Christian Peeters

Directeur de Recherche au CNRS

(French National Scientific Research Agency)

I study and compare societies across the ant phylogeny, especially the evolutionary divergence of castes (queens, workers, soldiers) and strategies of colony foundation.

Institute of Ecology and Environmental Sciences (iEES), Sorbonne University, Paris

PDFs of all my publications can be downloaded from RESEARCHGATE

Biomechanics of load transport in insects: why are ants stronger?

I collaborate with Evan Economo (Okinawa Institute of Science and Technology, Japan) and Roberto Keller (Lisbon Museum) to study the evolutionary redesign of the flightless thorax of ant workers. Using X-ray microtomography and 3D segmentation, we compare adaptations of the muscles and skeleton in queens and workers across several lineages.

Wing muscles occupy a considerable volume in the thorax in all flying insects, and their evolutionary loss enabled modifications of neck, leg and petiole muscles, resulting in greater mandible strength and better transmission of force to all legs.

These results were presented at IUSSI in Guarujà Brazil (August 2018). (An article is in preparation)

©Deb Yarrow

Engineering a novel lifestyle: Melissotarsus and Rhopalomastix ants chew tunnels in healthy wood to accommodate their scale insect partners

Ants cannot eat leaves but they can feed on plants via sap-sucking insects. Thousands of arboreal species interact with scale insects, obtaining honeydew in return for protection against predators and weather. Family Diaspididae is the largest and most advanced among scale insects, yet they engage in mutualisms with only two genera of ants, partly because they do not excrete honeydew. Melissotarsus and Rhopalomastix are sister genera with minute workers exhibiting a combination of unique morphological adaptations, including powerful mandibles and silk glands. This lineage engineered a novel ecological niche : they live permanently inside living trees, where adults and larvae feed on the diaspidids (meat and secretions). In Melissotarsus, mid- and hindlegs are modified as anchors for tunnelling, hence workers cannot walk on flat surfaces. Such extreme specialization of ant workers is possible because the queen caste can disperse by flight and walk on the outside of host trees during colony foundation.

This project is a collaboration with Johan Billen (histology), Evan Economo (micro-tomography), Roberto Keller (morphology) and Danièle Matile (identification of diaspidids, Muséum National d'Histoire Naturelle Paris)

Winglessness, body size and queen-worker divergence in ants

Peeters & Ito 2015 Myrmecological News

Ant colonies are organized similarly to those of wasps and bees: reproductive altruism, age polyethism, and complex communication. Yet ants exhibit more species, have a much higher total biomass, and their lifestyles and diet are more diverse. Hence, factors additional to sociality must be involved in this evolutionary diversification.

We argue that loss of flight permitted extensive changes in body size of ant workers and queens. Wingless helpers revolutionized colonial economy because they are cheaper to manufacture. Flightlessness also removed constraints on the evolution of minute workers (head width 1 mm or less); these exist in 229 / 286 ant genera examined but not in social wasps and bees. Miniaturisation involves simplification of tissues and organs (compound eyes, sting apparatus, ovaries, exoskeleton), and minute workers are cheaper per capita. Comparison of ovariole numbers in 106 genera indicates reduction of ovaries in minute workers, and complete loss in six genera of Ponerinae and eight genera of Myrmicinae. Body size influences trophic ecology, but also the pattern in which a colony's finite energy budget is "packaged", allowing increases in colony size if adaptive. Minute workers together with big queens enabled the evolution of claustral independent colony foundation that is predominant in three large subfamilies (Dolichoderinae, Formicinae and Myrmicinae). Winglessness allows this divergence of costs between workers and queens, but also novel activity schedules and adaptations for defence. Highly dimorphic queens and workers promoted the evolution of mosaic phenotypes (soldiers and ergatoid queens), which added to colonial complexity (Molet et al. 2012 American Naturalist). We speculate that cheaper workers caused a shift away from a carnivorous diet to carbohydrates such as honeydew.

Wasp and bee workers – infertile just as in ants – need to fly and this constrained extensive divergence from queens, which prevented bigger colonies. The winglessness of ant helpers maximized the benefits of having two morphological castes.

Loss of aerial dispersal in ant reproductives

Even though flight is restricted in ants to a brief dispersal event, winged queens have been selected against in many independent lineages. Thousands of species have permanently wingless (or short-winged) reproductives that occur together with or instead of winged queens (Peeters 2012). This change in morphology is associated with a shift from independent colony foundation to colony fission (i.e. reproductive and nestmate workers disperse on foot).

Brian Fisher (California Academy of Sciences) and I currently study the (wingless) secondary reproductives of Metapone (subfamily Myrmicinae) in Madagascar.

Queen-worker divergence in ovaries across ants

Fuminori Ito (Kagawa University, Japan) and I have dissected the ovaries of queens and workers in more than 450 species belonging to 135 genera (13 subfamilies). We show (manuscript in preparation) a remarkable variation in ovariole numbers among ant queens, which is an unknown adaption in social wasps and bees (Apis excepted). Workers have as many ovarioles as queens in most poneroid species, but formicoid workers have much fewer. Ovaries are completely lost in six genera of Ponerinae and eight genera of Myrmicinae.

Convergent evolution of soldiers in over 15 ant genera

Soldiers are a third caste, additional to workers and queens. Their function varies strikingly across species (food storage, defence, seed-milling) and is not part of the definition, i.e. "soldier" is a strictly morphological term.

Soldiers have distinctive heads relative to conspecific workers. Powerful mandibles can be specialized for defence but also wood-boring (Gesomyrmex) or seed-milling (Acanthomyrmex). In Cephalotes and Colobopsis, door-shaped (phragmotic) heads are shared with queens.

Soldiers also have larger abdomens than workers. In several genera, their ovaries are queen-like and they lay trophic eggs (e.g. Acanthomyrmex, Cataglyphis bombycina, Crematogaster (Orthocrema), Gesomyrmex). In other genera, liquid nutrients are stored in an expanded crop (e.g. Carebara, Pheidole).

Ant soldiers are a mosaic of queen and worker morphological traits (Molet et al. 2012).

What to call the different actors in ant societies? queens (flying or not), workers (reproductive or not), soldiers

Ant societies consist of reproductive and sterile adults that show tremendous diversity of phenotypes. These include permanently wingless queens and a soldier caste that evolved convergently in many genera. Myrmecologists describe ant castes with terminology based on form, or function, or both; moreover terms are used inconsistently in the literature. Since morphology changes less readily than behaviour, I emphasise morphological definitions to facilitate comparative studies and an understanding of the evolutionary origin of castes.

Economie coloniale et diversification morphologique des castes

Relatif aux autres insectes sociaux, les fourmis montrent une vaste gamme de dimorphisme entre les castes reine et ouvrière. La spécialisation morphologique des adultes femelles détermine la dispersion et reproduction des colonies (reines), ainsi que le partage des tâches dans les sociétés établies (ouvrières). La perte du vol est universelle chez les ouvrières, et sporadique parmi les reines. L’élimination des muscles alaires chez les reines non-volantes permet d’économiser les resources coloniales. Des pressions de sélection pour la réduction du coût des ouvrières ont mené à l’amincissement de la cuticule ainsi qu’à une diminution extrême de la taille individuelle (Peeters & Ito 2015). Une caste soldat a évolué en plus chez plusieurs genres avec ouvrières minuscules, permettant l’étude des compromis dans l’allocation des ressources entre différentes parties du corps.