Tetraponera aethiops worker showing the location of the clypeus in green (Scanning Electron Micrograph, Roberto Keller/AMNH)

When looking at an arthropod from our vertebrate perspective it is easy to forget that we are looking right at the animal’s skeleton. While our own vertebrate skeleton consists of a series of internal compact pieces with sponge-like cores that support an external layer of muscles and entrails (all nicely wrapped in skin), the reverse is true for arthropods. The arthropod skeleton consists of a series of external plates and hollow tubes that form enclosed spaces within which the internal musculature system attaches¹. One consequence of this peculiar body architecture is that most of what we see on the outer surface of this exoskeleton is but a reflection of what is going on on the inside– minute external pits correspond to places where the cuticle folds in to form internal pillars, and innocent looking shallow furrows on the surface are large internal walls where powerful muscles originate. A simple examination of the exoskeleton, therefore, can tell us a lot about particular functions and consequently about an insect’s behavior.

The clypeus (in green) on the turtle ant Procryptocerus, with a characteristic brush on its anterior border (Scanning Electron Micrograph, Roberto Keller/AMNH).

A good example of this is provided by the clypeus in ants and its wide diversity of forms across the different species in the family. The clypeus corresponds to an unpaired skeletal plate lying right at the center of an insect face. It is normally located lower in the head just in front to where the antennae are inserted, but in many ant groups it quite commonly extends in between the antennal sockets. The anterior border of the clypeus is involved in two important articulations relating to the movement of the mouthparts. The central part forms a wide hinge with the mouth’s “lid” or labrum, allowing the latter to move forward and backwards to open and close the preoral cavity where the intricate ant tongue is stored when retracted (the actual opening of the mouth lies internally at the back-end of this preoral cavity). The sides of the clypeal border, on the other hand, form deep cavities where the anterior condyle of each mandible articulate.

Those articulations occur externally. But what is going on the inside of the clypeus? The inner surface on the clypeus provides attachment to a set of muscles that originate right at the anterodorsal section of a special elastic chamber located just before the mouth known as cibarium. When these muscles contract the cibarium expands producing a suction action. It is basically the sucking pump of the insect, and the bigger the clypeus the bigger the pump muscles and the larger the sucking force. You may have probably noticed the big goofy snout in cicadas; well it is nothing but the hypertrophied clypeus attesting to the large sucking pump of these dedicated suckers. Ants never reach such extremes, but the clypeus can be quite large in some groups.

Clypeus (in green) on a Onychomyrmex doddi worker. Species in this genus display a convergent army ant like behavior (Scanning Electron Micrograph, Roberto Keller/AMNH)

In clades of chiefly predatory ants, like amblyoponines, the clypeus is never large and has become rather reduced in the more specialized genera like Apomyrma and Onychomyrmex. The same pattern occurs more or less among ponerines.

The large clypeus (in green) on a Formica fusca worker (left antenna removed. Scanning Electron Micrograph, Roberto Keller/AMNH).

Oh, but formicines and dolichoderines, those ants are such big suckers. Those are the ants you will most commonly see wandering between flowers looking for nectar and tending aphids for honeydew (that is, they suck ass big time²). Myrmecines ants have large clypeus in general, and it is not surprising to see a correlation between tending other insects and having a well developed clypeus in genera like Crematogaster.

The clypeus (in green, maybe) in a Neotropical army ant Labidus coecus worker (the clypeus is there, I swear. Left antenna removed. Scanning Electron Micrograph, Roberto Keller/AMNH)

Now army ants, the ultimate specialized predators of the insect world, they are not suckers at all. All clades can be easily characterized by having almost no clypeus, so that the antennal sockets seem to fall off their heads forward.

The swollen clypeus (in green) on an Acanthoponera minor worker (left antenna removed. Scanning Electron Micrograph, Roberto Keller/AMNH)

One genus that intriges me is Acanthoponera. Species in this genus have a very large and swollen clypeus for what you will expect given the group’s phylogenetic position in between other major clades of ants. I don’t think much is known about the biology of this genus other than it is a nocturnal ant. But I bet you this ant is sucking around something.

