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1. Treadmilling and dynamic protrusions in fire ant rafts

   https://doi.org/10.1098/rsif.2021.0213  

   Wagner, Robert J.; Such, Kristen; Hobbs, Ethan; Vernerey, Franck J. (June 2021, Journal of The Royal Society Interface)

   null (Ed.)

   Fire ants ( Solenopsis invicta ) are exemplary for their formation of cohered, buoyant and dynamic structures composed entirely of their own bodies when exposed to flooded environments. Here, we observe tether-like protrusions that emerge from aggregated fire ant rafts when docked to stationary, vertical rods. Ant rafts comprise a floating, structural network of interconnected ants on which a layer of freely active ants walk. We show here that sustained shape evolution is permitted by the competing mechanisms of perpetual raft contraction aided by the transition of bulk structural ants to the free active layer and outward raft expansion owing to the deposition of free ants into the structural network at the edges, culminating in global treadmilling. Furthermore, we see that protrusions emerge as a result of asymmetries in the edge deposition rate of free ants. Employing both experimental characterization and a model for self-propelled particles in strong confinement, we interpret that these asymmetries are likely to occur stochastically owing to wall accumulation effects and directional motion of active ants when strongly confined by the protrusions' relatively narrow boundaries. Together, these effects may realize the cooperative, yet spontaneous formation of protrusions that fire ants sometimes use for functional exploration and to escape flooded environments. 

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2. Catch bond kinetics are instrumental to cohesion of fire ant rafts under load

   https://doi.org/10.1073/pnas.2314772121  

   Wagner, Robert J; Lamont, Samuel C; White, Zachary T; Vernerey, Franck J (April 2024, Proceedings of the National Academy of Sciences)

   Dynamic networks composed of constituents that break and reform bonds reversibly are ubiquitous in nature owing to their modular architectures that enable functions like energy dissipation, self-healing, and even activity. While bond breaking depends only on the current configuration of attachment in these networks, reattachment depends also on the proximity of constituents. Therefore, dynamic networks composed of macroscale constituents (not benefited by the secondary interactions cohering analogous networks composed of molecular-scale constituents) must rely on primary bonds for cohesion and self-repair. Toward understanding how such macroscale networks might adaptively achieve this, we explore the uniaxial tensile response of 2D rafts composed of interlinked fire ants (S. invicta). Through experiments and discrete numerical modeling, we find that ant rafts adaptively stabilize their bonded ant-to-ant interactions in response to tensile strains, indicating catch bond dynamics. Consequently, low-strain rates that should theoretically induce creep mechanics of these rafts instead induce elastic-like response. Our results suggest that this force-stabilization delays dissolution of the rafts and improves toughness. Nevertheless, above 35 $\%$strain low cohesion and stress localization cause nucleation and growth of voids whose coalescence patterns result from force-stabilization. These voids mitigate structural repair until initial raft densities are restored and ants can reconnect across defects. However mechanical recovery of ant rafts during cyclic loading suggests that—even upon reinstatement of initial densities—ants exhibit slower repair kinetics if they were recently loaded at faster strain rates. These results exemplify fire ants’ status as active agents capable of memory-driven, stimuli-response for potential inspiration of adaptive structural materials. 

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3. Agitated ants: regulation and self-organization of incipient nest excavation via collisional cues

   https://doi.org/10.1098/rsif.2022.0597  

   Avinery, Ram; Aina, Kehinde O.; Dyson, Carl J.; Kuan, Hui-Shun; Betterton, Meredith D.; Goodisman, Michael A.; Goldman, Daniel I. (May 2023, Journal of The Royal Society Interface)

   Ants are millimetres in scale yet collectively create metre-scale nests in diverse substrates. To discover principles by which ant collectives self-organize to excavate crowded, narrow tunnels, we studied incipient excavation in small groups of fire ants in quasi-two-dimensional arenas. Excavation rates displayed three stages: initially excavation occurred at a constant rate, followed by a rapid decay, and finally a slower decay scaling in time as t −1/2 . We used a cellular automata model to understand such scaling and motivate how rate modulation emerges without global control. In the model, ants estimated their collision frequency with other ants, but otherwise did not communicate. To capture early excavation rates, we introduced the concept of ‘agitation’—a tendency of individuals to avoid rest if collisions are frequent. The model reproduced the observed multi-stage excavation dynamics; analysis revealed how parameters affected features of multi-stage progression. Moreover, a scaling argument without ant–ant interactions captures tunnel growth power-law at long times. Our study demonstrates how individual ants may use local collisional cues to achieve functional global self-organization. Such contact-based decisions could be leveraged by other living and non-living collectives to perform tasks in confined and crowded environments. 

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4. Better tired than lost: Turtle ant trail networks favor coherence over short edges

   https://doi.org/10.1371/journal.pcbi.1009523  

   Chandrasekhar, Arjun; Marshall, James A.; Austin, Cortnea; Navlakha, Saket; Gordon, Deborah M (October 2021, PLoS computational biology)

   Creating a routing backbone is a fundamental problem in both biology and engineering. The routing backbone of the trail networks of arboreal turtle ants (Cephalotes goniodontus) connects many nests and food sources using trail pheromone deposited by ants as they walk. Unlike species that forage on the ground, the trail networks of arboreal ants are constrained by the vegetation. We examined what objectives the trail networks meet by comparing the observed ant trail networks with networks of random, hypothetical trail networks in the same surrounding vegetation and with trails optimized for four objectives: minimizing path length, minimizing average edge length, minimizing number of nodes, and minimizing opportunities to get lost. The ants’ trails minimized path length by minimizing the number of nodes traversed rather than choosing short edges. In addition, the ants’ trails reduced the opportunity for ants to get lost at each node, favoring nodes with 3D configurations most likely to be reinforced by pheromone. Thus, rather than finding the shortest edges, turtle ant trail networks take advantage of natural variation in the environment to favor coherence, keeping the ants together on the trails. 

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5. Scale insect (Hemiptera: Coccomorpha) morphology is transformed under trophobiosis

   https://doi.org/10.1093/aesa/saad033  

   Sodano, James; Oufiero, Christopher E; Schneider, Scott A; LaPolla, John S (December 2023, Annals of the Entomological Society of America)

   Solis, Alma (Ed.)

   Abstract Ants (Hymenoptera: Formicidae) have great potential to exert influence over the morphological evolution of their obligate mutualist partners. Obligately myrmecophilic mealybugs are noted for their unusual morphology, and while this is often attributed to their relationship with ants, a quantitative assessment of this link is lacking. We address this need by evaluating morphological change among mealybugs as a function of ant association. This study considers the associates of 2 independent ant clades—Acropyga Roger, 1862 ants associated with root mealybugs from the families Xenococcidae and Rhizoecidae and herdsmen ants from the Dolichoderus cuspidatus (Smith, F., 1857) species-group associated with mealybugs from the tribe Allomyrmococcini (Pseudococcidae)—and compares them to free-living or potentially myrmecophilic species sampled from among the mealybugs and root mealybugs. We use a combination of geometric morphometric and linear datasets to evaluate characteristics of body shape, body size, leg metrics, and ostiole development. Obligate myrmecophily significantly influences both body shape and size. Myrmecophilous mealybugs are smaller than their free-living counterparts and are either pyriform or rotund in shape rather than oval. Ant-associates from Rhizoecidae also have significantly reduced anterior pairs of ostioles compared to free-living species. Ostioles are involved in defense against natural enemies and mutualist ants typically protect their partners, presumably supplanting the need for structures like ostioles among myrmecophilous species. We discuss the influence ants have on the evolution of their associates in the context of domestication and offer avenues for future exploration. 

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