Homology Weekly: Mouthparts

Tuesday, April 21st, 2009 | Ants, Homology Weekly, Morphology

Frontal part of the head in an Anochetus emarginatus worker, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

This image shows the mouthparts of a trap-jaw ant in resting position. The only structures really visible are the prominent elongated mandibles (in yellow) that project forward. The rest of the pieces, laying immediately below, are retracted inside the preoral cavity.

Fully extended mouthparts in an Anochetus emarginatus worker, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

And this is how the mouthparts look fully extended, when the ant is sticking its tongue out. There are four different sets of structures here: the labrum (in green); the mandibles (in yellow); the maxillae (in orange); and the labium (in red). Each set corresponds originally, both phylogenetically and ontogenetically, to a pair of structures, although only the last three are modified limbs properly (each correspond to a pair of head appendages).

These are very complex structures, each part deserving its own separate discussion. But I wanted to thrown these images here now to serve as reference for future posts, and mention a few important generalities.

Ants display the unmodified general architecture of a biting insect. The mouthparts of adult ants are typical for what is found when comparing different insect groups, and one can readily homologize each part with a corresponding structure in a grasshopper or a beetle for example. Even the most derived mouthpart morphologies found within ants, like that of the trap-jaw ant pictured here, preserve this general pattern.

However, ants do have some uniquely derived features. They are truly prognathous insects, something uncommon within Hymenoptera (but not exclusive). While in bees and in most parasitic and stinging wasps the mouthparts hang down below the head pointing to the ground, in ants they are directed forward, always pointing to the front.

Ant prognathy, however, is not only a function of the fact that the whole head is tilted forward. Examine the image above and you will notice that the labrum, maxillae and labium are fully extended while the mandibles remain closed. That is, unlike other Hymenoptera, prognathous or not, in ants the mandibles do not fold right on top of the rest of the mouthparts at rest. Instead the main body of the mandible “steps-up” immediately after the mandible’s articulation (the rounded yellow piece at the far right), thus laying out of the way from the remaining structures.

The much derived Anochetus pictured here provides an extreme example illustrating this, but the exceptional modification is universally shared within the family. It is another unique ant synapomorphy. Obvious as it may seem once explained, I have to confess it took me a while (a few years actually) to figure out what was happening structurally in ants that was different from the non-formicid outgroups. But since then, after the explanation clicked, I cannot look at an ant without seeing it.

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Tags: Anochetus emarginatus, Labium, Labrum, Mandibles, Maxillae, Mouthparts, Prognathous

9 Comments to Homology Weekly: Mouthparts

Alex
April 22, 2009

Apparently, Roberto, you have a fan base.

Roberto Keller
April 22, 2009

I’m not sure about the one of the best blogs around comment, but I’m glad some people find these posts interesting. Wait a second, you are posting there too…

Devin Bowen
April 24, 2009

Awesome blog! These are GREAT photos illustrating the mouth parts. Previously I’ve only seen the drawings in The Ants.

Question: how to you distend the mouth parts when mounting specimens? I’ll take a reference if you don’t want to explain it.

Roberto Keller
April 25, 2009

Devin – I start from specimens stored in ethanol (96%). Fishing around under the stereoscope, most of the time you will find one that got fixed with the mouthparts distended. If this was the case, the mouthparts will not collapse during air-drying. Still it is a good idea to leave the ant in fresh 96% ethanol for 10 to 15 minutes before placing it on its back over a piece of filter paper (coffee filter paper works fine) and away from the strong heat of the stereoscope light, so evaporation occurs slowly.

If I can’t find a specimen with its mouthparts naturally extended, if all in the sample decided to die without doing me a favor (bastards), I just directly dissect the mouthparts out (the labiomaxillary complex goes out in one piece in hymenopterans at least), air-dry them, and place them in SEM stubs.

In any event, I never had to submit the mouthparts through the tedious step of critical-point drying (CPD) before the scanning electron microscopy.

If you need to distend the mouthparts in situ, clearing whole specimens in hot 10% KOH for a few minutes will sometimes do the trick. However, the skeleton may become too soft and won’t retain its shape when dried unless you use CPD.

Check out this recent paper for some other clearing techniques that may do the trick (I haven’t try them myself):
Stüben, M. & Linsenmair, K.E. 2008. Advances in insect preparation: bleaching, clearing and relaxing ants (Hymenoptera: Formicidae). Myrmecol. News 12: 15-21; published Online Earlier 6 October 2008 pdf here.

Hope this helps.

The internet has things in it « Myrmecos Blog
April 28, 2009

[...] Roberto Keller explains ant mouths. [...]

Devin Bowen
April 30, 2009

ahhh, thanks for this info. I have about 7 sitting in 200 proof. I’ll check them out. I’m an undergrad interested in specializing in ants.

My school’s SEM isn’t working so I’m not going to be needing to CPD them. I’m just using the dissector to get pics for a project.

thanks again!

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