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