Homology Weekly: Dentiform Clypeal Setae
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)
7 Comments to Homology Weekly: Dentiform Clypeal Setae
You read my mind! I was hoping you’d do these.
Ha. I’m torn between doing posts about general insect structures and how they occur in ants, or posts about features specific and unique to the group.
I guess I can take suggestions from readers.
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January 21, 2009
January 21, 2009
[…] Roberto Keller has wonderfully detailed SEMs of the clypeal pegs of amblyoponine ants. […]
JochenB May 6, 2009
So you are not convinced that they are involved in prey-catching behavior as they suspect here:
http://www.ncbi.nlm.nih.gov/pubmed/12769958?dopt=Abstract
?
JochenB– Insofar as these structures enhance the holding of prey to be stung yes. Otherwise I’m not quite convinced they are directly involved. The paper by Gronenberg et al. you link is one of the reasons why.
They provide good histological evidence that these teeth-like structures are mechanoreceptive sensillae, thus strengthening the homology case that they are indeed modified setae. And, as they also point out, such type of “oral” hairs are commonly present throughout the family rather than being unique and novel in Amblyoponinae. No doubt that in all ants such sensillae tell the insect that something is right in front of them, and thus will incite a response (e.g., close the mandibles).
In groups where these sensillae are a crucial component of the prey-catching mechanism, trap-jaw ants for example, the structures are rather elongated and thin, projecting forward away from the head as deadly triggers (as you probably know and illustrated on these pictures of Anochethus). Amblyoponinae shows quite the opposite- the dentiform setae are minute in most cases and very close to the cuticular surface. An object needs to be in the ant’s face for it to excite these pegs. On the other hand, all Amblyoponinae have long and thin setae along the mandibles, clypeus and labrum that project way beyond the teeth-like pegs (with the exception of Amblyopone pluto and Apomyrma stygia which have these setae hypertrophied in the clypeus and labrum respectively). As Gronenberg et al. point out in the case of Mystrium, the dentiform setae doesn’t come in contact with the prey since the mandibles are already closed for the snap mechanism to function.
So we are left with the situation in Amblyoponinae, where having “oral” mechanoreceptive sensillae is a plesiomorphic feature, but having such structures modified into blunt, short teeth evolved later. My guess is that they do secondarily function rather like teeth, providing some traction for the mandibles that do not have wide masticatory margins that meet when closed.
[…] 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 […]
Dave May 12, 2009
Excellent SEMs! Similar peg-like setae are present on various parts of the body (leg segments, dorsal shield) that come into contact with their phoretic carrier in Antennoseius mites (Mesostigmata) that hitch rides on carabid beetles. You can see a low mag (and not so elegant as yours) example in Fig. 2 of Beaulieu et al. 2008 (Zootaxa 1961: 37-57).
One neat thing about the Antennoseius mites is that they have two adult forms: a ‘normal’ free-living mite that lacks the peg setae and a phoretomorph with the cone setae that searches out the beetles for a ride. Thus, the homology of the setae is readily determined (in these mites, every seta has a designation).
We assume that the cone setae help the mites hold on (in the case of Fig. 2 by pressing against the underside of an elytron). More mysterious are similar setae with a long distal process (Fig. 6) usually found on the venter of a basal limb segment. If you get a chance to look at this figure, I’d be interested if you see similar things in ants.
January 19, 2009