Homology Weekly: Tentorial Pits
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
- If you have ever been biten by a major worker of an Atta leaf-cutter ant or Eciton army ant you surely know what I mean ↩
6 Comments to Homology Weekly: Tentorial Pits
Hey Roberto,
Thanks for these posts. I’ve really enjoyed following the “homology weekly” series.
By the way, given that you mention “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”, what would be the function of different sculpturing patterns in the cuticle? I’ve often wondered what could be their adaptive significance.
Marcio– Actually, external cuticular sculpture is one of those few features that I think don’t have much of an impact inside. It is quite possible, of course, that certain patterns give the cuticle more rigidity without an increase in thickness. The few studies on morphogenesis of sculpture I know of show that some of the different patterns one observe are the result of the way the secretory cells in the epidermis grow (e.g., Fusco, Brena & Minelli 2000. Zoologischer Anzeiger 239:91-102).
Now, it is indeed puzzling how plastic sculpture patterns are and how high its rate of evolution is, to the point that in hyperdiverse ant genera (e.g., Pheidole) sculpture is basically the only morphological feature that will help you tell species apart.
P.S. Oh, and I’m glad you like the series.
Carol Hart June 23, 2009
I also love your series–it is so much more informative than any textbook illustrations that I’ve, at least, come across. And I hope you will do post more mouth part examples for us sometime!
Marcio June 24, 2009
Hi Roberto,
How about the spread of gland products throughout the cuticle? I always wondered if the grooves are more efficient in spreading liquid through its effects on surface tension. I don’t know… it’s one of those things that seem too obvious to lack an adaptive advantage…
Marcio– Curious thing is, the cuticle around the exit orifice of some of the most prominent glands in ant (i.e., the metapleural gland and the metatibial gland) is always smooth, even when the rest of the body is covered by strong sculpture. 🙂
In any event, I’m not such a big fan of adaptative explanations. It is not that I don’t think they are important, I just have a hard time thinking in those terms.
Carol– Thank you for the kind words.
Craig Pemberton August 5, 2010
I know spider’s aren’t your specialty but I was wondering if you could enlighten us about the four pronounced pits on the dorsal surface of some spiders’ abdomens.
June 22, 2009