Hairs distinguish castes and sexes in larvae of a fungus-growing ant

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Rachelle Adams next to one of her favorite trees in Panama. She has been climbing over the roots of this tree for over 20 years (© Jamie Mah).

In the recent paper “Hairs distinguish castes and sexes: Identifying the early ontogenetic building blocks of a fungus-farming superorganism (Hymenoptera: Formicidae)” by Adams et al.* published in Myrmecological News, the authors address a rarely studied topic: the morphology and development of larvae. They surveyed larval morphology for the fungus-growing ant Acromyrmex echinatior using various combinatory quantitative traits and provide a key for the 3rd to 5th instar larvae. Here, first author Rachelle M.M. Adams answers some questions and shares some pictures.

* Rachelle M.M. Adams, Rasmus S. Larsen, Nicoletta Stylianidi, Dave Cheung, Bitao Qiu, Stephanie K. Murray, Guojie Zhang, Jacobus J. Boomsma

An interview compiled by Alice Laciny, Patrick Krapf, and Felix Oberhauser

MNB: Could you tell us a bit about yourself?
RMMA: I am an Assistant Professor at The Ohio State University. My lab group studies symbiotic interactions involving fungus-growing ants (https://megalomyrmex.osu.edu/). Our work integrates the fields of evolution, behavior, chemical ecology, systematics, microbiology, and taxonomy.

MNB: Could you briefly outline the research you published in Myrmecol. News in layman’s terms?
RMMA: We document the four larval developmental stages (instars) of large and small workers and the five instars of gyne and male larvae of Acromyrmex echinatior ants. From this we were able to write a key for the 3rd to 5th instar larvae where sex and developmental stage can be unambiguously identified. This key is also available as a downloadable application: https://megalomyrmex.osu.edu/apps/acro-larva-key/

Collecting colonies near Gamboa, Panama (Sabrina Amador, Stephanie Murray, Rachelle Adams, Julian Roberts) (© David Girbino).

MNB: What is the take-home message of your work?
RMMA: We found that hair morphology used with other measurements can reliably differentiate young larvae. Our methods can be used to identify the sex and instar of larvae without killing them.

MNB: What was your motivation for this study?
RMMA: The goal of our study was to identify morphological traits of ant larvae that distinguish caste, sex, and age using a typical z-staking system and stereo microscope. With the ability to identify these early larval traits, future work can address gene regulation and phenotypic plasticity of individuals as well as examine social conflict among colony members.

Caption picture above: Early discovery of larval differences in body shape and hair morphology using a stereo microscope ( © Rachelle Adams). Top row scanning electron microscopy (SEM) images validate our early findings (© David Cheung).

MNB: What was the biggest obstacle you had to overcome in this project?
RMMA: Rearing the larvae and keeping track of individuals. Because the larvae are kept within the spongy fungus garden, they can be hard to see. Also, unlike other social insects that have brood cells, ants move their young around the nest while they develop.

MNB: Do you have any tips for others who are interested in doing related research?
RMMA: Be patient and persistent. Working with live ants always has challenges but it also makes the work fun.  

MNB: Where do you see the future for this particular field of ant research?
RMMA: I hope to see more projects like this one. I was not expecting to see distinct hair morphology among castes and sexes! Many questions remain. Why have different hair morphologies evolved? Can the ants detect differences between castes and sexes because of the hairs? How pervasive is this among the many ant species?

Example of how different the larvae look after they are killed in ethanol (© Rachelle Adams).

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