Bombus caliginosus (Frison, 1927), nomen protectum Bombus flavifrons Smith, 1866, in Lord, 1866: 343 (appendix) [♀]. Preoccupied, not Bombus flavifrons Cresson, 1863. Neotype: ♀ (designated here; Fig. 1). Canada, British Columbia, Vancouver Island, Victoria [Island], Butchart Gardens, 30 May 1973, by S. Gittelman [American Museum of Natural History, no. AMNH_BEE 00120464]. Bombus columbicus Dalla Torre, 1890: 139. Replacement name for Bombus flavifrons Smith, 1866. Nomen oblitum. Bremus caliginosus Frison, 1927: 376 [♂]. Syn. nov. Holotype: ♂. USA, California, Arcata, 16 September 1920, by C. D. Duncan [Illinois Natural History Survey, no. 179514]. Bremus caliginosus var. tardus Frison, 1927: 380 [♂]. Synonymy of Bremus caliginosus Frison, 1927 by Stephen (1957: 100). Holotype: ♂. USA, California, Carmel, 20 July 1921, by L. S. Slevin [California Academy of Sciences no. 2438]. Remark. The original description of Bombus flavifrons Smith was of a female with a body length of 8 ½ lines (ca 18 mm), consistent with the neotype selected here (Fig. 1). Diagnosis. Females of B. caliginosus have a longer malar space (length: width = 0.90, Fig. 2 a) than those of B. vosnesenskii (length: width = 0.71, Fig. 2 b) and in many specimens sternum 4 of the former has a weak apical (posterior) band of pale hairs, at least laterally (Thorp et al. 1983; Williams et al. 2014) (Fig. 1 d), though as cautioned by Williams et al. (2014) this is also true for some specimens of B. vosnesenskii. Williams et al. (2014) also noted that tergum 4 of B. caliginosus often has some dark hairs basally, especially medially, while in B. vosnesenskii the black hairs are lacking, or few. Stephen (1957) noted that the relative lengths of flagellomeres 1 and 3 also differed, flagellomere 1 longer than 3 (3: 2) in B. caliginosus, but the lengths are subequal in B. vosnesenskii. Williams et al. (2014) made note of the size differences of punctures on the apical margin of the clypeus, being small in B. caliginosus, but large in B. vosnesenskii. Males of the two species can be also distinguished by the relative lengths of flagellomeres 1 and 3, the lengths being subequal in B. caliginosus, but flagellomere 1 shorter than 3 in B. vosnesenskii (Stephen 1957; Thorp et al. 1983). The males also differ in the shape of the apical recurved portion of the penis-valve head, narrowing apically in B. vosnesenskii, but remaining subparallel in B. caliginosus (Stephen 1957; Williams et al. 2014). Thorp et al. (1983) also provided illustrations noting differences in the shapes of sterna 7 and 8, the latter gradually narrowing apically (i. e., triangular) in B. caliginosus (their fig. 63), while widely truncate apically in B. vosnesenskii, with the lateral margins subparallel to base of process (their fig. 72).

Published as part of Sheffield, Cory S., Heron, Jennifer M. & Williams, Paul H., 2025, On the nomenclature of Bombus flavifrons Smith, 1866 (Hymenoptera, Apidae, Bombini), pp. 861-870 in Journal of Hymenoptera Research 98 on pages 861-870, DOI: 10.3897/jhr.98.163217