Abdomen of a Manica rubida worker. Roman numerals correspond to external abdominal segments. Propodeum shown in blue (Scanning Electron Micrograph, Roberto Keller/AMNH)

The propodeum is the subversive segment of the apocritan abdomen. As explained in a previous post, at some point during the evolution of Hymenoptera this first abdominal segment decided to part ways with its serial homologues and fuse with the thorax, forming a secondary tagma we call mesosoma. It is the Texas of the body’s segments so to speak.

An archetypal abdominal segment is composed of a dorsal plate (tergum) and a ventral one (sternum). The propodeum retains only a single dorsal piece shaped like a dome. It bears one large spiracle on each side (the largest in the ant’s body) and has a posterior cavity inside which the petiole (=II, second abdominal segment) articulates.

A common name given to the propodeum in the old myrmecological literature is “epinotum.” Through synonyms, this term should not be used since it derives from misidentification of this abdominal structure as homologous with a dorsal thoracic plate or notum.

Propodeum (in blue) armed with spines in an Acanthoponera minor worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

The constriction or “wasp-waist” that accompanies this abdominal modification has clear structural advantages over the primitive tubular form. Such narrow articulation allows for greater movement of the metasoma and thus better manipulation of the ovipositor and ultimately of the aculean sting. It is indeed tempting to hypothesize that the narrow waist acted as a precursor for the transformation of the ovipositor into a stinging weapon, since the latter innovation is  nested cladistically within the former (that is, it evolved before). However, possessing a narrow waist does have it drawbacks: the gut, dorsal vessel, ventral nerve cord, and the rest of the internal organs all have to pass through a bottle neck at the meso-metasoma junction. It is the Achilles’ heel of these insects. It is not surprising then, that in many unrelated ant clades the propodeum is armed with a pair of strong spines pointing backwards, protecting this narrow articulation.

Meso and metapleura (in yellow) and propodeum (in blue) on a Lasius (Acanthomyops) occidentalis male (Scanning Electron Micrograph, Roberto Keller/AMNH)

But why does the narrow constriction occurs between the first and second abdominal segments and not, for example, right between the thorax and abdomen? A possible explanation can be found when considering the role that the propodeum plays in the typical winged thorax. The propodeum is strongly fused with the lateral plates (the pleura) of the meso and metathorax, the wing bearing segments (shown in yellow above), these pieces together forming a rigid mesosomal box that provides strong structural support to the wings ventrally and  the dorsoventral flight muscles internally. The propodeum also forms a cavity that is occupied by the large longitudinal flight muscles and the posterior phragma where they attach. The extra space provided by the propodeal cavity permits an arrangement that wouldn’t be possible if the body were constricted immediately behind the last thoracic segment.

Barry Goldman asks in the comments which muscles are responsible for the movement of the metasoma. They are the same muscles that connect the first and second abdominal segments in the unconstricted abdomen of the non-apocritan Hymenoptera (e.g., sawfly). In winged ants the dorsal muscles that pull the petiole upward (the levator muscles) are sandwiched between the posterior phragma I mentioned above and the internal wall of the propodeum where these muscles attach. In the wingless workers the internal phragmata associated with the flight apparatus are atrophied and these levator muscles are larger and more powerful.

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.