Bumble Bee Body Parts
Male showing hairy rear tibia
Cheek Length
A more subtle field mark that can be very diagnostic is cheek length. Until one gets a good feel for this, practicing on known species to understand what you are looking for helps a lot. This is best done with a hand lens and a cold individual of a known species with an understood cheek length. The trick is “it is all relative”.
As I am interpreting (please correct me) cheek length is related to tongue length. And tongue length is related to how deep a flower can nectar be easily extracted: longer tongue–> deeper, more reliable nectar source (fewer “shorter tongued” competitors). Climate change may be driving tongue length back to being shorter, to the expense of the deeper tubed wildflowers–based on some preliminary studies from Mt Pennsylvania, CO.
The image above comes from Bumble Bees of the Western States. See Resources page for link information.