Chaoborus flavicans (Meigen, 1830)

Salmela, Jukka, Härmä, Oskari & Taylor, Derek J., 2021, Chaoborus flavicans Meigen (Diptera, Chaoboridae) is a complex of lake and pond dwelling species: a revision, Zootaxa 4927 (2), pp. 151-196 : 154-155

Chaoborus flavicans species complex

We found evidence for at least four species in the Chaoborus flavicans complex: Chaoborus flavicans , C. albipes , C. posio sp. n. and a lineage from Japan. The monophyly for each of these species is strongly supported by bootstrap values ( Fig. 15 ) and is not attributable to random branching under a coalescent null model (Rosenberg’s P(AB) <0.05 for all species in Table 1). The presence of the subordinate tooth between the second and fourth tooth in the larval mandible is a likely synapomorphy of the C. flavicans complex. Chaoborus crystallinus has been proposed as a sister species of C. flavicans ( Saether 1970, Borkent 1981; note that the close relationship between C. crystallinus and C. flavicans suggested by Berendonk et al. 2003 appears to be a mistake due to C. flavicans being confused for C. crystallinus ). However, we refrained from assessing the proposed sister species of C. flavicans complex in this study. Phylogenetic relationships beyond the species complex are not analysed in detail here as COI tends to lack resolution across species groups for Chaoborus ( Dupuis et al. 2008) . We also found a lack of resolution at the deeper nodes for Chaoborus in the present phylogeny ( Fig. 15 ). Characters that support a close relationship between C. flavicans and C. albipes are a slender male gonocoxite and gonostylus, a relatively short apical claw of paramere and a subapically constricted pupal respiratory organ. In contrast, C. posio sp. n. has several autapomorphic character states, such as a wide male gonocoxite and gonostylus, long apical claw of the male paramere and a club-shaped pupal respiratory organ. A high number of larval mandibular fan bristles and conspicuous lateral teeth of the mandibles are shared characters for C. albipes and C. posio sp. n. This sister group relationship also had strong support in our COI phylogeny. A distinct lineage from Japan appears to be more related to C. posio + C. albipes than to C. flavicans sensu stricto ( Fig. 15 ).

It is not known where and when the ancestor of the C. flavicans species complex evolved. Divergent selection for habitat seems to be important as species occupy different niches (ponds vs. lakes); Holarctic sister species that occupy the same niche are mostly allopatric (e.g., C. americanus and C. obscuripes, Borkent 1981 ). If the pattern of three phylogenetically independent shifts from pond to lake habitats within Holarctic Chaoborus ( Berendonk et al. 2003) is accepted, the ancestor of the C. flavicans complex was a pond-dwelling species and C. flavicans became a lake-dwelling taxon able to withstand high pressure in deep waters and fish predation. Note that the substantial genetic divergence between C. flavicans and other members of the complex (> 30%) indicates that divergence of the complex likely occurred well before the Pleistocene. Moreover, widespread species such as C. albipes and C. flavicans with two or more closely related geographic clades may also have been present before the Pleistocene.

Chaoborus tertiarius (von Heyden) , Upper Oligocene fossil species, is composed of two complete pupae, disarticulated pupal parts, larval mandibles, and anal fans ( Borkent 1978). This taxon may be ancestral to C. ( Chaoborus s.str.) and the subgenus C. ( Schadonophasma Dyar & Shannon 1924 ), or it may be ancestral, or a sister, to species of C. flavicans species complex ( Borkent 1978). Among the fossil material are two types of larval mandibles: the subordinate tooth is either present between teeth two and four or it branches from the second tooth ( Borkent 1978, figs. 3 A-H). However, Borkent admits that the subordinate tooth of the flavicans - type “was very difficult to see”, and hence it is uncertain if C. tertiarius is closely related to the C. flavicans species complex.

Diagnosis. Adults male. Penultimate flagellomere slightly longer than ultimate, or about equal in length. Flagellomeres pale or partly darkened, bases of whorls dark, giving annulated appearance. Scutellar stripes bare, orange brown–almost black, pleural sclerites pale yellow and with a varying degree of darker coloration. Legs straw yellow or grayish, apical tarsomeres may be somewhat darkened. Tergites yellowish brown or dark brown, bases of setae with a dark ring. Male epandrium either triangular or broadly rounded in shape, its length about 0.35–0.37 times the length of gonocoxite. Male paramere with a conspicuous apical claw, elongated, almost straight or curved.

Pupa. Outer rib of terminal process smooth, apical spines absent. Mid rib of terminal process usually darker than lateral ribs, margin of inner rib serrated along apical 2/3. Length/width ratio of 8 th segment ca. 0.36. Respiratory organ widest medially, constricted apically or not.

IV instar larva. Total length 7–15 mm (e.g. Parma 1971b). Labral blade elongated, degree of serration varied. Mandibular subordinate tooth (3) positioned at the vertex of tooth 2 and tooth 4. Mandibular lateral teeth either small and inconspicuous or larger and distinguishable. Dorsal process pointed.

