white scales. Scutum with dense long black setae, median stripe of narrow bronze scales and lateral stripe of narrow white scales; scutellum with narrow white scales. Postpronotum with a few bronze scales, otherwise broad white scales; postpronotal setae scattered on entire postpronotum; postprocoxal, subspiracular and postspiracular scale-patches present, hypostigmal scale-patch absent; postspiracular setae 10 or fewer; upper mesokatepisternal scale-patch not reaching anterior margin; mesepimeron with large white scale-patch. Coxae white-scaled; femora and tibiae dark with scattered white scales, tibiae with conspicuous black setae; tarsomeres dark-scaled. Wing veins usually entirely dark-scaled, a few pale scales sometimes present at the base of the costa and radius. Abdominal terga with broad basal white bands. Cerci long, tapering.

Male genitalia. Tergum IX lobes rounded. Gonocoxite with short setae predominating on inner side; basal mesal lobe well developed with a spine-like seta distinctly stouter than others; apical lobe of gonocoxite small, weakly developed. Gonostylus slender, somewhat broadened in middle. Claspette filament about as long as stem, with a unilateral wing; aedeagus elongate.

Larva. Body surface without spicules. Antenna very short, spiculate. Head wider than long. Setae 5,6-C always single, 7-C 3-branched. Comb scales 10–14 (6–16), each scale with long median apical spine and several short spicules at base. Siphon index 2.8–3.0; pecten spines close together, evenly spaced, each spine with one long lateral denticle; seta 1-S inserted slightly below middle of siphon, with 4–6 long branches. Saddle covers approximately half of segment X; ventral brush (seta 4-X) with 2 precratal setae; anal papillae at least 1.3 length of saddle.

Tanaka et al. (1979) described subspecies daisetsuzanus from specimens collected at an elevation of 2,300 to 3,100 m above sea level, from “Yukomambetsu, in Mt. Daisetsu [a volcanic mountain range], Hokkaido ”, Japan’s second largest island and northernmost prefecture. Tanaka et al. (1979) provided a complete description of the adult female, male genitalia and larva, and Tanaka (1999) provided a complete description of the pupa. Subspecies daisetsuzanus is adapted to similar extremely cold conditions as the nominotypical form. Tanaka et al. (1979) stated that “ Ae. (Och.) impiger daisetsuzanus and Ae. (Och.) hakusanensis [ Yamaguti & Tamaboko, 1954] appear to be relics of the Glacial epoch, now restricted to high mountains.” Aedes hakusanensis is found further south on Honshu Island. In a footnote, Tanaka et al. (1979) stated that “The nominal subspecies, Aedes (Ochlerotatus) impiger impiger ( Walker, 1848) does not occur in this region.”

Unusual to both nominal taxa, not mentioned elsewhere in the literature, the larval maxilla is highly modified and described by Tanaka et al. (1979) in great detail, their summary follows. “The characteristic larval maxilla is especially remarkable. All other Japanese species of Ochlerotatus studied have maxillae typical for browsers; their maxillae in general are moderately sclerotized; the mesostipes [galeastipes] slightly longer than wide, peach-shaped, with a narrow somewhat detached mesal area (lacinial sclerite - Gardner et al. 1973), bearing spine-like spicules along the mesal margin; the maxillary brush apical and moderately long; the stipital sensoria [seta 1-Mx] and lacinial seta 5-Mx [seta 2-Mx] located usually somewhat distad of middle; the palpostipes [maxillary palpus] much shorter than the mesostipes. The strongly sclerotized maxilla in general, very broad square mesostipes with a broad mesal detached area, somewhat ventrally located maxillary brush, distally removed stipital sensoria and lacinial seta 5-Mx, and very large palpostipes of impiger daisetsuzanus resemble the maxilla of predaceous Toxorhynchites . The maxilla of impiger daisetsuzanus may possibly be regarded as an intermediate type between browsers and predatory species. However, the mandible of impiger daisetsuzanus is not modified.” “The Japanese subspecies daisetsuzanus is identical with North American impiger in the male genitalia and most of the larval characteristics including the modified maxilla...”.

This leaves little doubt as to the close phylogenetic relationship of the two nominal taxa. Some differences, however, were pointed out by Tanaka et al. (1979) and Tanaka (1999), which are summarized in Table 1 . These differences were enough for the authors to suggest species status for daisetsuzanus , but because of the few specimens available, they chose to rank it as a subspecies.

Hokkaido Island was partially covered by ice at the last glacial maximum ( Sawagaki et al. 2004). We assume that populations of the nominal taxon impiger were isolated at that time and have been diverging genetically since [perhaps for 18,000 –20,000 years]. For this reason, since Tanaka et al. (1979) did not encounter the nominal form in the “Japanese Archipelago” and there are morphological differences which we believe will be sustained with examination of a larger number of specimens, we formally afford species status to Aedes (Ochlerotatus) daisetsuzanus Tanaka, Mizusawa & Saugstad, 1979 . Aedes daisetsuzanus is currently listed as a species in the Encyclopedia of Life.

The nominotypical form has two synonyms: Aedes (Ochlerotatus) nearcticus Dyar, 1919b and Aedes (Ochlerotatus) parvulus Edwards, 1921d . We retain both as synonyms of Ae. impiger .