Profile view of a Ponera pennsylvanica worker showing primary tagmosis. Red: head; yellow: thorax; blue: abdomen (Scanning Electron Micrograph, Roberto Keller/AMNH)

When a group of successive segments along the arthropod body form a distinct section (by fusion, for example), the division is called a tagma (pl. tagmata). In the case of hexapods (= insects plus their primitively wingless cousins) the body segments are arranged into the three familiar tagmata: head, thorax and abdomen.

The head (shown in red) contains the various segments bearing the antennae and mouthparts, being so tightly fused to form a cranium that it is difficult to determine the exact number of total segments involved. The thorax (shown in yellow) is the locomotor tagma of the body. It is composed of three segments bearing a pair of legs each and, in flying insects, the second and third thoracic segments support the wings. The abdomen (shown in blue) contains the remainder segments (up to twelve), and lacks appendages.

So far so good. But if you look at the image of the ant above you will notice a big patch of blue on the posterior upper corner of the middle section of the body, in what may otherwise appear to be the leg-bearing thorax. You may think, he screwed up the coloring, what an idiot (or something along those lines). The truth is that the area in blue does correspond to the first part of the abdomen. What happens is that at some point during the evolution of Hymenoptera (sawflies, wasps, bees and ants), natural selection decided that having a narrow waist was not only sexy for an insect, but it conferred great mobility to the abdomen in relation to the front part of the body. However, for reasons that will be dealt with in a another post, the “wasp waist” occurs between the first and second abdominal segments, with the first segment stuck to the back of the thorax. The clade formed by the descendants that inhered this novel morphology is known as the suborder Apocrita.

The result of this morphological innovation are two new tagmata composed of parts of the primary ones. Proper anatomical comparison among Apocritans, thus, requires us to have new names to refer to this secondary tagmosis. Notice that the two secondary tagamata overlap with the primary ones, as one is composed of the thorax plus the first segment of the abdomen while the other correspond to all the abdominal segments minus the first (see image below). As far as terminology goes, it would have been more convenient if the secondary tagmosis occurred hierarchically nested (for example, dividing one primary tagma into subsections), since we could have just used the primary tagmata terms  fitted with a convenient prefix.

Profile view of a Ponera pennsylvanica worker showing secondary tagmosis. Roman numerals refer to the external abdominal segments (Scanning Electron Micrograph, Roberto Keller/AMNH)

The father of modern myrmecology, William L. Brown Jr., introduced the term alitrunk to refer to the secondary tagma formed by the thorax plus the first abdominal segment. It literally means the body section to which the wings are attached. However, commonly working with the wingless worker ant cast, W. L. Brown Jr. also used the simpler term trunk in his publications. The term alitrunk is widely applied to ants, but it never really catch on within the rest of the hymenopterology community. Instead, the term mesosoma is used by most students of Apocrita, and everything posterior to the narrow waist is the metasoma. Mesosoma and alitrunk are thus synonyms.

I think mesosoma and metasoma are nice descriptive terms, and I favor this terminology over alitrunk, as it reflects the homology between the tagmosis found in ants with the rest of the wasps and bees belonging to the suborder.

Abdomen of a Leptogenys sp worker from Nepal, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

In most ants the hypopygium is a simple trapezoidal plate, yet commonly adorned by a set of long hairs pointing backwards. Hairs like these serve as tactile receptors: as arthropods, ants use them to sense the world around them in their otherwise numb medieval armored bodies. Predators, like the Leptogenys pictured here, relied on their sting (Stng) to capture prey and need to know where the rear end of their abdomen is moving.

Something interesting to note is that the hypopygium folds into itself around the protruding sting in exactly the same way as it does when forming an acidopore.

Rear end of a Leptogenys sp worker from Nepal (Scanning Electron Micrograph, Roberto Keller/AMNH)

There is a caveat with this anatomical concept, however. The hypopygium is not homologous among all insects, not even among all adult ants.  Primitively, the abdomen in insects consist of eleven segments, but the number of external, visible segments varies. Female ants have seven, male ants have nine. Since hypopygium denotes whatever the last visible abdominal segment is, it does not corresponds to the same body part between the sexes.

If you are asking yourself what happen to the other four abdominal segments in female ants relative to the eleven found in the insect groundplan, you are asking the right evolutionary question. I am not going to tell you just now, but here is a hint: it’s pointy and it hurts.