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.