bee biology

The names of bee segments: how Ⅱ became 1

Once upon a time in a dreary laboratory lit by a single dusty lamp, a small circle of entomologists leaned over a dead body. The roughly shaven men, each wearing a monocle and smelling of ale, spoke in low tones. Fumes of formaldehyde scented the air, fogging the minds of the men.

“To cause maximum confusion,” one whispered, “and to keep the secrets of the bee among ourselves, we should use Roman numerals!” Several nodded in agreement. But one, pulling at his chin, demurred.

“It would be even more confounding if we divided the segments into their component parts, numbered each series separately, and began with the most visible.”

“Yes!” Some agreed. “How cunningly obscure!”

Trouble is, the vote was tied. And today, much to the chagrin of budding bee lovers everywhere, we are stuck with two systems of naming segments, neither of which is completely satisfactory.

A segment of my imagination

Okay, you caught me. Everything up to this point is pure bunk except for the two naming systems. That part, unfortunately, is true. The problem originates from the insects themselves, and the way they develop.

All insects are made of segments. The segments, which are often visible in the late larval stages, develop in predictable ways into the three main body parts: head, thorax, and abdomen. But bees (and other Hymenoptera) are a bit weird. In most insects, the thorax is made from three segments, but the bee thorax has four. In truth, the fourth segment isn’t really a thoracic segment. Instead it is an abdominal segment. How can that be?

The plates make the structure

The definitions are based on structure. Thoracic segments are made of four plates: two sides, a top, and a bottom. But abdominal segments have only two plates: one on the top that curves down over the sides, and one on the bottom.

In bees, the first three segments of the thorax have all four parts, but the last segment has only two which means it’s actually an abdominal segment. This is where the naming problem begins. Since the fourth segment of the thorax is actually built like an abdominal segment, what do we call it?

Counting by abdominal segments

The fourth segment of the thorax is called the “first abdominal segment.” It is indicated by Roman numeral Ⅰ so the thorax is made of four segments Ⅰ, Ⅱ, Ⅲ, and Ⅰ. (Only an entomologist would think of that!) Moving over to the abdomen, the first segment you see is called the “second abdominal segment” and is given the Roman Numeral II. So the abdomen that you see consists of Roman numerals Ⅱ through Ⅶ.

The abdomen has even more segments, but these are not clearly visible because they’ve developed into specialized parts such as the stinger and the anus.

Petioles are not just for leaves

Leaf petiole

The petiole attaches a leaf to a stem.

We all know a petiole when we see one, right? The petiole is a skinny stalk that attaches a leaf to a stem. Similarly, the skinny stalk that attaches the thorax to the abdomen of a bee is also called a petiole. It is the “wasp waist” of ants, bees, and of course wasps. It is the scary-looking part that warns me of a nasty sting, even though it’s not the part that does the stinging.

In bees, the petiole is actually part of the first abdominal segment (the fourth thoracic segment) also known as Roman numeral Ⅰ. The anterior end of this segment is firmly attached to the thorax, almost like it’s glued in place. When we look at a diagram of a bee, this “glued on” portion is called the propodeum.

The first three segments of the thorax along with the fused first abdominal segment form what is called the “mesosoma.” The mesosoma is the middle part of the bee, what we think of when we say “thorax.” Using this system, we call the head the “prosoma” and the abdomen becomes the “metasoma.”

Counting by what we see

If we count segments by what we see instead of how they developed, we need a different system of numbering. The terms mesosoma and metasoma are useful for this purpose because they describe what we actually see on either side of the petiole.

When we look at the metasoma (or abdomen), we see a series of plates on the top (dorsal side) and another series of plates on the bottom (ventral side) of the bee. The top plates are called tergal segments and bottom plates are called sternal segments. The first segment we see is made of two plates, T1 and S1 for tergal segment 1 and sternal segment 1. So simple! And the numbering continues from front to back using the same pattern.

