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
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