Hypognathus condition in insects (left image from Wikimedia commons; right drawing modified after Snodgrass 1935)

In most insects, and certainly in all bees and most wasps, the mouthparts hang at the bottom of the head and, since each set of mouth pieces derives from a particular segment along the main axis of the body, they are positioned one after the other in a sequence from front to back: labrum¹; mandibles, maxillae, and labium. This head arrangement is known as the hypognathous condition, and can be considered the groundplan for insects.

In hypognathous insects the very first pair of appendages at the front of the head are the antennae, quite important since they are the primary tactile and smelling organs, so this makes sense (no pun intended).

Prognathus condition in insects (left image modified from ©Alex Wild; right drawing modified after Snodgrass 1935)

In ants, however, the very first thing at the front of the head are the mounthparts. Usually the large mandibles of these insects are at the forefront. But the term prognathy here (that is, “projecting jaws”) is really describing a functional condition that results from a significant structural rearrangement of the ant head: the entire head capsule is tilted almost 90 degrees forward, so what used to be the anterior region in a hypognathous insect is now the upper part of the head, and what used to be the bottom (the mouth) is now the front-most region.

In prognathous insects the antennae are no longer anterior in the head but are attached dorsally, and the different mouth pieces no longer run from front to back but are arranged from up to bottom (in a dorso-ventral axis). In fact, in relation to the rest of the elements in the head the mouthparts have not changed position. There is one important exception to this– the place where the head attaches to the neck and which has the hole by which the digestive tube, neural cord and the rest of the entrails go through has shifted from its ancestral place opposite to the antennal sockets to the upper back of the head, opposite to the mouth.

One way to envision this major structural rearrangement with more familiar examples is to compare the head of your cat or dog with your own head. Look at the head of your pet (even your goldfish will do). If you trace an imaginary line that passes right between the eyes (imaginary is the keyword here), that line will exit at the back of the head right through the hole that connects the head with the neck, the hole through which the neural cord coming from the brain passes (the foramen magnum in anatomical speech). Assuming your pet is standing straight in four legs, this imaginary line will continue parallel to the vertebral column all the way to the end and exit at the rear (the anatomical name of which I will spare for you). Now perform the same for your own head. In your case the imaginary line that went in-between your eyes will find a dead end at the back of your skull, right above the nape. In us, the “back” hole of the skull is located at the floor of the head. What happened is that as humans evolved up-rightness, the foramen magnum shifted position from back to bottom to balance our big heads and keep our faces looking to the front. This is also why if we lay chest down we look utterly ridiculous with our faces kissing the floor (or risk torticolis), as opposed to our cat or dog which will be graciously resting in front of the fireplace face-straight. Well, from wasps-like ancestors to ants the insect foramen magnum shifted exactly in the opposite way.

Now, in us jawed vertebrates (you and your pets) none of these conditions are called prognathous. This term has a very different meaning and refers to cases where either the upper jaw projects beyond the lower jaw or vice versa, something that can be appreciated in certain dog breeds like the Pug and in people that have subject themselves to large quantities of anabolic steroids, like certain current Governor of California.

Rhytidoponera metallica (April Nobile http://www.antweb.org/)

Back to insects, I have heard entomologists deny that prognathy is a diagnostic feature of ants, a synapomorphy. It is true that in some ants their mandibles seem to point downward, but this is just because having the neck attachment way in the upper back does provides the head articulation with a wider range of play. Again, the important point is not in which direction do the mouthparts project, but where is the location of the foramen magnum within the insect head.

Detach head of a Figitidae wasp (Via Mattias Forshage http://www.morphbank.net/)

Case in point. If you want to take a SEM of the base of the mouthparts in a regular wasps, the first thing you are forced to do is to detach the head from the body and place it face-down into the mounting stub. Your SEM will nicely show both the mouthparts and the foramen magnum.

Underside of head of a worker of Cerapachys nitidulus (Scanning Electron Micrograph, Roberto Keller/AMNH)

I have taken hundreds of SEMs of ant mouthparts and I never had to detach the head. The only thing you need to do is flip the ant, legs up, and you will have a unobstructed view of the base of the mouthparts. You won’t be able to see the foramen magnum in the same image though!

Notes and references

The meeting is organized by young researches, and this year will be specially interesting because there will be a discussion about creating a national society of evolutionary biologists.

I will be talking about the evolution of mouthparts within ants, covering some fascinating new discoveries that I haven’t share here yet but will blog about some time in the near future. In the mean time, here are a couple of my slides.

Media sources: antweb.org; Roberto Keller/AMNH.

Media sources: Wiki Commons; Alex Wild (http://www.alexanderwild.com/); R.E. Snodgrass 1935.

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.