Converting between systems

If you compare the two numbering systems, you will see that the second abdominal segment, or Ⅱ, is made of plates T1 and S1. This can be confusing, but there is usually no need to convert between the two systems. Just like measuring in metric or “English” (U.S. Customary System), you don’t normally need to convert between them, just pick one and run with it.

The more modern references use the second system, including Michener’s Bees of the World, second edition (2007). That is, they use the words mesosoma for the thorax and metasoma for the abdomen and call the first (most anterior) abdominal plates T1 and S1. This system is especially helpful when collecting data from citizen scientists and others who are not trained in insect morphology. If you are corresponding by email and ask for the coloration of abdominal segments 1-6, you don’t want to explain that the first one is actually the second. Say what?

The way of the future

Although I think the second system is the way of the future, bee biologists or those interested in bee identification need to understand both. This subject came up recently when someone on Facebook asked me what to call the wispy waist of a bee. To say “petiole” does not explain that it’s actually a part of the abdomen that is firmly attached to the thorax. And on and on.

I can’t actually imagine a room full of conniving entomologists trying to keep the secrets of beedom to themselves, but they probably weren’t too worried about confusing the public either. And certainly the original dudes weren’t concerned about email or citizen scientists. So while the bee has not changed, we humans live in a world where Ⅱ has become 1, so we need to adapt.

Honey Bee Suite

The petiole connects the thorax with the abdomen. It is incredibly narrow considering how important it is. © Rusty Burlew.


  • Hi.
    Interesting article. I never thought about learning the bee anatomy until now.

    Just to clarify, by English system do you need Imperial one? I do live in the UK and never heard about English system so wanted to clarify if I understand it correctly.

    • Peter,

      The official name is U.S. Customary System, but everyone I know calls it “English.” You know, like English muffins (which I hear you don’t have either).

  • I enjoyed this tremendously. And the reason I could enjoy it so much is because I don’t need to pass any tests.

    Also “we see a serious of plates”. Was it truly that serious?

  • English muffins were invented by an American whose name was English. That’s why we don’t have them!

    PS We’re waiting for someone to invent American muffins………!

    • Alun,

      According to Wikipedia, they were invented by an Englishman (Samuel Bath Thomas) who emigrated to the United States and started a bakery. Wikipedia says, “They are called ‘English muffins’ to distinguish them from American muffins, which are larger and sweeter miniature baked cakes, called in Britain and Australia ‘fairy cakes.'”

      Fairy cakes?

      Whatever, I have English muffins almost every morning for breakfast, along with honeycomb, of course.

  • And then there is this: “The scutellum is the posterior portion of either the mesonotum or the metanotum of an insect thorax; however, it is used almost exclusively in the former context, as the metanotum is rather reduced in most insect groups. In the Hemiptera, and some Coleoptera, the scutellum is a small triangular plate behind the pronotum and between the forewing bases.[1] In Diptera and Hymenoptera the scutellum is nearly always distinct, but much smaller than (and immediately posterior to) the mesoscutum.” (Wikipedia)

    Makes remembering numbers not so bad.

  • Hi Rusty,

    This was fascinating and awesomely interesting. I will have to read it a couple of times to learn the terms properly. Can you do a labelled drawing like you did with the pollen baskets or give a link to an existing one please?

    • Lindy,

      Thanks. My husband has the same request, so I will see if I can label a real bee or make a diagram. I wrote it on my “to do” list.

  • Rusty,

    In the UK an American muffin is NOT the same as a fairy cake. Wikipedia has this wrong. A fairy cake is a small sponge cake and is invariably iced.

    An American muffin, usually now just called a muffin, is what we all know and love.

    And yes we do have the English muffin, which is a yeast leavened thing and cooked on a griddle not baked in the oven. In the UK, supermarkets now usually call it a “toasting muffin” .

    And hey, back on topic, thank you for the very clear explanation of bee segments. I always learn something new about bees on your blog. But not always about muffins!

  • So, this fourth thoracic segment is basically a bit that’s attached to the thorax, a bit that’s attached to the abdomen, and the petiole in between? (And it’s constructed of an upper and lower plate, rather than the usual four thoracic plates) Am I understanding this correctly?

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