The anterior tentorial pits (arrows) in a Tetraponera aethiops worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

The head of an ant in frontal view has a couple of holes usually located in the area between the mouth and the place where the antennae are inserted. These holes look intriguing from the outside– Are they part of a sensing organ? Do they secrete a special chemical signal or defense substance through them? Are they use for breeding? The answer is more mundane than that. As I mentioned in an earlier post, most of what one sees in the outer surface of the arthropod’s exoskeleton does not have an external function, but is rather a symptom of the inside working in these wonderful machines. These particular holes mark the places where the cuticle invaginates to form the internal skeleton of the insect cranium known as the tentorium. The external holes produced by these invaginations are thus termed the tentorial pits.

The tentorium is the H-like structure of the internal skeleton of the head, marked in red as it looks in the inside. Tetraponera attenuata worker, left antenna removed (Scanning Electron Micrograph, Roberto Keller/AMNH)

In ants, the tentorium consists of two elongated arms or apophyses that start on the front of the head, right at the anterior pits, and extend towards the back to where the head attaches to the neck. The arms fuse with each other half-way through before parting again, forming an H-like pattern. In the image above, I painted in red how does the tentorium normally looks like internally. The tentorium is the place of attachment for some of the muscles that move the mouthparts and dilate the first section of the digestive tube. It also plays an important role as a support antagonist to the powerful muscles that close the mandibles in ants: these huge muscles originate back at the inside of the nape and connect forward to the base of the mandibles via strong tendons. Without the tentorium the head would probably collapse under the bite’s pressure¹.

Acropyga ant workers are minute individuals displaying a very reduced external morphology. Arrows point to the anterior tentorial pits (Scanning Electron Micrograph, Roberto Keller/AMNH)

The couple of anterior tentorial pits are always located right at the posterior margin the clypeus and, maybe due to a functional constrain, are very conserved in terms of their absolute position in the head. Knowing this is handy when you are doing comparative morphology, because these pits are always present regardless of how reduced other features of the head can become, so they are wonderful landmarks when it comes to understanding what went on with head morphology during the evolution of the group. In the minute Acropyga pictured above, for example, the clypeus is completely fused to the rest of the head. However we can not only know that the clypeus is still there, but also that it remains quite large due to where the tentorial pits are located.

The antennal sockets in Leptanilloides biconstricta lay very close to the anterior margin of the head. Arrows point to the anterior tentorial pits (Scanning Electron Micrograph, Roberto Keller/AMNH)

Also in the minute Leptanilliodes, the position of the tentorial pits tells us that the antennal insertions are very close to the front of the head not only due to extreme reduction of the clypeus but also because the antennal sockets have further migrated forward, passing the imaginary line that can be drawn between the pits (dotted line).

The antennae of Discothyrea ants sit on a shelf-like projection of the front of the head. Note the forward position of the antennal socket in relation to the large tentorial pit (arrow; left antenna removed. Scanning Electron Micrograph, Roberto Keller/AMNH)

The example I like best, however, is the location of the tentorail pits in Discothyrea and Probolomyrmex. In these genera the antennae are inserted in a shelf-like projection of the anterior part of the head that is otherwise completely fused and devoid of any line or suture. Looking at the position of the large tentorial pits one can appreciate just how much this peculiar modification protrudes forward, as the full antennal apparatus sits well beyond the tentorial pits.

Notes

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

Pars stridens (in yellow) on the fourth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Many insects produce chirping sounds by rubbing body parts against each other in a behavior know as stridulation. The structures involved have modifications specialized for this purpose thus forming a stridulatory organ.

In the case of ants, the stridulatory organ is composed of a scraper or plectrum that rubs against an area of the tegument where the sculpture is modified into a series of microscopic parallel ribs, the pars stridens.

Detail of the pars stridens (in yellow) on the forth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

The organ occurs dorsally at the point of articulation between the third and fourth segments of the abdomen: the plectrum lies underneath the posterior region of the third abdominal plate, while the pars stridens is located on the anterior part of the fourth plate. The only known exceptions to this location occur in Nothomyrmecia macrops, where the stridulatory organ is at the same articulation but ventral, and in the genus Rhytidoponera, that has both a dorsal and a ventral organ on those same segments.

Abdomen of a Pachycondyla villosa worker showing the location of the stridulatory organ (arrow; Scanning Electron Micrograph, Roberto Keller/AMNH)

Abdomen of a Tetraponera aethiops worker showing the location of the stridulatory organ (arrow; Scanning Electron Micrograph, Roberto Keller/AMNH)

Looking at the the images above you can notice that the stridulatory organ can be present irrespectively of the amount of abdominal constriction occurring between those segments. The sound is produced by pulling down the section of the abdomen from the fourth segment back. So far as we know, however, ants don’t have any type of specialized hearing organ, but they are sensitive to vibrations transmitted through the ground.

Considering that this organ is absent in many different ant clades leaves no doubt that it has been lost repeatedly during the family’s evolution. However, this same pattern of presence/absence also suggest that the organ has been gained repeatedly during the diversification of the group, its similarity in structure and position among all the species where it occurs notwithstanding.

The primary function of stridulation in ants is communication, with some studies showing that workers trapped by burying soil can incite nestmates to dig them out by stridulating. A secondary function was discovered in leaf-cutter ants, where stridulation by workers will cause their bodies to vibrate in a way that makes cutting leafs with the mandibles easier- sort of like an electric carving knife.

Finally, in a paper published in this week’s Science magazine, Barbero and coworkers describe how the larva and pupae of a social parasitic butterfly mimics the stridulation sounds produced by queens in order gain entrance and acceptance with the host colony. Thus suggesting that acoustic communication in ants may be more important than we previously thought.

Further reading

Markl, H. 1965. Stridulation in Leaf-Cutting Ants. Science 149 (3690), 1392. [DOI: 10.1126/science.149.3690.1392]

Tautz, J., F. Roces, B. Hölldobler. 1995. Use of a Sound-Based Vibratome by Leaf-Cutting Ants. Science 267 (5194), 84.[DOI:10.1126/science.267.5194.84]

Barbero, F., J.A. Thomas, S. Bonelli, E. Balletto, and K. Schönrogge. 2009. Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite. Science 323 (5915) 782. [DOI: 10.1126/science.1163583]