Divergent ecological responses to typhoon disturbance revealed via landscape-scale acoustic monitoring

Climate change is increasing the frequency, intensity, and duration of extreme weather events across the globe. Understanding the capacity for ecological communities to withstand and recover from such events is critical. Typhoons are extreme weather events that are expected to broadly homogenize ecological dynamics through structural damage to vegetation and longer-term effects of salinization. Given their unpredictable nature, monitoring ecological responses to typhoons is challenging, particularly for mobile animals such as birds. Here, we report spatially variable ecological responses to typhoons across terrestrial landscapes. Using a high temporal resolution passive acoustic monitoring network across 24 sites on the subtropical island of Okinawa, Japan, we found that typhoons elicit divergent ecological responses among Okinawa's diverse terrestrial habitats, as indicated by increased spatial variability of biological sound production (biophony) and individual species detections. This suggests that soniferous communities are capable of a diversity of different responses to typhoons. That is, spatial insurance effects among local ecological communities provide resilience to typhoons at the landscape scale. Even though site-level typhoon impacts on soundscapes and bird detections were not particularly strong, monitoring at scale with high temporal resolution across a broad spatial extent nevertheless enabled detection of spatial heterogeneity in typhoon responses. Further, species-level responses mirrored those of acoustic indices, underscoring the utility of such indices for revealing insight into fundamental questions concerning disturbance and stability. Our findings demonstrate the significant potential of landscape-scale acoustic sensor networks to capture the understudied ecological impacts of unpredictable extreme weather events.

Targeted treatment of injured nestmates with antimicrobial compounds in an ant society

Infected wounds pose a major mortality risk in animals. Injuries are common in the ant Megaponera analis, which raids pugnacious prey. Here we show that M. analis can determine when wounds are infected and treat them accordingly. By applying a variety of antimicrobial compounds and proteins secreted from the metapleural gland to infected wounds, workers reduce the mortality of infected individuals by 90%. Chemical analyses showed that wound infection is associated with specific changes in the cuticular hydrocarbon profile, thereby likely allowing nestmates to diagnose the infection state of injured individuals and apply the appropriate antimicrobial treatment. This study demonstrates that M. analis ant societies use antimicrobial compounds produced in the metapleural glands to treat infected wounds and reduce nestmate mortality.

The feeding apparatus of ants: an overview of structure and function

Ants are a dominant family of eusocial terrestrial insects with a diversity of ecologies, lifestyles and morphologies. Ant diet preferences range from strict carnivory through omnivory to almost complete herbivory in species feeding on seeds or exudates of plant-sucking insects. While several studies have investigated ant feeding performance on different substrates, comparatively little is known about the functional morphology of the structures involved in food uptake or their diversification across the ants. To take stock of our current knowledge, we give an overview of how adult ants ingest food, followed by a morphological description of the mouthparts, preoral space and cephalic sucking pump. The mandibles are the most prominent mouthparts and have received considerable attention in the literature, so we focus on the maxillae and labium here. We present current hypotheses for the movement patterns of these parts and discuss morphological differences among ants that may be related to their ecological diversity. Finally, we give short comparisons of the ant condition with some other insects and vertebrates, as well as an outlook summarizing gaps in our knowledge. This sets the stage for future studies elucidating the connections between ant feeding mechanisms and mouthpart evolution. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Collective and harmonized high throughput barcoding of insular arthropod biodiversity: Toward a Genomic Observatories Network for islands

Current understanding of ecological and evolutionary processes underlying island biodiversity is heavily shaped by empirical data from plants and birds, although arthropods comprise the overwhelming majority of known animal species, and as such can provide key insights into processes governing biodiversity. Novel high throughput sequencing (HTS) approaches are now emerging as powerful tools to overcome limitations in the availability of arthropod biodiversity data, and hence provide insights into these processes. Here, we explored how these tools might be most effectively exploited for comprehensive and comparable inventory and monitoring of insular arthropod biodiversity. We first reviewed the strengths, limitations and potential synergies among existing approaches of high throughput barcode sequencing. We considered how this could be complemented with deep learning approaches applied to image analysis to study arthropod biodiversity. We then explored how these approaches could be implemented within the framework of an island Genomic Observatories Network (iGON) for the advancement of fundamental and applied understanding of island biodiversity. To this end, we identified seven island biology themes at the interface of ecology, evolution and conservation biology, within which collective and harmonized efforts in HTS arthropod inventory could yield significant advances in island biodiversity research.

Richness and resilience in the Pacific: DNA metabarcoding enables parallelized evaluation of biogeographic patterns

Islands make up a large proportion of Earth's biodiversity, yet are also some of the most sensitive systems to environmental perturbation. Biogeographic theory predicts that geologic age, area, and isolation typically drive islands' diversity patterns, and thus potentially impact non-native spread and community homogenization across island systems. One limitation in testing such predictions has been the difficulty of performing comprehensive inventories of island biotas and distinguishing native from introduced taxa. Here, we use DNA metabarcoding and statistical modelling as a high throughput method to survey community-wide arthropod richness, the proportion of native and non-native species, and the incursion of non-natives into primary habitats on three archipelagos in the Pacific - the Ryukyus, the Marianas and Hawaii - which vary in age, isolation and area. Diversity patterns largely match expectations based on island biogeography theory, with the oldest and most geographically connected archipelago, the Ryukyus, showing the highest taxonomic richness and lowest proportion of introduced species. Moreover, we find evidence that forest habitats are more resilient to incursions of non-natives in the Ryukyus than in the less taxonomically rich archipelagos. Surprisingly, we do not find evidence for biotic homogenization across these three archipelagos: the assemblage of non-native species on each island is highly distinct. Our study demonstrates the potential of DNA metabarcoding to facilitate rapid estimation of biogeographic patterns, the spread of non-native species, and the resilience of ecosystems.

