Gigantiops destructor (via Michael Branstetter - www.antweb.org)

The lateral eyes of adult insects (and most Arthropods) known as compound eyes, are like no other visual organs found in animals. You can think of our vertebrate eye as a simplified, one-lens photographic camera with a sensor composed of millions of light sensitive cells (and a blind spot, mind you). Well, that’s nothing. Each insects eye is composed of several small photographic cameras, each with its own lens and light sensitive cells (albeit, commonly only six of these). These units are called ommatidia (sing. ommatidium), and the image if formed by the combined information from all of them.¹

An interesting property of this peculiar anatomical arrangement is that compound eyes exhibit modularity– each ommatidium acts as an independent, yet fully functional building block that can be repeated multiple times to form a whole eye in different configurations. In layman’s terms, the eyes of insects are built out of sets of identical Lego pieces.

Compound eyes of a queen (a) and a worker (b) of the citronella ant Lasius (=Acanthomyops) occidentalis. The images are shown at the same scale (Scanning Electron Micrograph, Roberto Keller/AMNH)

Ommatidia do vary in size from species to species, but the diversity in size and shape of the compound eye as a whole comes primarily from the number and position of these elements. This can be easily appreciated by comparing the different castes in ants, since queens of a given species have large, well-developed eyes while in workers the eyes are smaller due to the fewer number of elements, even though the ommatidia in both castes are equal in size (see image above).

Blind as an ant. The eye-less worker of the Old World army ant Aenictus binghami (Scanning Electron Micrograph, Roberto Keller/AMNH)

Compared to their flying counterparts, both within the family as well as among bees and wasps, worker ants have in general poor vision. It is not uncommon for this caste to have no compound eyes at all, a characteristic that has evolved multiple times independently across the ant family tree.

As always in biology, they are notable exceptions. Genera like Myrmecia, Harpegnathos and Gigantiops (Greek for “mighty eyes”; see image opening this post) have huge eyes and excellent vision. I don’t have field experience with ants in the first two genera, but I once encountered Gigantiops ants in the Venezuelan Amazon. Let me tell you, if you are used to staring at live ants from a few centimeters away unnoticed, approaching a large ant that suddenly stops what she is doing to turn and stare at you in return is quite frightening.

So, how few ommatidia does the eyes of worker ants can have? Let’s see:

A worker of the small ponerine Cryptopone gilva (Scanning Electron Micrograph, Roberto Keller/AMNH)

Above is a worker of the leaf-litter inhabitant Cryptopone gilva with four ommatidia.

A worker of the tiny formicine Acropyga sp (Scanning Electron Micrograph, Roberto Keller/AMNH)

Workers of the tiny formicine ants in the genus Acropyga can have three ommatidia.

A worker of the elusive Concoctio concenta, from Gabon (Scanning Electron Micrograph, Roberto Keller/AMNH)

Workers of Concoctio concenta from west central Africa have compound eyes with just two ommatidia (this image is from the holotype, by the way). But, how about eyes with just one ommatidium?

Eciton burchelli (via Myrmecos Blog. © Alex Wild)

At first glance workers in the army ant genus Eciton seem to fit the bill: each eye has just a huge single lens. But external close inspection already reveals that this is not one enlarged ommatidium. Rather, the single dome-shaped lens is formed by the fusion of several ommatidia²:

The domed compound eye of an Eciton worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

A detailed histological study of these modified eyes done by Werringloer in 1932³ revealed that it is not only the external facets that are fused. Internally the photoreceptor cells from the vestigial ommatidia are also united into a single light sensor, pretty much like the retina in the eyes of vertebrates and cephalopods.

In 1974 Bill Brown described some worker ants in a species he named Proceratium avium that also have huge single-faceted eyes⁴. I have yet to look at these ants in detail, but given what we know from the eyes of Eciton my guess is that the eyes in P. avium are also a fused set of several ommatidia.

Whether these vestigial eyes in workers are the result of re-evolution of eyes from blind ancestors will have to be the subject of a future post.

Notes and references

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