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.