Breakdown in seasonal dynamics of subtropical ant communities with land-cover change

Concerns about widespread human-induced declines in insect populations are mounting, yet little is known about how land-use change modifies both the trends and variability of insect communities, particularly in understudied regions. Here, we examine how the seasonal activity patterns of ants-key drivers of terrestrial ecosystem functioning-vary with anthropogenic land-cover change on a subtropical island landscape, and whether differences in temperature or species composition can explain observed patterns. Using trap captures sampled biweekly over 2 years from a biodiversity monitoring network covering Okinawa Island, Japan, we processed 1.2 million individuals and reconstructed activity patterns within and across habitat types. Forest communities exhibited greater temporal variability of activity than those in more developed areas. Using time-series decomposition to deconstruct this pattern, we found that sites with greater human development exhibited ant communities with diminished seasonality, reduced synchrony and higher stochasticity compared with sites with greater forest cover. Our results cannot be explained by variation in regional or site temperature patterns, or by differences in species richness or composition among sites. Our study raises the possibility that disruptions to natural seasonal patterns of functionally key insect communities may comprise an important and underappreciated consequence of global environmental change that must be better understood across Earth's biomes.

Generalized mutualisms promote range expansion in both plant and ant partners

Mutualism improves organismal fitness, but strong dependence on another species can also limit a species' ability to thrive in a new range if its partner is absent. We assembled a large, global dataset on mutualistic traits and species ranges to investigate how multiple plant-animal and plant-microbe mutualisms affect the spread of legumes and ants to novel ranges. We found that generalized mutualisms increase the likelihood that a species establishes and thrives beyond its native range, whereas specialized mutualisms either do not affect or reduce non-native spread. This pattern held in both legumes and ants, indicating that specificity between mutualistic partners is a key determinant of ecological success in a new habitat. Our global analysis shows that mutualism plays an important, if often overlooked, role in plant and insect invasions.

Global determinants of insect mitochondrial genetic diversity

Understanding global patterns of genetic diversity is essential for describing, monitoring, and preserving life on Earth. To date, efforts to map macrogenetic patterns have been restricted to vertebrates, which comprise only a small fraction of Earth's biodiversity. Here, we construct a global map of predicted insect mitochondrial genetic diversity from cytochrome c oxidase subunit 1 sequences, derived from open data. We calculate the mitochondrial genetic diversity mean and genetic diversity evenness of insect assemblages across the globe, identify their environmental correlates, and make predictions of mitochondrial genetic diversity levels in unsampled areas based on environmental data. Using a large single-locus genetic dataset of over 2 million globally distributed and georeferenced mtDNA sequences, we find that mitochondrial genetic diversity evenness follows a quadratic latitudinal gradient peaking in the subtropics. Both mitochondrial genetic diversity mean and evenness positively correlate with seasonally hot temperatures, as well as climate stability since the last glacial maximum. Our models explain 27.9% and 24.0% of the observed variation in mitochondrial genetic diversity mean and evenness in insects, respectively, making an important step towards understanding global biodiversity patterns in the most diverse animal taxon.

Behavioural individuality determines infection risk in clonal ant colonies

In social groups, infection risk is not distributed evenly across individuals. Individual behaviour is a key source of variation in infection risk, yet its effects are difficult to separate from other factors (e.g., age). Here, we combine epidemiological experiments with chemical, transcriptomic, and automated behavioural analyses in clonal ant colonies, where behavioural individuality emerges among identical workers. We find that: (1) Caenorhabditis-related nematodes parasitise ant heads and affect their survival and physiology, (2) differences in infection emerge from behavioural variation alone, and reflect spatially-organised division of labour, (3) infections affect colony social organisation by causing infected workers to stay in the nest. By disproportionately infecting some workers and shifting their spatial distribution, infections reduce division of labour and increase spatial overlap between hosts, which should facilitate parasite transmission. Thus, division of labour, a defining feature of societies, not only shapes infection risk and distribution but is also modulated by parasites.

Parallel And Divergent Morphological Adaptations Underlying The Evolution of Jumping Ability in Ants

Jumping is a rapid locomotory mode widespread in terrestrial organisms. However, it is a rare specialization in ants. Forward jumping has been reported within four distantly related ant genera: Gigantiops, Harpegnathos, Myrmecia, and Odontomachus. The temporal engagement of legs/body parts during jump, however, varies across these genera. It is unknown what morphological adaptations underlie such behaviors and whether jumping in ants is solely driven directly by muscle contraction or additionally relies on elastic recoil mechanism. We investigated the morphological adaptations for jumping behavior by comparing differences in the locomotory musculature between jumping and non-jumping relatives using X-ray micro-CT and 3D morphometrics. We found that the size-specific volumes of the trochanter depressor muscle (scm6) of the middle and hind legs are 3-5 times larger in jumping ants, and that one coxal remotor muscle (scm2) is reduced in volume in the middle and/or hind legs. Notably, the enlargement in the volume of other muscle groups is directly linked to the legs or body parts engaged during the jump. Furthermore, a direct comparison of the muscle architecture revealed two significant differences between jumping vs. non-jumping ants: First, the relative Physiological Cross-Sectional Area (PCSA) of the trochanter depressor muscles of all three legs were larger in jumping ants, except in the front legs of Odontomachus rixosus and Myrmecia nigrocincta; second, the relative muscle fiber length was shorter in jumping ants compared to non-jumping counterparts, except in the front legs of O. rixosus and M. nigrocincta. These results suggest that the difference in relative muscle volume in jumping ants is largely invested in the area (PCSA), and not in fiber length. There was no clear difference in the pennation angle between jumping and non-jumping ants. Additionally, we report that the hind leg length relative to body length was longer in jumping ants. Based on direct comparison of the observed vs. possible work and power output during jumps, we surmise that direct muscle contractions suffice to explain jumping performance in three species, except for O. rixosus, where the lack of data on jumping performance prevents us from drawing definitive conclusions for this particular species. We suggest that increased investment in jumping-relevant musculature is a primary morphological adaptation that separates jumping from non-jumping ants. These results elucidate the common and idiosyncratic morphological changes underlying this rare adaptation in ants. まとぅみ (Okinawan language-Uchinaaguchi) (Japanese) РЕЗЮМЕ (Kazakh) ZUSAMMENFASSUNG (German).

