Bibio

Fall-flying Bibio View in CoL species

( Figs. 4–12 View FIG View FIG View FIG )

Several species of Bibio have late summer to late fall emergence times and thus have been previously referred to as the “fall-flying species group” (e.g. Fitzgerald 1997a). While the phylogenetic relationships between these species is unclear and they may not represent a monophyletic group, they are all morphologically quite similar and have sometimes proved difficult to distinguish from each other. From the Nearctic region, Fitzgerald (1997a) included Bibio autumnalis Fitzgerald , Bibio longipes Loew , and Bibio slossonae Cockerell in this group, though B. flukei Hardy may also belong here. When taking into account the influence of range on phenology there may be other lateseason species that could be added, however, the four species under consideration here all have males with the hind basitarsus strongly swollen, hind femur slender until about the middle and then widening towards apex.

Of these four species B. flukei is the most distinct, differing by the deeply cleft epandrium of the male terminalia ( Fitzgerald 1996) and the form of the hind tibia which is slender on the basal 1/2 and rapidly dilated on the apical 1/2 (Fig. 5). B. flukei is most similar to B. autumnalis as they share rather distinctive leg coloration ( Figs. 4 View FIG –5); both the hind femur and hind tibia basally yellow and distally darkened. However, B. autumnalis is differentiated by the shallowly cleft epandrium, hind tibia evenly expanded apically ( Fig. 4 View FIG ), and smaller size (wing 4mm versus 6–6.5 mm in B. flukei ). At present, both these species seem to have fairly limited distributions in the southern Rocky Mountains ( Fitzgerald 1996).

Males of B. slossonae and B. longipes were both described from the eastern Nearctic (Nova Scotia, Canada and Washington, D.C., USA respectively) with black legs though Hardy (1945) noted that the hind tibia in B. longipes may sometimes be “rutilous.” Hardy (1945) distinguished males of B. longipes from B. slossonae by color of the pile of the mesonotum and legs and this system was followed by later authors (e.g. Fitzgerald 1997a). However, pile color has been shown to be a notoriously unreliable character in distinguishing Bibio species ( Fitzgerald & Skartveit 1997) and series of specimens of B. longipes or B. slossonae may include specimens with pile that is black, yellow, or intermixed. Similarly, Hardy (1945) stated that females of B. longipes and B. slossonae could be distinguished by the color of the mesonotum; orange in B. longipes and black in B. slossonae . However, some series of specimens have been collected with female mesonotal color ranging from orange to orange with black markings and other series collected with mesonotal color ranging from orange with black markings to black, so the reliability of this character is also in question. Due to these observations these two species are now indistinguishable based on the characters outlined in the literature and it is not clear if they are even distinct.

Both sexes of B. longipes and B. slossonae might be distinguished by general habitus, as “ B. slossonae ” appears to have a smaller head and thorax relative to the antennae and wings, compared to “ B. longipes . ” These observed differences warrant further study in order to evaluate their consistency and usefulness. Examples of the differences in habitus based on internet images are as follows:

1) “ B. longipes ” habitus, with female eyes seemingly smaller relative to body, and thorax thicker relative to wing width in both sexes: male and female: https://www.inaturalist.org/photos/332106876?size=large female: https://www.inaturalist.org/photos/332292704?size=large male: https://www.inaturalist.org/photos/25335804?size=large male: https://www.inaturalist.org/photos/158301375?size=large 2) “ B. slossonae ” habitus, with female eyes larger relative to body, and thorax narrower relative to wing width in both sexes (wings fold a bit differently as a result at rest): female: https://www.inaturalist.org/photos/171093034?size=large male: https://www.inaturalist.org/photos/55689548?size=large female: https://www.inaturalist.org/photos/96569118?size=large

B. longipes View in CoL (synonymized with the European B. lepidus Loew View in CoL by Skartveit (2006)) also occurs in Europe alongside the extremely similar B. clavipes Meigen. In View in CoL continental Europe, B. longipes View in CoL and B. clavipes View in CoL seem to intergrade ( Duda 1930, Skartveit 2006) though Skartveit (2006) notes they appear to be “distinct in the western- and northernmost parts of Europe and should be treated as separate species;” initial phylogenetic work suggested they are sisterspecies ( Skartveit & Willassen 1996). European and North American populations of B. longipes View in CoL show some curious differences. B. longipes View in CoL females were described with an orange mesonotum and reddish legs ( Figs. 9–10 View FIG ) while European females of B. longipes View in CoL have a consistently black mesonotum and dark (brown to black) legs (J. Skartveit pers. comm. with SF, May 2024) or reddish legs ( Freeman and Lane, 1985; England). Skartveit (2006 & pers. comm.) noted that the synonymy of B. longipes View in CoL and B. lepidus View in CoL was based on the study of a male syntype of B. longipes View in CoL so no female specimens had been considered; this is not atypical as most of the taxonomic species concepts in the genus are based on males.

At this time, it is unclear how to distinguish B. longipes View in CoL from B. slossonae View in CoL in the Nearctic region consistently and a detailed comparison of both the types as well as a comparison to Palearctic species is probably needed. Additionally, a series of specimens from Marys Peak Oregon (SFC) show a mix of characters that also brings into question the validity of B. autumnalis View in CoL as a distinct species; males with legs ranging from entirely brown, to brown with hind femur pale basally, to legs with both hind femur and tibia pale basally (as B. autumnalis View in CoL ; Fig. 5), mesonotal pile black (as in B. slossonae View in CoL ) and female with mesonotum ranging from orange (as in B. longipes View in CoL ) to orange with dark fascia. The male terminalia of these species is extremely similar and no distinguishing features have yet been identified. While an integrative taxonomic revision of the Holarctic fall-flying species is beyond the scope of the current study, it is the hope of the authors that raising these issues and providing some initial data, such as photos of some of the type specimens, may provide a starting point and a stimulus for further work.