The five segmented gaster (in yellow, arabic numeral) in a Cerapachys nitidulus worker. Roman numerals refer to abdominal segments (Scanning Electron Micrograph, Roberto Keller/AMNH)

Gaster is a morphological term that is very useful and yet imprecise for the purpose of comparative anatomy as it is currently used in ants. It comes from the Greek for “belly” and it refers to the collection of segments in the metasoma that remain after the pedicel of ants and wasps. It is the bulbous part of the body that hosts the insect viscera.

The problem when applying this term comes from the fact that ants can have a pedicel or waist composed of one or two metasomal segments (either abdominal segment II or II-III), and so, depending on the group, the ant gaster is a belly made up of either four or five external segments.

Four segmented gaster (in yellow, arabic numerals) in a Manica rubida worker. Roman numerals refers to abdominal segments (Scanning Electron Micrograph, Roberto Keller/AMNH)

It may seem harmless to say that formicine ants, for example, have a five segmented gaster while myrmicine ants (see image above) have a four segmented one. The imprecision arises, however, whenever there is a reference to an individual gastral segment. The first segment of the gaster in a carpenter ant is not homologous with the first segment of the gaster in a fire ant. The former correspond to the third abdominal segment (III) while the latter refers to the forth (IV).  This peculiarity is not only the result of defining the tagma by what segments it excludes (that is, metasoma minus the segments of the pedicel), but by the fact that the excluded segments are anterior to it, and hence there is a shift in anatomical reading frame.

Note that the shift is purely nominal, there is no insertion/deletion of body segments along the insect axis between the different group of ants.

Luckily, authors are consistent when referring to gastral segment on a given publication, even if they don’t define gaster beforehand, so there is no internal ambiguity. The headache problem comes when one is compiling information from different publications, by different authors across all ant groups. Without context, reading that something is or occurs in gastral segment number X is ambiguous until you scan the rest of the publication for an explanation. Moreover, it may seem obvious that in groups in which the pedicel is clearly two segmented (e.g., Myrmicinae, Pseudomyrmicinae) the second gastral segment will correspond to the Vth abdominal for example. But the author may have decided to stick to a conservative terminology and consider that the gaster starts after the petiole (abdominal II) regardless of how modified the third abdominal segment is. And I won’t even go into groups that have genera with one and two segmented pedicels (the New World army ants, Ecitoninae) or groups in which it is difficult to decided if the pedicel has one or two segments (Cerapachinae, see the image of Cerapachys above).

This brings us to the topic of cybertaxonomy. We humans can scan a publication and resolve an ambiguity in anatomical terminology. Machines can’t (or at least not easily). This is the sort of challenges faced by initiatives like Plazi that aim to translate legacy data into digital format.

Moral of the story? Gaster is fine when referring to the whole section of the ant’s body. For individual segments, in comparative anatomy, it is always better to stick with the abdominal segmentation number.

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.