Evidence of a thoracic crop in workers, soldiers, and queens of Carebara perpusilla ants (Formicidae: Myrmicinae)

The ability to share and store food is paramount in group-living animals, allowing a finely tuned distribution of resources over time and individuals and an enhanced survival over periods of food scarcity. Ants have several ways to store food: one of them is their gastral crop, also known as a "social stomach." Nutrients in the crop can be regurgitated to nestmates through oral trophallaxis (mouth-to-mouth) or proceed to the midgut by opening the proventriculus, a valve connecting the crop to the midgut. However, some ants are also known to have a so-called "thoracic crop," an extension of the esophagus that allows for additional storage space. In this study, we provide the first evidence of a thoracic crop in the genus Carebara, in reproductive (queen) and sterile (soldier and worker) castes. We discuss how the ant body plan allowed for the evolution of a novel food storage structure in the mesothorax.

Morphology of the metapleural gland and its associated novel atrial cone gland in Strumigenys ants

The metapleural gland is a unique feature of the ant phenotype, but diversity in its anatomy and function across the ants is not well documented or understood. We studied the morphology of the metapleural gland in 20 mainly Oriental Strumigenys species using histology, scanning and transmission electron microscopy, and micro-CT. The gland is formed by a cluster of class-3 cells, their secretion is transported through a bundle of ducts into the sclerotized atrium and is guided along a series of parallel cuticular ridges towards the atrial opening. Among the examined species, queens have more gland cells than conspecific workers, while the examined males do not have the gland. The social parasite S. mutica has the most developed metapleural gland. In addition, we describe a novel class-1 atrial cone gland which is associated with the metapleural gland. The epithelium of this cone gland forms an invagination into the ventral atrium of the metapleural gland. The cuticular cone may be lacking in some Afrotropical and Neotropical species, although these may still contain the epithelial gland. The functional and evolutionary drivers of morphological variation in the exocrine system across species form interesting questions for future work.

The impact of land use on non-native species incidence and number in local assemblages worldwide

While the regional distribution of non-native species is increasingly well documented for some taxa, global analyses of non-native species in local assemblages are still missing. Here, we use a worldwide collection of assemblages from five taxa - ants, birds, mammals, spiders and vascular plants - to assess whether the incidence, frequency and proportions of naturalised non-native species depend on type and intensity of land use. In plants, assemblages of primary vegetation are least invaded. In the other taxa, primary vegetation is among the least invaded land-use types, but one or several other types have equally low levels of occurrence, frequency and proportions of non-native species. High land use intensity is associated with higher non-native incidence and frequency in primary vegetation, while intensity effects are inconsistent for other land-use types. These findings highlight the potential dual role of unused primary vegetation in preserving native biodiversity and in conferring resistance against biological invasions.

GABI-I: The global ant biodiversity informatics-island database

Island systems are known to harbor disproportionate amounts of geographically restricted biodiversity and to experience high rates of species loss, and they ultimately represent critical systems with significant conservation values. However, knowledge of the biodiversity value of insular systems remains highly fragmented and incomplete for many groups of organisms, especially insects. This gap limits our understanding of their global significance for biodiversity and inhibits prioritization for future exploration and conservation efforts. Here, we developed a new database to present current knowledge on ants on 2678 islands globally, based on nominal species, and provide information on their native or exotic status. In total, this database contains 7010 ant species (44.6% of the known global ant fauna) that have been recorded on islands globally from 449,232 records. In addition, this database identifies 108 large islands (area > 200 km² ) that have received no ant sampling efforts globally. This new data set provides the most comprehensive understanding of ant diversity and composition on islands globally, opening up new opportunities to address questions on a multitude of research questions and fields related to biogeography, ecology, and evolution. This data set also provides a roadmap for future exploration and conservation actions in connection with ants on islands as well as overall ant diversity, with updates available as new records and taxonomic updates are published. There are no copyright restrictions on this database and users should cite this data paper in publications when using the data.

The global distribution of known and undiscovered ant biodiversity

Invertebrates constitute the majority of animal species and are critical for ecosystem functioning and services. Nonetheless, global invertebrate biodiversity patterns and their congruences with vertebrates remain largely unknown. We resolve the first high-resolution (~20-km) global diversity map for a major invertebrate clade, ants, using biodiversity informatics, range modeling, and machine learning to synthesize existing knowledge and predict the distribution of undiscovered diversity. We find that ants and different vertebrate groups have distinct features in their patterns of richness and rarity, underscoring the need to consider a diversity of taxa in conservation. However, despite their phylogenetic and physiological divergence, ant distributions are not highly anomalous relative to variation among vertebrate clades. Furthermore, our models predict that rarity centers largely overlap (78%), suggesting that general forces shape endemism patterns across taxa. This raises confidence that conservation of areas important for small-ranged vertebrates will benefit invertebrates while providing a "treasure map" to guide future discovery.

