What does it mean to be an eyeless ant?

In the comments, Rob Clack asks:

I’ve just read about Martialis on Panda’s Thumb and have a question. If I interpret it correctly, your cladogram shows Martialis to be the sister group of all living ants. Since it was blind and many living genera are not, that presumably implies that vision evolved independently within modern ants. I would therefore expect there to be some significant differences between modern ant eyes and those of other hymenoptera.

I assume I’m missing something.

Rob is referring to this post, going straight to the problem that Martialis seemingly poses for our understanding of ant evolution.  Was the ancestor of all ants blind?

The honest answer is that we don’t know.  There are, however, a couple observations relevant to Rob’s comment.

First, most “eyeless” ant species are blind only in the worker caste.  In these ants the reproductive males and females commonly have eyes used for orientation while mating and dispersing.  As way of an example, check out Antweb’s gallery of Centromyrmex bequearti.

Although the workers are blind, they retain the requisite genetic machinery for functional eyes. They must, because their parents have eyes, as do their brothers and some of their sisters.  The workers have just turned off the eye program during development.

In theory then, it is possible that a blind species could re-evolve normal eyed workers just by up-regulating the existing eye developmental program.  A much easier task than creating eyes de novo.

Second, we have excellent examples of ants that have re-evolved eyes from a blind ancestor.  And as Rob suggests, the re-created eyes are indeed odd.  Consider the army ant Eciton burchelli:

eciton19a

The eyes of worker Eciton are made of a single enlarged lens like those of, say, spiders.  Most other sighted ants have the “normal” insect condition of multi-faceted eyes with many lenses:

saltator13s

We know from studies of the evolution of army ants that Eciton, although it forages in the open, descended from ants that were largely subterranean and almost certainly blind. The oddness of Eciton’s eyes could be a consequence of a circuitous route evolution took when proto- Eciton emerged above ground and needed to see again.

These two observations, the sightedness of reproductive ants and the weirdness of re-evolved eyes, are somewhat at odds with respect to their implications for Martialis.  After all, Eciton males have normal multi-faceted eyes.  So why didn’t Eciton workers recruit the male eye program when they needed eyes?   Is there something about the developmental process that precludes it?  And if so, does this mean that an eyeless Martialis really does pose difficulties for our understanding of ant evolution?

Of course, whatever we know or don’t know about ant evolution, it’s worth noting that Martialis is not the Ur-Ant.  Martialis is quite likely to have lost its eyes somewhere in the intervening 120 or so million years. If so, then the question is moot anyway.  All the same, we have a lot yet to learn about the eyesight of ants.

7 thoughts on “What does it mean to be an eyeless ant?”

  1. And if so, does this mean that an eyeless Martialis really does pose difficulties for our understanding of ant evolution?

    Not in the least, because Martialis is the sister group to other ants, not the ancestor. As you say, both Martialis and other ants would have evolved in different ways from their common ancestors, and eyelessness is probably a derived feature of Martialis.

    I think it’s also worth noting that phylogenetic trees may give misleading indications of ancestral traits if certain features have appeared multiple times in closely related lineages. “Loss” characters are particularly prone to such effects. So, for instance, if a flighted bird is nested among a number of flightless relatives, it’s probably more likely that the relatives became flightless independently than that the flighted species re-evolved flight from flightless ancestors. Ants developed subterranean habits (closely associated with eyelessness) on multiple occasions.

    I didn’t know about the army ant eyes – that’s really interesting, especially if the ‘eye genes’ are still present in the species. Do ants possess median ocelli like other insects? Do the army ant eyes necessarily correspond to the primary eyes of other ants, or could they be enlarged and displaced median ocelli?

  2. Some worker ants do possess median ocelli; they are not uncommon among the formic acid sprayer formicine ants (e.g., wood ants), and they are found here and there among the rest of the ant family.

    Army ants workers don’t have median ocelli. However, that Eciton eye correspond to the primary compound eye of other insect seems to be well established by close inspection of its morphology. Internally, the retina if formed by the aggregation of the photosensory cells from many atrophied ommatidia, and the eye is connected to the part of the ant brain where normal developed compound eyes are in other insects (and queen and male ants in general). Externally, although the lens looks like a shiny single piece (as in the image above), high magnification reveals a multitude of vestigial ommatidial lenses:
    http://roberto.kellerperez.com/misc/Eci_hamatum-eye.jpg

    That is, not only are Eciton’s eyes primary compound-type of eyes, they are a fusion of the multiple units of an insect eye rather than a overgrown single ommatidium!

  3. Wow, thanks Roberto!

    I had always assumed from outward appearance that the Eciton eye was an enlarged ommatidium. Fascinating- is that observation published anywhere?

    Christopher-

    Of course, you are correct. What I neglected to mention in this post is that the presumed next branch up the ant tree- the leptanillinae- are also eyeless in the worker caste, suggesting the parsimonious conclusion that the ant ancestor was blind too.

  4. Really interesting post and even more interesting discussion. I don’t have any material contribution, other than to let you know I find these evolutionary discussions among specialists very entertaining.

    Now if you want to extend the discussion to beetles…
    😉

    regards — ted

  5. Alex-

    I have to confess, I did thought each eye on Eciton was an enlarged, glorified ommatidium until I happen to take that scanning electron micrograph on the link above. An excellent study of its anatomy was published in:

    Werringloer, A. 1932. Die Sehorgane und Sehzentren der Dorylinen nebst Untersuchungen über die Facettenaugen der Formiciden. Z. Wiss. Zool. 141,432-524.

    I don’t remember if he mentions the fact that vestigial facets can be seen on the corneal surface though (I don’t have my copy of the paper with me, and I am yet to scan it for antbase.org).

  6. Mike from Ottawa

    This is a fascinating discussion. Thanks guys.

    BTW, the Eciton with what looks like a single eye but made up of many separate ones reminds me of the holochroal eye of most types of trilobites where there are many separate eyes in a compound eye but they are covered by a single cornea, which, when well-preserved looks like simply a single large eye.

  7. Pingback: Homology Weekly: Compound Eyes | Archetype

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