Note the white hairs on the stergites of this Obscure bumble bee, Bombus caliginosis. The Yellow-faced bumble would have black stergites.

Stergites and Tergites

Stergite is the term for the underside of the abdomen.

Like the dorsal tergites, stergites are numbered from thorax to end of abdomen:  S1, S2, S3, S4, S5 and S6.  The primary stergites used for identification are T4 and T5.

The color of the T4 andT5 tergites are key for separating two very similar species:  Yellow-faced bumble bee, Bombus vosnesenskii, and the Obscure (Fogbelt) bumble bee,  Bombus caliginosis.

bumble bee body parts for field ID identification
Note the white hairs on stergites S4, S5 and S6 on this Obscure bumble bee, Bombus caliginosis

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