Toward global integration of biodiversity big data: a harmonized metabarcode data generation module for terrestrial arthropods

Metazoan metabarcoding is emerging as an essential strategy for inventorying biodiversity, with diverse projects currently generating massive quantities of community-level data. The potential for integrating across such data sets offers new opportunities to better understand biodiversity and how it might respond to global change. However, large-scale syntheses may be compromised if metabarcoding workflows differ from each other. There are ongoing efforts to improve standardization for the reporting of inventory data. However, harmonization at the stage of generating metabarcode data has yet to be addressed. A modular framework for harmonized data generation offers a pathway to navigate the complex structure of terrestrial metazoan biodiversity. Here, through our collective expertise as practitioners, method developers, and researchers leading metabarcoding initiatives to inventory terrestrial biodiversity, we seek to initiate a harmonized framework for metabarcode data generation, with a terrestrial arthropod module. We develop an initial set of submodules covering the 5 main steps of metabarcode data generation: (i) sample acquisition; (ii) sample processing; (iii) DNA extraction; (iv) polymerase chain reaction amplification, library preparation, and sequencing; and (v) DNA sequence and metadata deposition, providing a backbone for a terrestrial arthropod module. To achieve this, we (i) identified key points for harmonization, (ii) reviewed the current state of the art, and (iii) distilled existing knowledge within submodules, thus promoting best practice by providing guidelines and recommendations to reduce the universe of methodological options. We advocate the adoption and further development of the terrestrial arthropod module. We further encourage the development of modules for other biodiversity fractions as an essential step toward large-scale biodiversity synthesis through harmonization.

Minute workers and large soldiers in the subterranean ant Carebara perpusilla: Musculoskeletal consequences of Haller's rule in the thorax

Many organismal traits vary with body size, often reflecting trade-offs in the face of size-dependent constraints. For example, Haller's rule, the allometric pattern whereby smaller organisms have proportionally larger brains, can have carry-on effects on head design as the brain competes for space with other structures. Ant species with polymorphic worker castes are interesting cases for helping us understand these allometric effects. Here, we examine the effects of miniaturization on the ant power core, the mesosoma (thorax), with particular attention to how the scaling of nervous system structures affects the skeletomuscular elements involved with load bearing and locomotion. Using X-ray computed microtomography (microCT), we studied the thorax of Carebara perpusilla, an African ant species that has minute workers (1.5 mm-long) and larger soldiers (3.0 mm-long), allowing strong intraspecific comparisons. We find that the thoracic nervous system is relatively larger in minute workers, similar to Haller's rule, with consequences on the skeletomuscular organisation. Minute workers have relatively smaller petiole muscles and indirect head muscles, but relatively larger external trochanter muscles and direct head muscles. We link these allometric trade-offs to miniaturization and division of labor, and discuss how thorax design underlies the success of minute ants.

Pheidoleklaman sp. nov.: a new addition from Ivory Coast to the Afrotropical pulchella species group (Hymenoptera, Formicidae, Myrmicinae)

In this study the taxonomy of the Pheidolepulchella species group is updated for the Afrotropical region and the new species P.klaman sp. nov. described. It is integrated into the existing taxonomic system by an updated identification key for the whole group and an update of the known distribution ranges of its members. High quality focus stacking images are provided, with X-ray micro-CT scanned digital 3D representations, of major and minor worker type specimens.

Warm and arid regions of the world are hotspots of superorganism complexity

Biologists have long been fascinated by the processes that give rise to phenotypic complexity of organisms, yet whether there exist geographical hotspots of phenotypic complexity remains poorly explored. Phenotypic complexity can be readily observed in ant colonies, which are superorganisms with morphologically differentiated queen and worker castes analogous to the germline and soma of multicellular organisms. Several ant species have evolved 'worker polymorphism', where workers in a single colony show quantifiable differences in size and head-to-body scaling. Here, we use 256 754 occurrence points from 8990 ant species to investigate the geography of worker polymorphism. We show that arid regions of the world are the hotspots of superorganism complexity. Tropical savannahs and deserts, which are typically species-poor relative to tropical or even temperate forests, harbour the highest densities of polymorphic ants. We discuss the possible adaptive advantages that worker polymorphism provides in arid environments. Our work may provide a window into the environmental conditions that promote the emergence of highly complex phenotypes.

Evidence for the evolution of eusociality in stem ants and a systematic revision of †Gerontoformica (Hymenoptera: Formicidae)

It is generally assumed that Cretaceous stem ants were obligately eusocial, because of the presence of wingless adult females, yet the available evidence is ambiguous. Here, we report the syninclusion of a pupa and adult of a stem ant species from Mid-Cretaceous amber. As brood are immobile, the pupa was likely to have been transported by an adult. Therefore, the fossil substantiates the hypothesis that wingless females were cooperators, thus these were true ‘workers’. Re-examination of all described Cretaceous ant species reveals that winged–wingless diphenism – hence a variable dispersal capacity – may have been ancestral to the total clade of the ants, and that highly specialized worker-specific phenotypes evolved in parallel between the stem and crown groups. The soft-tissue preservation of the fossil is exceptional, demonstrating the possibility of analysing the development of the internal anatomy in stem ants. Based on the highest-resolution µ-CT scans of stem ants to date, we describe †Gerontoformica sternorhabda sp. nov., redescribe †G. gracilis, redefine the species group classification of †Gerontoformica, and provide a key to the species of the genus. Our work clarifies the species boundaries of †Gerontoformica and renders fossils relevant to the discussion of eusocial evolution in a way that has heretofore been intractable.

