Homology Weekly: Compound Eyes
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
- To be honest, I have never know if this visual organ is called compound eye because it is composed of several ommatidia or because each ommatidium is composed of several elements. This has never disturb my sleep though. ↩
- This post was inspired in part by this post by Alexander the great-blogger and the comments therein ↩
- Werringloer, A. 1932. Die Sehorgane und Sehzentren der Dorylinen nebst Untersuchungen über die Facettenaugen der Formiciden. Z. Wiss. Zool. 141,432-524. ↩
- Brown, W. L. 1974. A remarkable new island isolate in the ant genus Proceratium (Hymenoptera: Formicidae). Psyche 81: 70-83. pdf (1Mb) via antbase.org. ↩
9 Comments to Homology Weekly: Compound Eyes
Me gustó mucho!! y las primeras hormigas tenían pocos omatidia o ya “venían” con ojos mas complejos?
Gracias Ek.
Todo parece indicar que las hormigas obreras ancestrales tenían ojos compuestos grandes y bien desarrollados, similares a los de la casta reina.
Un buen ejemplo es Sphecomyrma freyi. Fósiles en ámbar de esta especie extinta del Cretáceo muestran individuos, que claramente son obreras, con ojos bien desarrollados.
Why no blogging? « Myrmecos Blog
November 4, 2009
November 4, 2009
[…] In the meantime, check out the new ant articles at Myrmecological News. And don’t miss Roberto Keller’s discussion of ant eyes. […]
Warren November 5, 2009
The Myrmecos bump is working – I came from Alex’s site. Interesting piece!
Thank you Warren. I’m always more than happy to cover for Myrmecos’s slack busy schedule.
Rodrigo Feitosa November 7, 2009
Hi Roberto,
Congratulations for this wonderful piece of ant morphology!
Do you know the interesting case of the Neotropical leaf litter ant species Cryptomyrmex longinodus? These ants were described by Fernández & Brandão (2003) from workers collected in Amazonian Brazil. In some workers, the ommatidia lenses are virtually absent, although pale pigments are clearly visible under the reflective light, but not under SEM images. Could it represent a step towards eye loss in the evolution of this group?
All the best!
– Rodrigo. Thank you. Originally I was going to mention something along those lines (with an image): in ant species with apparently blind workers there is often something going on in the cuticle where the eyes should be. As F. Fernández (2004) describes for Cryptomyrmex, there is a “bulging area” in the cuticle.
Functionally, once you are down to a few vestigial ommatidia it makes no difference if the cuticle immediately above the working photoreceptors is shaped into lenses or not– at this point the eyes are only sensing light intensity.
Now, the evolution of blindness in workers is a tricky subject in ants because, as I am sure you know, with a few exceptions the queen caste always has well developed eyes (as is the case in Cryptomyrmex). This is relevant given that the difference in caste morphology within females is the results of differential expression during development due to environmental cues. I think we still don’t have a good understanding of the plasticity of this kind of features of the caste polymorphic system.
Great post, Roberto. Your first note got a chuckle out of me. Also, I can attest to the wonderful vision of Myrmecia ants – combined with their aggressive behaviour, they are the bane of any photographer who gets too close. All the more challenging, I guess 😛
Very interesting point about the developmental evolution of blindness in workers as well!
I’m dying to visit Australia and experience Myrmecia ants live (and the rest of the Australian fauna). I have heard/read many accounts about how aggressive these ants can be. For example, from this guy.
Fortunately, Gigantiops in the field look back at you with a “What are you looking at?” look, rather than the “You want a piece of me, punk!” look of Myrmecia ants.
October 29, 2009