queen ants

Queen ants are a beautiful example of how form evolves with function

Thanks to expanded muscles in the front of the thorax, this Anochetus worker can hoist a heavy load.

Roberto Keller,  Christian Peeters, and Patrícia Beldade have published an intriguing new paper about ant thorax morphology. Worker ants lack wings, of course, so their thoraces are smaller. But this new study notes there is more to the thorax than mere reduction. Ant workers might not fly, but they do lift things with their heads, so the first segment- the pronotum- is enlarged to strengthen the neck. It’s essentially the factoid “ants can lift 50 times their own weight!!”, explained.

But I’m not blogging the paper for the worker necks. Workers are not the best part.

Rather, Keller et al also looked at queen ants, and here is where the study gets clever. Consider:

Queen ants of most species fall into one of two body types.

Queens that hunt need strong, weight-bearing necks, so the thoracic segment anchoring neck muscles is enlarged as it is for workers. Queens that stay in the nest and don’t gather food don’t need neck strength, so the corresponding segment is reduced. Meanwhile, the segment associated with muscles that are metabolized to raise early workers is enlarged.

Thoracic tergite #1 (T1), in blue, is much larger in queen ants that leave the nest to forage, while the second tergite (T2), in orange, is enlarged in queens that do not leave the nest. (Modified from Figure 2 of Keller et al).

While this pattern of claustral/non-claustral was generally known, Keller et al perform a more rigorous accounting. First, they quantified the phenomenon by measuring many species. Remarkably, the data did not form a continuum of values; rather, queens sort cleanly into one type or the other. Thus, queen ants really do come in two forms. It’s not just our imagination.

Then- and here’s the fun part- the authors tested for an evolutionary association of form and function. When thorax shape and colony-founding behavior are mapped on an evolutionary tree inferred from genetic data, we see two traits changing in concert rather than randomly. Ancestrally, queens forage and have an enlarged T1. Several times in evolution queens have shifted to claustral, non-foraging founding. When they do, the morphology also shifts.

That ant shape and ant behavior are intertwined should not be surprising, but I mention it because this study forms an especially nice example.

The form/function association has a practical use for ant-keepers, too. If you’ve caught a young queen and want to know if she requires feeding to raise her first workers, you can look at her thorax. A big T1 needs feeding, a small T1 is claustral and does not.

Source: Keller RA, Peeters C, Beldade P. 2014. Evolution of thorax architecture in ant castes highlights trade-off between flight and ground behaviors. eLife 2014;3:e01539 http://dx.doi.org/10.7554/eLife.01539


Her Royal Highness

Crematogaster lineolata queen with a retinue of workers. (Vermillion River Observatory, Illinois)
Crematogaster lineolata queen with a retinue of workers. (Vermillion River Observatory, Illinois)

This weekend we took a trip with some entomology students to the Vermillion River Observatory.  The astronomical function of the observatory has long been abandoned, but the site remains as a lovely nature reserve and one of the closest patches of decent forest habitat to where we live in Champaign-Urbana.

The acrobat ant Crematogaster lineolata was one of many ants we encountered, and in this nest the queen was right up near the surface.  She lingered long enough for me to get a few shots before she disappeared into the labyrinth of tunnels.

photo details: Canon mp-e 65mm 1-5x macro lens on a Canon EOS 50D
ISO 100, f/13, 1/250 sec, flash diffused through tracing paper

How to Identify Queen Ants

Atta texana queen and worker

Ant queens are those individuals in a nest that lay the eggs.  They’re pretty important, of course, as without reproduction the colony dwindles and disappears.

Understandably, ant-keepers have an interest in making sure their pet colonies have queens.  Conversely, pest control folks trying to get rid of ant colonies need to be sure that they’ve eliminated queens.  Whether your interest is live ants or dead ants, I’ll give some pointers in this post for recognizing queens. (more…)