A Roadmap to Reconstructing Muscle Architecture from CT Data

Skeletal muscle is responsible for voluntary force generation across animals, and muscle architecture largely determines the parameters of mechanical output. The ability to analyze muscle performance through muscle architecture is thus a key step towards better understanding the ecology and evolution of movements and morphologies. In pennate skeletal muscle, volume, fiber lengths, and attachment angles to force transmitting structures comprise the most relevant parameters of muscle architecture. Measuring these features through tomographic techniques offers an alternative to tedious and destructive dissections, particularly as the availability of tomographic data is rapidly increasing. However, there is a need for streamlined computational methods to access this information efficiently. Here, we establish and compare workflows using partially automated image analysis for fast and accurate estimation of animal muscle architecture. After isolating a target muscle through segmentation, we evaluate freely available and proprietary fiber tracing algorithms to reconstruct muscle fibers. We then present a script using the Blender Python API to estimate attachment angles, fiber lengths, muscle volume, and physiological cross-sectional area. We apply these methods to insect and vertebrate muscle and provide guided workflows. Results from fiber tracing are consistent compared to manual measurements but much less time-consuming. Lastly, we emphasize the capabilities of the open-source three-dimensional software Blender as both a tool for visualization and a scriptable analytic tool to process digitized anatomical data. Across organisms, it is feasible to extract, analyze, and visualize muscle architecture from tomography data by exploiting the spatial features of scans and the geometric properties of muscle fibers. As digital libraries of anatomies continue to grow, the workflows and approach presented here can be part of the open-source future of digital comparative analysis.

Species-range-size distributions: Integrating the effects of speciation, transformation, and extinction

The species-range size distribution is a product of speciation, transformation of range-sizes, and extinction. Previous empirical studies showed that it has a left-skewed lognormal-like distribution. We developed a new mathematical framework to study species-range-size distributions, one in which allopatric speciation, transformation of range size, and the extinction process are explicitly integrated. The approach, which we call the gain-loss-allopatric speciation model, allows us to explore the effects of various speciation scenarios. Our model captures key dynamics thought to lead to known range-size distributions. We also fitted the model to empirical range-size distributions of birds, mammals, and beetles. Since geographic range dynamics are linked to speciation and extinction, our model provides predictions for the dynamics of species richness. When a species-range-size distribution initially evolves away from the range sizes at which the likelihood of speciation is low, it tends to cause diversification slowdown even in the absence of (bio)diversity dependence in speciation rate. Using the mathematical model developed here, we give a potential explanation for how observed range-size distributions emerge from range-size dynamics. Although the framework presented is minimalistic, it provides a starting point for examining hypotheses based on more complex mechanisms.

Climate and land-use interactively shape butterfly diversity in tropical rainforest and savanna ecosystems of southwestern China

Human-induced habitat conversion and degradation, along with accelerating climatic change, have resulted in considerable global biodiversity loss. Nevertheless, how local ecological assemblages respond to the interplay between climate and land-use change remains poorly understood. Here, we examined the effects of climate and land-use interactions on butterfly diversity in different ecosystems of southwestern China. Specifically, we investigated variation in the alpha and beta diversities of butterflies in different landscapes along human-modified and climate gradients. We found that increasing land-use intensity not only caused a dramatic decrease in butterfly alpha diversity but also significantly simplified butterfly species composition in tropical rainforest and savanna ecosystems. These findings suggest that habitat modification by agricultural activities increases the importance of deterministic processes and leads to biotic homogenization. The land-use intensity model best explained species richness variation in the tropical rainforest, whereas the climate and land-use intensity interaction model best explained species richness variation in the savanna. These results indicate that climate modulates the effects of land-use intensity on butterfly alpha diversity in the savanna ecosystem. We also found that the response of species composition to climate varied between sites: specifically, species composition was strongly correlated with climatic distance in the tropical rainforest but not in the savanna. Taken together, our long-term butterfly monitoring data reveal that interactions between human-modified habitat change and climate change have shaped butterfly diversity in tropical rainforest and savanna. These findings also have important implications for biodiversity conservation under the current era of rapid human-induced habitat loss and climate change.

Mandibular morphology, task specialization and bite mechanics in Pheidole ants (Hymenoptera: Formicidae)

Ants show remarkable ecological and evolutionary success due to their social life history and division of labour among colony members. In some lineages, the worker force became subdivided into morphologically distinct individuals (i.e. minor versus major workers), allowing for the differential performance of particular roles in the colony. However, the functional and ecological significance of these morphological differences are not well understood. Here, we applied finite element analysis (FEA) to explore the biomechanical differences between major and minor ant worker mandibles. Analyses were carried out on mandibles of two Pheidole species, a dimorphic ant genus. We tested whether major mandibles evolved to minimize stress when compared to minors using combinations of the apical tooth and masticatory margin bites under strike and pressure conditions. Majors performed better in pressure conditions yet, contrary to our expectations, minors performed better in strike bite scenarios. Moreover, we demonstrated that even small morphological differences in ant mandibles might lead to substantial differences in biomechanical responses to bite loading. These results also underscore the potential of FEA to uncover biomechanical consequences of morphological differences within and between ant workers.