Key to species of Chaoborus flavicans species complex

Note. All species key out as C. flavicans in the key by Saether (1972).

adult males

1 Gonocoxite and gonostylus stout in structure, length:width ratio 2.64 (2.4–3) and 9.43 (8–10.7), respectively ( Fig. 14a ); apical claw of paramere relatively long ( Fig. 14 c–e).............................................. C. posio Salmela sp. n.

- Gonocoxite and gonostylus slender, length:width ratio ca. 3–3.5 (2.37–3.9) and 12.5–13.7 (10.2–16), respectively ( Figs. 6a , 11a ); apical claw of paramere relatively short ( Figs. 6 c–e, 11c–e)................................................ 2

2 Paramere medially bent and constricted; apical claw rather narrow, moderately curved ( Figs. 6 , 8a ).... C. flavicans (Meigen)

- Paramere medially almost straight or gently curved; apical claw stout, curved ( Figs. 8b , 11 )...................................................................................................... C. albipes (Johannsen stat. rev.)

pupae Separation of pond-dwelling C. flavicans and C. albipes pupae may be obscured by intraspecific variation

1 Respiratory organ club-shaped, lacking subapical constriction, relatively short (844 (770–930) µm, Fig. 9d................................................................................................ C . posio Salmela sp. n.

- Respiratory organ with subapical constriction, either slender or voluminous ( Figs. 9 a–c)............................. 2

2 Length of respiratory horn Ẑ 1000 µm (920–1360), either slender or voluminous ( Figs. 9 a–b)......... C. flavicans (Meigen)

- Length of respiratory organ mostly <1000 µm, may exceed 1000 µm (690–1050); slender ( Fig. 9c )................................................................................................ C. albipes (Johannsen stat. rev)

larvae Serration of the labral blade may vary in all species.

1 Mandibular lateral teeth inconspicuous; uppermost tooth shorter than subordinate tooth ( Figs. 10 a–b); number of mandibular fan bristles ±16....................................................................... C. flavicans (Meigen)

- Mandibular lateral teeth conspicuous; uppermost tooth about as long as subordinate tooth ( Figs. 10d,f ); number on mandibular fan bristles almost always>16........................................................................... 2

2 Number of mandibular fan bristles>20, usually>22 (21–29) ( Fig. 10h ); labral blade finely serrated, rather wide (length:width ratio 3,7 (3–5 Fig. 10g )............................................................... C. posio Salmela sp. n.

- Number of mandibular fan bristles up to 25, usually 15–21; labral blade almost devoid of serration, slender (length:width ratio 5.59 (4.38–6.85, Fig. 10e )..................................................... C. albipes (Johannsen, stat. rev.)

Figures

FIGURE 6. Chaoborus flavicans (Meigen), male (Finland, Kärkölä). a) hypopygium (epandrium removed, scale bar 430 µm), b–e) paramere viewed in different positions (scale bar 140 µm). ac=apical claw, gst=gonostylus, gx=gonocoxite, pa=paramere.

FIGURE 8. Male parameres of a) Chaoborus flavicans (Meigen) (Finland, Kärkölä), b) C. albipes (Johannsen) (Finland, Joroinen) and c) C. posio Salmela sp. n. (Finland, Utsjoki).

FIGURE 9. Pupal respiratory organs. Chaoborus flavicans (Meigen), a) slender type from a pond population (Finland, Urjala), b) voluminous type from a lake population (Canada, Ontario, Kenora), c) C. albipes (Finland, Savonlinna), d) C. posio Salmela sp. n. (Finland, Posio).

FIGURE 10. Larval mandibles (a,b,d,f;h showing whole mandible with mandibular fan bristles) and labral blades (c,e,g) of Chaoborus flavicans (Meigen) (a,c, lake population, Finland, Arrajärvi, b, pond population, Finland, Rovaniemi), C. albipes (Johannsen) (d,e, Finland, Savukoski) and C. posio Salmela sp. n. (f,g,h Finland, Posio). I–IV mandibular teeth (III=subordinate tooth), Lt=lateral teeth. Scale bars a: 200, d: 180 and f: 130 µm. Lengths of labral blades ca. 250 µm.

FIGURE 11. Chaoborus albipes (Johannsen), male (Finland, Joroinen). a) hypopygium (epandrium removed, scale bar 400 µm), b–e) paramere viewed in different positions (scale bar 130 µm). ac=apical claw, gst=gonostylus, gx=gonocoxite, pa=paramere.

FIGURE 14. Chaoborus posio Salmela sp. n., a) hypopygium (scale bar 500 µm), b–e) paramere viewed in different positions (scale bar 170 µm). ac=apical claw, gst=gonostylus, gx=gonocoxite, ep=epandrium, pa=paramere.

FIGURE 15. A maximum likelihood phylogram of the Chaoborus flavicans species complex based on the DNA barcoding region (COI) of mitochondrial DNA. The best fit substitution model of TVM+F+R4 was used. Support values exceeded 90 (transfer bootstrap support) except where asterisks are given. Clade sample sizes are provided. The light gray shading indicates the C. flavicans complex and the dark gray box indicates C. albipes (also a member of the C. flavicans complex). The tree was outgroup rooted using species of Monchlonyx. Triangles represent collapsed clades to save space.