Toxorhynchites (Lynchiella) rutilus (Coquillett)

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 : 120-121

Toxorhynchites (Lynchiella) rutilus (Coquillett)

subspecies rutilus (Coquillett, 1896) —original combination: Megarhinus rutila [sic]. Distribution: Extreme southeastern United States— Florida, Georgia (mainly coastal), North Carolina ( Harrison et al. 2016), South Carolina (coastal) ( Jenkins 1949; Darsie & Ward 2005).

subspecies septentrionalis ( Dyar & Knab, 1906a)—original combination: Megarhinus septentrionalis (subspecific status by Jenkins & Carpenter 1946). Distribution: Eastern North America, northward from northern Florida to Canada (southwestern Ontario) and westward to Kansas, Oklahoma and Texas ( Jenkins 1949; Wood et al. 1979; Darsie & Ward 2005).

Coquillett (1896) indicated that Tx. rutilus , as Megarhinus rutila , was described and named based on three males and five females from North Carolina and Georgiana, Florida. According to Stone & Knight (1957c), the original collection included two specimens from Georgiana , Florida and seven from undisclosed localities in Florida, but none from North Carolina. They selected a male from Georgiana bearing a type label to be the lectotype. Georgiana is located on Merritt Island of Brevard County on the eastern side of central Florida.

Dyar & Knab (1906a) described and named Tx. septentrionalis , also as a species of Megarhinus Robineau-Desvoidy, 1827 , from a collection of 13 males and 11 females from localities in the District of Columbia, Louisiana, Maryland, Mississippi, Missouri, North Carolina, Virginia and West Virginia. Stone & Knight (1957c) considered a male from Woodstock, Virginia bearing a type label to be the holotype.

The subspecific status of septentrionalis was established by Jenkins & Carpenter (1946), and reinforced by Jenkins (1949) based on conclusions drawn from a study that included specimens form a narrow zone of overlap and apparent integration with the type form. Jenkins described the evidence for integration as follows.

In the zone of overlap of ranges of the two subspecies, all types of integrations occur. In the northern part of the zone in Georgia and South Carolina, the male intergrades have segments [tarsomeres] 2 and 3 of the fore tarsi with white areas or with isolated white scales surrounded by purple. Two typical intergrades from Myrtle Beach , South Carolina, Carpenter and Jenkins (1945) [i.e. 1946], have the following markings on the fore tarsi: Specimen 1.-2nd segment [foretarsomere 2] with basal three-fourths silvery white; 3rd segment [foretarsomere 3] with three white scales. Specimen 2.- 2nd segment, (left) with six white scales, (right) with 4 white scales; 3rd segment, (left) with 3 white scales, (right) entirely purple. In the southern part of the zone of overlap in Florida, the fore tarsi of the intergrade males are usually typical T. r. rutilus with the second and basal two-thirds of the third segment silvery white, but interspersed with several dark purple scales. The intergrades were observed from Jacksonville, Tallahassee , and Bushnell , Florida.

The amount of white on the fifth segment [tarsomere 5] of the hind tarsi of the males is variable. There is a decided tendency for this segment to be mostly white in Florida T. r. rutilus specimens, and mostly dark purple in the main part of the range of T. r. septentrionalis . This is quite variable and a specimen of the subspecies from Falls Church, Virginia has this segment almost entirely white. The type of intergradation described for the fore tarsi and often observed on the fifth hind tarsal segment [hindtarsomere 5]. An unusual amount of white was observed on the fore tarsi of males from Duval Co., Leon Co., and Orange Co., Florida that had the 2nd, 3rd and about half of the 4th segments [foretarsomeres 2, 3 and 4] silvery white. A few specimens exhibited white scales on the apical tip of the tibiae.

Jenkins summarized his findings and previous observations ( Jenkins & Carpenter 1946) in the form of conclusions:

The tropical genus Toxorhynchites is represented in the United States by two closely related forms which are considered to be subspecies for the following reasons.

1. The distributions of the two forms overlap in part of their ranges.

2. A complete series of intergrades between the two forms occurs throughout the zone of overlap in ranges.

3. The only known character separating the two forms is a color difference of the males which is subject to variation.

4. No significant differences have been found to separate the two forms in the male genitalia, larvae, pupae, or females.

5. The life histories, habitats, and breeding habits of the two forms have been found to be similar.

It is important to note that conclusions 3 and 4 have been repeated, apparently without further detailed morphological study, by later authors ( Carpenter & LaCasse 1955; Wood et al. 1979; Harrison et al. 2016). Conclusion 5 is meaningless as it applies to species of Toxorhynchites in general.

It is surprising to find that the subspecific status of septentrionalis has been accepted by taxonomists without question since the studies of Jenkins & Carpenter (1946) and Jenkins (1949), and has not since been re-examined using genetic and molecular tools. There are four records of DNA sequences in GenBank: 28S rRNA, MT-CYB and MT-ND5, Colony, Florida Medical Entomology Laboratory, recorded as Tx. rutilus ( Krzywinski et al. 2001 a); mt - COI (partial), mt -COII and tRNA -Leu, York County, Pennsylvania, recorded as Tx. rutilus ( Mitchell et al. 2002) ; mt -COI, Ontario, Canada, recorded as Tx. rutilus ( Cywinska et al. 2006) ; and 18S rDNA, Stonington, Connecticut, recorded as Tx. r. septentrionalis ( Shepard et al. 2006) . Unfortunately, because the researchers did not produce the same molecular sequences, it is not possible to conduct a meaningful analysis of relationships. That aside, the identity of the records attributable to Tx. rutilus are problematic. The records from Canada and Pennsylvania must certainly be based on specimens of septentrionalis , and it is not possible to know whether the Florida colony was derived from specimens of rutilus sensu stricto or intergrades. It should be noted, however, that researchers who conducted ecological studies of treehole mosquitoes in eastern North America ( Bradshaw & Holzapfel 1975, 1977; Chambers 1985) did not recognize septentrionalis .

As there is no indication of intermediates in the geographically separated populations, there is little doubt that specific traits are being maintained outside the zone of introgression. This is evidence for independent species cohesion, and we believe that molecular systematic studies will reveal that the two forms are separate species that hybridize in a narrow zone where the two species overlap. This supports the taxonomic conclusion that the type form and septentrionalis are distinct species; therefore, we hereby formally return septentrionalis to its original rank as a species: Toxorhynchites (Lynchiella) septentrionalis ( Dyar & Knab, 1906a), which is currently listed as a species in the Encyclopedia of Life.

Toxorhynchites septentrionalis has a single synonym, Megarhinus herrickii Theobald, 1906 (type locality: Mississippi State, USA), synonymy by Howard et al. (1917). As noted by Belkin (1968), Theobald based the description of herrickii on Herrick’s (1905) description of Megarrhinus portoricensis , apparently without having seen any specimens—probably the ultimate reason for naming the species after Herrick. Megarhinus herrickii remains a junior subjective synonym of Tx. septentrionalis .