Head and mandible shapes are highly integrated yet represent two distinct modules within and among worker subcastes of the ant genus Pheidole

Ants use their mandibles for a wide variety of tasks related to substrate manipulation, brood transport, food processing, and colony defense. Due to constraints involved in colony upkeep, ants evolved a remarkable diversity of mandibular forms, often related to specific roles such as specialized hunting and seed milling. Considering these varied functional demands, we focused on understanding how the mandible and head shape vary within and between Pheidole subcastes. Using x-ray microtomography and 3D geometric morphometrics, we tested whether these structures are integrated and modular, and how ecological predictors influenced these features. Our results showed that mandible and head shape of majors and minor workers tend to vary from robust to slender, with some more complex changes related to the mandibular base. Additionally, we found that head and mandible shapes are characterized by a high degree of integration, but with little correlation with feeding and nesting habits. Our results suggest that a combination of structural (allometric) constraints and the behavioral flexibility conferred by subcaste dimorphism might largely buffer selective pressures that would otherwise lead to a fine-tuning between ecological conditions and morphological adaptation.

Genomic Signature of Shifts in Selection in a Subalpine Ant and Its Physiological Adaptations

Understanding how organisms adapt to extreme environments is fundamental and can provide insightful case studies for both evolutionary biology and climate-change biology. Here, we take advantage of the vast diversity of lifestyles in ants to identify genomic signatures of adaptation to extreme habitats such as high altitude. We hypothesized two parallel patterns would occur in a genome adapting to an extreme habitat: 1) strong positive selection on genes related to adaptation and 2) a relaxation of previous purifying selection. We tested this hypothesis by sequencing the high-elevation specialist Tetramorium alpestre and four other phylogenetically related species. In support of our hypothesis, we recorded a strong shift of selective forces in T. alpestre, in particular a stronger magnitude of diversifying and relaxed selection when compared with all other ants. We further disentangled candidate molecular adaptations in both gene expression and protein-coding sequence that were identified by our genome-wide analyses. In particular, we demonstrate that T. alpestre has 1) a higher level of expression for stv and other heat-shock proteins in chill-shock tests and 2) enzymatic enhancement of Hex-T1, a rate-limiting regulatory enzyme that controls the entry of glucose into the glycolytic pathway. Together, our analyses highlight the adaptive molecular changes that support colonization of high-altitude environments.

Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants

Evolutionary innovations underlie the rise of diversity and complexity-the 2 long-term trends in the history of life. How does natural selection redesign multiple interacting parts to achieve a new emergent function? We investigated the evolution of a biomechanical innovation, the latch-spring mechanism of trap-jaw ants, to address 2 outstanding evolutionary problems: how form and function change in a system during the evolution of new complex traits, and whether such innovations and the diversity they beget are repeatable in time and space. Using a new phylogenetic reconstruction of 470 species, and X-ray microtomography and high-speed videography of representative taxa, we found the trap-jaw mechanism evolved independently 7 to 10 times in a single ant genus (Strumigenys), resulting in the repeated evolution of diverse forms on different continents. The trap mechanism facilitates a 6 to 7 order of magnitude greater mandible acceleration relative to simpler ancestors, currently the fastest recorded acceleration of a resettable animal movement. We found that most morphological diversification occurred after evolution of latch-spring mechanisms, which evolved via minor realignments of mouthpart structures. This finding, whereby incremental changes in form lead to a change of function, followed by large morphological reorganization around the new function, provides a model for understanding the evolution of complex biomechanical traits, as well as insights into why such innovations often happen repeatedly.

Evidence for a thoracic crop in the workers of some Neotropical Pheidole species (Formicidae: Myrmicinae)

The ability of ant colonies to transport, store, and distribute food resources through trophallaxis is a key advantage of social life. Nonetheless, how the structure of the digestive system has adapted across the ant phylogeny to facilitate these abilities is still not well understood. The crop and proventriculus, structures in the ant foregut (stomodeum), have received most attention for their roles in trophallaxis. However, potential roles of the esophagus have not been as well studied. Here, we report for the first time the presence of an auxiliary thoracic crop in Pheidole aberrans and Pheidole deima using X-ray micro-computed tomography and 3D segmentation. Additionally, we describe morphological modifications involving the endo- and exoskeleton that are associated with the presence of the thoracic crop. Our results indicate that the presence of a thoracic crop in major workers suggests their potential role as repletes or live food reservoirs, expanding the possibilities of tasks assumed by these individuals in the colony. Our contribution emphasizes the utility of combining data from external and internal morphology to better understand functional and behavioral mechanisms.

Ants of the Hengduan Mountains: a new altitudinal survey and updated checklist for Yunnan Province highlight an understudied insect biodiversity hotspot

China's Hengduan Mountain region has been considered one of the most diverse regions in the northern hemisphere. Its stunning topography with many deep valleys and impassable mountain barriers has promoted an astonishing diversification in many groups of organisms including plants, birds, mammals, and amphibians. However, the insect biodiversity in this region is still poorly known. Here, the first checklist of ant species from the Southern Hengduan Mountain region is presented, generated by sampling ant diversity using a wide array of collection methods, including Winkler leaf litter extraction, vegetation beating, and hand collection. 130 species/morphospecies from nine subfamilies and 49 genera were identified. Among them, 17 species from 13 genera represent new records for Yunnan province, and eight species are newly recorded for China. Moreover, we believe 41 novel morphospecies (31% of the total collected taxa) will prove to be new to science. These results highlight the rich ant fauna of this region and strongly support its status as a biodiversity hotspot. The current ant species checklist for the whole of Yunnan Province was updated by recording 550 named species from 99 genera. Taken together, our results suggest that the Yunnan ant fauna still remains under-sampled, and future sampling will likely yield many more species, among them many undescribed ones.

