Baetis fuscatus (Linnaeus, 1761)

Webb, J. M., Jacobus, Luke M. & Sullivan, S. P., 2018, The state of systematics of North American Baetis Leach, 1815 (Ephemeroptera: Baetidae), with recommendations for identification of larvae, Zootaxa 4394 (1), pp. 105-127 : 109-110

Baetis fuscatus group

The Baetis fuscatus group is distinguished from the other North American species groups by labial palps with a distinctly curved outer margin and with the length of segment two <2× the basal width of segment three. Most specimens have an inverted, dark U-shaped marking on the pronotum (in some specimens the ‘U’ may be enclosed in dark pigment, forming a pale spot, e.g., Fig. 12) and a quadrate dark marking on each of the femora (Fig. 34). Robust setae are absent from the antennal scapes and pedicels, paraprocts, gills, and abdominal sterna and terga. Gills have lengths that are <2× their widths, and they have serrate margins. All species in the fuscatus group are morphologically similar, and specific differences are based mostly on color patterns.

Baetis notos: The larva of this species is differentiated from the flavistriga complex species by the combination of ‘crown’ shaped pale markings on abdominal terga (Fig. 13) and the absence of a medial dark band on the caudal filaments. Additionally, the femora lack the distinct quadrate dark marking of other B. fuscatus group species, but have a more indistinctly shaped dark marking of variable size instead. This species was initially thought to be restricted to the southwestern United States, but it has since been reported from California, Oregon, Washington, and Idaho ( Lester et al. 2002; Meyer & McCafferty 2007a; b, 2008).

Baetis flavistriga complex: Baetis flavistriga , including all of its synonyms, has been reported from all regions of North America. One species, B. phoebus , was recently removed from synonymy with B. flavistriga (Zhou et al. 2010) based on molecular evidence and morphological differences in male adults. Larvae of B. phoebus appear to differ from other flavistriga complex species by the absence or reduction of dark tracheae in the gills and by the smaller, elongate medial pale markings on the abdominal terga ( Ide 1937; Bergman & Hilsenhoff 1978), but more specimens with their identification confirmed through DNA will be required to determine whether these characters are of actual discriminatory use. Until species boundaries within the flavistriga complex are resolved species-level identifications of larvae are not possible and the use of flavistriga complex is appropriate ( Klubertanz 2016). The distribution of B. phoebus and B. flavistriga cannot be accurately determined at this time, because many records of B. flavistriga are likely attributable to B. phoebus , and DNA barcoding results (Webb et al. 2012) show B. phoebus to be widespread from Colorado to Nova Scotia to Saskatchewan.

Specimens of the B. flavistriga complex from several localities in far western Washington and Oregon have a color pattern similar to that of B. notos , including having tergum 5 similarly marked as the other terga (Fig. 8) (rather than paler than adjacent segments as seen in other flavistriga complex species, as in Figs. 11, 12), and having the more elongate and less quadrate dark markings on the femora (Figs. 35, 36); in contrast to B. notos , however, these far western specimens have a dark medial band on the caudal filaments (Fig. 10) and the pale abdominal macula are slightly less fused (Fig. 9). In late instar specimens, a purple transverse band is apparent on the posterior margins of the abdominal terga (Fig. 9), presumably indicative that the adult will also have the band. Adults of B. notos do not have banded abdominal terga, so these specimens from western Washington probably represent a different species than B. notos . The only described flavistriga complex adults with banded abdominal segments are B. quebecensis , a synonym of B. phoebus , and B. caurinus , currently a synonym of B. flavistriga ( McCafferty 2007) . Baetis phoebus has not been reported west of the Rocky Mountains, and the larvae (Fig. 12) have a color pattern typical of the flavistriga complex. Baetis caurinus , on the other hand, was described from northwestern Oregon by Edmunds & Allen (1957b), and the larva remains unknown. The banded-tailed, notos -like larvae from western Washington and Oregon might prove to be associated with B. caurinus , and if this is the case, then the synonymy of B. caurinus with B. flavistriga should be reconsidered.

The larva of B. rusticans remains unknown, but DNA barcoding of adults shows them to be nested within sequences of B. flavistriga (Webb et al. 2012) ; based on this, we tentatively place B. rusticans within the B. flavistriga complex and predict the larvae will be similar to those of B. phoebus and B. flavistriga . This species has been reported from northeastern North America and Texas (Myers et al. 2008).

Baetis intercalaris complex: The intercalaris complex as defined herein consists of only a single accepted nominal taxon, B. intercalaris McDunnough, 1921 . However, multiple deeply divergent mtDNA haplogroups, comprising at least six BINs (http://www.boldsystems.org/index.php/Taxbrowser_Taxonpage?taxid=9583), have been observed (Webb et al. 2012) that may represent putative species. Most specimens encountered have abdominal terga with 3 round pale markings near the posterior margins (Figs. 15, 16), but one distinct haplogroup sequenced by Webb et al. (2012) had much reduced markings (Fig. 14). Specimens from Oklahoma, Pennsylvania, and Wisconsin that consistently differ in color pattern from typical B. intercalaris in both adult and larval stages were hypothesized to be environmental variants of B. intercalaris (Waltz et al. 1996) , but it is possible they represent a distinct species. Larvae are most readily distinguished from those of the flavistriga complex by color pattern, although the third segment of the labial palp is reported to be more bulbous medially (Wiersema et al. 2004).