Basic Bumble Bee Anatomy

Bumble bees share the same body pattern as other insects, with three distinct parts: head, thorax (middle section where the legs and wings attach), and abdomen. The abdomen is divided into several segments called tergites. In identification guides, these are often abbreviated to T1, meaning tergite 1, through T6, meaning tergite 6. When identifying a bumble bee, you need to look at the hair color on each body part. This can help differentiate between species as well as females from males.

Using the above diagram as an example, we see 1) that there are black hairs on the front of the face between the antennae, and yellow hairs on the top of the head, 2) the thorax is mostly yellow with some black hairs in the center, and 3) the first tergite (T1) on the abdomen is yellow, and tergites 2-6 (T2-T6) are black. Based on the color pattern of all three body parts, we would identify the species as the common eastern bumble bee (Bombus impatiens).

Female vs. Male Bumble Bees

Learning to differentiate between female and male bumble takes practice. Field observations can help, for example if you see a bumble bee carrying pollen you know it is a female, because males don't carry pollen. Seasonality is also a good factor to consider because males are typically not active on the landscape until mid-to-late summer. But how else can you differentiate females from males?

Perhaps the most reliable way to differentiate females from males is by counting the number of tergites (abdominal segments). Females (shown below in top photo) have six tergites, while males (bottom photo) have seven tergites. Their antennae differ in the same way, with males possessing an extra thirteenth antennal segment compared to the females’ twelve. Counting these is incredibly difficult, however when viewed side-by-side, you can often see a difference in the overall antenna length. Males also tend to have longer, shaggier hair and smaller, slimmer bodies compared to their female counterparts—This makes males look extra fluffy!

The final way in which male and female bumble bees differ is in the middle section of their hindlegs, you want to look for the presence or absence of a pollen basket, also called corbicula (singular) or corbiculae (plural). Female bumble bees usually have a corbicula on each hindleg that they use to carry and transport pollen back to their nest. When the corbiculae are empty, the leg segment appears flat, shiny, and angular. When the corbiculae are full, the pollen is clearly visible as yellow or orange lumps on the hindlegs.

Males, whose purpose is geared towards reproduction rather than resource acquisition, do not possess corbiculae. As a result, the midsection of their hindleg is rounded and hairy. So, if you observe a bumble bee with pollen on its legs, you’ll be able to tell right away that it’s a female! However, we did say that females usually have corbiculae on their hindlegs. They don’t always have one – that complication can be attributed to…

Cuckoo Bumble Bees!

Cuckoo bumble bees break one of the ID rules we just established regarding males vs females: the shape of the hindleg. Female cuckoo bumble bees do not possess corbiculae, so the midsections of their hindlegs are rounded and hairy like male bumble bees.

Much like males, cuckoo females lack corbiculae because they simply don’t need them. Cuckoo bumble bees are social parasites, meaning they require another colony to provide it with resources. A cuckoo female will infiltrate an existing colony of its host bumble bee species, oust the colony's foundress queen, and rely on the colony’s existing workers to feed and raise the cuckoo’s brood. This strategy eliminates the need for cuckoos to gather their own resources, which renders corbiculae unnecessary. Cuckoos also possess enlarged heads as well as stingers that curve around the underside of their abdomen. These features presumably help them overthrow the colony's foundress queen.

Luckily for us, the other distinctions between sexes still apply, such as the number of tergites, the length of the antennae, and the length of their hair. There are no cuckoo bumble bees, male or female, that can carry pollen. If you find a bumble bee with six tergites but no corbiculae, you’ve found a cuckoo!