Bombus (Thoracobombus) nigrodorsalis Franklin, 1907

Williams, Paul H., Sagot, Philippe, Martínez-López, Oscar, Barajas, Ricardo Ayala-, Mérida-Rivas, Jorge A. & Vandame, Rémy, 2024, A new wave of Mesoamerican bumblebees? Revising the weisi-complex to reject numts and pseudospecies (Apidae: Bombus), Zootaxa 5514 (4), pp. 301-318 : 311

Bombus (Thoracobombus) nigrodorsalis Franklin, 1907 , sensu stricto stat. rev.

Bombus nigrodorsalis Franklin, 1907:90 . Holotype queen Mexico USNM (images at USNMENT01044337), examined.

< Bombus nigrodorsalis > subsp. laticollis Franklin, 1907:91. Holotype queen Mexico USNM (images at USNMENT00534364), examined.

[ B.<ombus> weisi (part A: nigrodorsalis ) [ Mexico] Brasero et al., 2019:190.]

Diagnosis. Female. (cf. B. weisi s. l.) Metasomal T 2 in the middle between the pits (with hair bases) with the surface strongly sculptured, irregular and dull. Short hair between the antennal base and the ocelli predominantly yellow; hair of the thoracic dorsum usually anteriorly with a transverse yellow band; metasomal T3 entirely yellow; hind basitarsus posterior long fringe with at least the tips orange.

Male. (cf. B. weisi s. l.) Genitalia with the gonostylus shorter than long, rectangular, with the inwardly projecting inner basal process strongly sclerotised for a length less than half of the breadth of the volsella ( Fig. 4 ); volsella distally (posteriorly) beyond the gonostylus strongly narrowed or constricted proximally to the inner distal process; penis-valve head distally (posteriorly) slightly pointed and slightly incurved towards the midline. Hair of metasomal T3 laterally yellow without a distinct lateral patch of black.

Description. Female. Queen body length 15‒17 mm, worker 13 mm. Head with the oculo-malar area length / breadth ratio 1.6. Thorax with the mid basitarsus distal posterior corner acutely and spinosely pointed. Hair long, uneven, and black; on the head above the antennal socket the shorter hair yellow; pronotum and anterior scutum either often yellow, forming a transverse band connected with the yellow hypoepimeral area, or this band reduced with black in some queens and most workers; scutellum posteriorly with a yellow fringe, which may be much reduced with black; lateral propodeum yellow; hind tibia with the ends of the corbicular fringes and on the hind basitarsus orange-brown; all leg bases and hind femur with yellow ventrally; metasomal T1‒3 entirely yellow; T4 black, with a narrow medial posterior fringe of rufous orange; T5 black anteriorly and in the posterior half orange; T6 orange.

Male. Body length 11‒13 mm. Hair long, uneven, and black; mandibular ventral fringe with long curved hair black; face with many mixed short hair yellow; pronotum and anterior scutum either yellow, forming a transverse band connected with the yellow hypoepimeral area, or often this band reduced with black; mesepisternum, hypoepimeral area and metepisternum with yellow; lateral propodeum yellow; hind tibia with dorsal fringe longer than the breadth of the tibia; hind basitarsus with keirotrichia entirely orange; metasomal T1‒3 yellow; T4‒5 posterior fringe often mixed with yellow; T 6 in the posterior half with orange; T7 entirely orange; sterna with long pale hairs on posterior fringes but S6 short and orange. Genitalia shown in Fig. 4 .

Material examined. ECOSUR: 596 specimens. Data and barcodes for specimens from Figs 2‒3 are available from https://www.ecosur.mx/beesofmesoamerica/ and barcodes are available from a Supplementary File .

Distribution. Bombus nigrodorsalis is endemic to Mexico ( Fig. 6 ), distributed from Chihuahua in the north (29.189° N) to Oaxaca in the south (16.149° N). It is absent south of the Tehuantepec Isthmus and from beyond Mexico. The altitudinal range is 2000–3634 m asl, in areas with pine forest. This species has been collected from July to January, with only two queens collected in April and June. Males were collected from late September to mid-January. Bombus nigrodorsalis is broadly sympatric with B. weisi in some regions of Mexico, but within those regions B. nigrodorsalis is present at higher elevations and has a geographical range that extends further to the north.

Figures

FIGURE 2. Support for apparent species’ gene coalescents from unique haplotypes for COI barcode-like sequences including numts (nuclear copies of mitochondrial sequences), obtained from an evolutionary tree estimated with MRBAYES and using maximum likelihood to fit Poisson-Tree-Process (PTP) models. Numbers at each node show the probability that all daughter sequences to the right are parts of a single unique species. Branches change from blue to red at the node with the best fit (maximum local probability) for the change from inter-species to intraspecies branching PTP models (species’ coalescent nodes). Sequence labels include: sequence length in number of bases; taxon name; country in capitals (if not shown then MEXICO) and province; sequence identification code (E# = ECOSUR#). Sequence labels are followed by the percentage of the bases A and T at the third codon position, where low scores (≤97.6%) are likely to indicate numts. Labels for sequences interpreted as likely lower-divergence numts are shown in orange and for likely higher-divergence numts are shown in red (see Fig. 1). The scale bar represents branch length on the tree in substitutions per nucleotide.

FIGURE 3. Support for species’ gene coalescents from unique haplotypes for COI barcode (orthologous) sequences excluding likely numts (orange and red bars and labels in Figs 1‒2), obtained from an evolutionary tree estimated with MRBAYES and using maximum likelihood to fit Poisson-Tree-Process (PTP) models. Numbers at nodes, treebranch colours, sequence labels, and scale bar as in Fig. 2.

FIGURE 6. Map showing the distributions of samples examined for this study of the weisi-complex: (white spots) taxon nigrodorsalis s. str. (=B. nigrodorsalis stat. rev.); (grey spots) taxon montezumae (=B. weisi stat. rev.); and (black spots) taxon weisi s. str. (=B. weisi stat. rev.). Larger symbols with spots in their centres show locations from which specimens were sequenced for each taxon. Spherical projection with international boundaries as recognized by the UN shown as grey lines. Map projected in ArcGIS using the World_Shaded_Relief basemap © 2014 ESRI.

FIGURES 4‒5. Male genitalia of (4) B. nigrodorsalis and (5) B. weisi from the right lateral aspect, with posterior extremities at the top of the images and dorsal parts to the right of the images.