The loss of flight in ant workers enabled an evolutionary redesign of the thorax for ground labour

Background

Explanations for the ecological dominance of ants generally focus on the benefits of division of labour and cooperation during foraging. However, the principal innovation of ants relative to their wasp ancestors was the evolution of a new phenotype: a wingless worker caste optimized for ground labour. Ant workers are famous for their ability to lift and carry heavy loads, but we know surprisingly little about the morphological basis of their strength. Here we examine the consequences of the universal loss of flight in ant workers on skeletomuscular adaptations in the thorax for enhanced foraging on six legs.

Results

Using X-ray microcomputed tomography and 3D segmentation, we compared winged queens and wingless workers in Euponera sikorae (subfamily Ponerinae) and Cataglyphis savignyi (subfamily Formicinae). Workers are characterized by five major changes to their thorax: i) fusion of the articulated flight thorax (queens) into a rigid box optimized to support the muscles that operate the head, legs and abdomen, ii) redesign of internal cuticular structures for better bracing and muscle attachment, iii) substantial enlargement of the neck muscles for suspending and moving the head, iv) lengthening of the external trochanter muscles, predominant for the leg actions that lift the body off the ground, v) modified angle of the petiole muscles that are key for flexion of the abdomen. We measured volumes and pennation angles for a few key muscles to assess their increased efficacy. Our comparisons of additional workers across five genera in subfamilies Dorylinae and Myrmicinae show these modifications in the wingless thorax to be consistent. In contrast, a mutillid wasp showed a different pattern of muscle adaptations resulting from the lack of wing muscles.

Conclusions

Rather than simply a subtraction of costly flight muscles, we propose the ant worker thorax evolved into a power core underlying stronger mandibles, legs, and sting. This contrasts with solitary flightless insects where the lack of central place foraging generated distinct selective pressures for rearranging the thorax. Stronger emphasis is needed on morphological innovations of social insects to further our understanding of the evolution of social behaviours.

The Ants (Hymenoptera, Formicidae) of Sri Lanka: a taxonomic research summary and updated checklist

An updated checklist of the ants (Hymenoptera: Formicidae) of Sri Lanka is presented. These include representatives of eleven of the 17 known extant subfamilies with 341 valid ant species in 79 genera. Lioponeralongitarsus Mayr, 1879 is reported as a new species country record for Sri Lanka. Notes about type localities, depositories, and relevant references to each species record are given. Accounts of the dubious and some undetermined species from Sri Lanka are also provided. 82 species (24%) are endemic whereas 18 species that are non-native to Sri Lanka are recorded. The list provides a synthesis of the regional taxonomical work carried out to date and will serve as a baseline for future studies on the ant fauna of this biodiversity hotspot.

Macroevolutionary integration of phenotypes within and across ant worker castes

Phenotypic traits are often integrated into evolutionary modules: sets of organismal parts that evolve together. In social insect colonies, the concepts of integration and modularity apply to sets of traits both within and among functionally and phenotypically differentiated castes. On macroevolutionary timescales, patterns of integration and modularity within and across castes can be clues to the selective and ecological factors shaping their evolution and diversification. We develop a set of hypotheses describing contrasting patterns of worker integration and apply this framework in a broad (246 species) comparative analysis of major and minor worker evolution in the hyperdiverse ant genus Pheidole. Using geometric morphometrics in a phylogenetic framework, we inferred fast and tightly integrated evolution of mesosoma shape between major and minor workers, but slower and more independent evolution of head shape between the two worker castes. Thus, Pheidole workers are evolving as a mixture of intracaste and intercaste integration and rate heterogeneity. The decoupling of homologous traits across worker castes may represent an important process facilitating the rise of social complexity.

Colonize, radiate, decline: Unraveling the dynamics of island community assembly with Fijian trap-jaw ants

The study of island community assembly has been fertile ground for developing and testing theoretical ideas in ecology and evolution. The ecoevolutionary trajectory of lineages after colonization has been a particular interest, as this is a key component of understanding community assembly. In this system, existing ideas, such as the taxon cycle, posit that lineages pass through a regular sequence of ecoevolutionary changes after colonization, with lineages shifting toward reduced dispersal ability, increased ecological specialization, and declines in abundance. However, these predictions have historically been difficult to test. Here, we integrate phylogenomics, population genomics, and X-ray microtomography/3D morphometrics, to test hypotheses for whether the ecomorphological diversity of trap-jaw ants (Strumigenys) in the Fijian archipelago is assembled primarily through colonization or postcolonization radiation, and whether species show ecological shifts toward niche specialization, toward upland habitats, and decline in abundance after colonization. We infer that most Fijian endemic Strumigenys evolved in situ from a single colonization and have diversified to fill a large fraction of global morphospace occupied by the genus. Within this adaptive radiation, lineages trend to different degrees toward high elevation, reduced dispersal ability, and demographic decline, and we find no evidence of repeated colonization that displaces the initial radiation. Overall these results are only partially consistent with taxon cycle and associated ideas, while highlighting the potential role of priority effects in assembling island communities.