Notes

Red Hot Chilli Peppers? No, dentiform setae in the labrum of an Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Just as the anterior margin of an ant’s cranium can sometimes be armed with rows of dentiform clypeal setae (that is, especially modified hairs), the lid that closes the insect’s mouth called labrum can bear identical structures. The image above shows two of these specialized teeth-like pieces (in red) flanking an empty broad socket where a third piece used to be inserted.

The labrum and part of the clypeus of an Onychomyrmex doddi worker. A row of dentiform setae adorn the labrum (in red) and the clypeus (in yellow; Scanning Electron Micrograph, Roberto Keller/AMNH)

Although these dentiform setae vary in size and shape quite considerably from species to species, when they are present in different parts of the body within an individual they show the same morphology, suggesting that they are the result of a similar developmental program that switches on at the different positions.

Head of the African subterranean ant Apomyrma stygia showing the hypertrophied dentiform setae in the labrum (in red; Scanning Electron Micrograph, Roberto Keller/AMNH)

Moreover, there is a interesting similarity between dentiform setae that have developed in similar but apparently independent (non-homologous) conditions. The hugely grown dentiform setae restricted to the labrum in Apomyrma stygia are identical to the similarly hypertrophied ones found in Amblyopone pluto but that occur exclusively in the clypeus (see last image on this post). Though not sisters, these two taxa belong to the same Amblyoponinae clade.

Lastly, a couple of comments regarding a recent paper describing the ant fossil Gerontoformica, which has similar detiform setae in both the clypeus and labrum. Nel and coworkers¹ mention that dentiform setae in extant ants are found either in the clypeus or in the labrum but never in combination. This is not the case as can be seen in the example of Onychomyrmex pictured above. There is also the suggestion that Probolomyrmex, a non-amblyoponine genus, has dentiform setae on the labrum. However, close inspection revels that this is also not the case. Rather, most setae covering the body in this genus, including the few stout ones on the labrum surface, are short and scale-like and can easily be confused with pegs at low magnification.

So far, in extant taxa these peculiar dentiform setae arming the clypeus and/or labrum are only known to occur within the subfamily Amblyoponinae.

References

Anterior part of the head of an Australian Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Among the many interesting features found in members of the subfamily Amblyoponinae is the presence of unique teeth-like structures at the anterior margin of the ant’s cranium. They are arranged in one or two parallel rows, right above the opening of the oral cavity, in a plate called clypeus.

Dentiform clypeal setae on a Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Close examination reveals that the teeth-like structures are overgrown modified setae, that is, insect hairs or bristles with a socket at their bases rather than spiny projections of the cuticle (as is the case of the teeth in the ant’s mandible). The longitudinal ridges or striations are a common feature of most setae, formed during development as they elongate. Similar setae are sometimes found also in the labrum, the lid that closes the oral cavity from above.

Nobody really knows the function of these stout dentiform setae, but given their location one hypothesis is that they help secure prey or nestmates when transported between the mandibles: the mandibles in this group are elongated and slender, without a clear masticatory margin. The objects being carried are “hugged” against the frontal margin of the clypeus.

Though presence of these specialized setae is a synapomorphy of ambyoponines, they have been secondarily lost in some derived lineages within the group. I surveyed these structures as part of my morphological work on ant phylogeny, and found them to be quite diverse. The setal apex can be acute, blunt, truncated or slender and oblique, and each seta can arise from a tubercle-like process or right from the flat cuticle as in the Onychomyrmex pictured above. They vary in a way that seems to hold good phylogenetic potential, but I am not using that information right now because my taxon sampling is too broad within this group.

Lastly, it is interesting to note that some lower cretaceous ant fossils not belonging to the ambyoponines display similar dentiform setae in the clypeus. Whether they are homologous to the ones found in this subfamily remains to be tested.

Below, a few more images of these fascinating setae for you enjoyment.

Dentiform clypeal setae on a Concoctio concenta worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Dentiform clypeal setae on a Amblyopone armigera worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Hypertrophied dentiform clypeal setae on a Amblyopone pluto worker (Scanning Electron Micrograph, Roberto Keller/AMNH)