Distal leg structures of the Aculeata (Hymenoptera): A comparative evolutionary study of Sceliphron (Sphecidae) and Formica (Formicidae)

The distal parts of the legs of Sceliphron caementarium (Sphecidae) and Formica rufa (Formicidae) are documented and discussed with respect to phylogenetic and functional aspects. The prolegs of Hymenoptera offer an array of evolutionary novelties, mainly linked with two functional syndromes, walking efficiently on different substrates and cleaning the body surface. The protibial-probasitarsomeral cleaning device is almost always well-developed. A complex evolutionary innovation is a triple set of tarsal and pretarsal attachment devices, including tarsal plantulae, probasitarsomeral spatulate setae, and an arolium with an internal spring-like arcus, a dorsal manubrium, and a ventral planta. The probasitarsal adhesive sole and a complex arolium are almost always preserved, whereas the plantulae are often missing. Sceliphron has retained most hymenopteran ground plan features of the legs, and also Formica, even though the adhesive apparatus of Formicidae shows some modifications, likely linked to ground-oriented habits of most ants. Plantulae are always absent in extant ants, and the arolium is often reduced in size, and sometimes vestigial. The arolium contains resilin in both examined species. Additionally, resilin enriched regions are also present in the antenna cleaners of both species, although they differ in which of the involved structures is more flexible, the calcar in Sceliphron and the basitarsal comb in Formica. Functionally, the hymenopteran distal leg combines (a) interlocking mechanisms (claws, spine-like setae) and (b) adhesion mechanisms (plantulae, arolium). On rough substrate, claws and spine-like setae interlock with asperities and secure a firm grip, whereas the unfolding arolium generates adhesive contact on smooth surfaces. Differences of the folded arolium of Sceliphron and Formica probably correlate with differences in the mechanism of folding/unfolding.

TREEasy: An automated workflow to infer gene trees, species trees, and phylogenetic networks from multilocus data

Multilocus genomic data sets can be used to infer a rich set of information about the evolutionary history of a lineage, including gene trees, species trees, and phylogenetic networks. However, user-friendly tools to run such integrated analyses are lacking, and workflows often require tedious reformatting and handling time to shepherd data through a series of individual programs. Here, we present a tool written in Python-TREEasy-that performs automated sequence alignment (with MAFFT), gene tree inference (with IQ-Tree), species inference from concatenated data (with IQ-Tree and RaxML-NG), species tree inference from gene trees (with ASTRAL, MP-EST, and STELLS2), and phylogenetic network inference (with SNaQ and PhyloNet). The tool only requires FASTA files and nine parameters as inputs. The tool can be run as command line or through a Graphical User Interface (GUI). As examples, we reproduced a recent analysis of staghorn coral evolution, and performed a new analysis on the evolution of the "WGD clade" of yeast. The latter revealed novel patterns that were not identified by previous analyses. TREEasy represents a reliable and simple tool to accelerate research in systematic biology (https://github.com/MaoYafei/TREEasy).

Genomic and phenomic analysis of island ant community assembly

Island biodiversity has long fascinated biologists as it typically presents tractable systems for unpicking the eco-evolutionary processes driving community assembly. In general, two recurring themes are of central theoretical interest. First, immigration, diversification, and extinction typically depend on island geographical properties (e.g., area, isolation, and age). Second, predictable ecological and evolutionary trajectories readily occur after colonization, such as the evolution of adaptive trait syndromes, trends toward specialization, adaptive radiation, and eventual ecological decline. Hypotheses such as the taxon cycle draw on several of these themes to posit particular constraints on colonization and subsequent eco-evolutionary dynamics. However, it has been challenging to examine these integrated dynamics with traditional methods. Here, we combine phylogenomics, population genomics and phenomics, to unravel community assembly dynamics among Pheidole (Hymenoptera, Formicidae) ants in the isolated Fijian archipelago. We uphold basic island biogeographic predictions that isolated islands accumulate diversity primarily through in situ evolution rather than dispersal, and population genomic support for taxon cycle predictions that endemic species have decreased dispersal ability and demography relative to regionally widespread taxa. However, rather than trending toward island syndromes, ecomorphological diversification in Fiji was intense, filling much of the genus-level global morphospace. Furthermore, while most endemic species exhibit demographic decline and reduced dispersal, we show that the archipelago is not an evolutionary dead-end. Rather, several endemic species show signatures of population and range expansion, including a successful colonization to the Cook islands. These results shed light on the processes shaping island biotas and refine our understanding of island biogeographic theory.

A Morphological Integration Perspective on the Evolution of Dimorphism among Sexes and Social Insect Castes

Many species have evolved alternate phenotypes, thus enabling individuals to conditionally produce phenotypes that are favorable for reproductive success. Examples of this phenomenon include sexual dimorphism, alternative reproductive strategies, and social insect castes. While the evolutionary functions and developmental mechanisms of dimorphic phenotypes have been studied extensively, little attention has focused on the evolutionary covariance between each phenotype. We extend the conceptual framework and methods of morphological integration to hypothesize that dimorphic traits tend to be less integrated between sexes or social castes. In the case of social insects, we describe results from our recent study of an ant genus in which workers have major and minor worker castes that perform different behavioral repertoires in and around the nest. In the case of birds, we describe a new analysis of a family of songbirds that exhibits plumage coloration that can differ greatly between males and females, with apparently independent changes in each sex. Ant head shape, which is highly specialized in each worker caste, was weakly integrated between worker castes, whereas thorax shape, which is more monomorphic, was tightly integrated. Similarly, in birds, we found a negative association between dimorphism and the degree of integration between sexes. We also found that integration decreased in fairy wrens (Malurus) for many feather patches that evolved greater dichromatism. Together, this suggests that the process of evolving increased dimorphism results in a decrease in integration between sexes and social castes. We speculate that once a mechanism for dimorphism evolves, that mechanism can create independent variation in one sex or caste upon which selection may act.

Evolution of a multifunctional trait: shared effects of foraging ecology and thermoregulation on beak morphology, with consequences for song evolution

While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function not only in foraging but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however, most focus on a single function. Hence, we quantified relative contributions of different functions over an evolutionary timescale. We measured beak shape using geometric morphometrics and compared this trait with foraging behaviour, climatic variables and song characteristics in a phylogenetic comparative study of an Australasian radiation of songbirds (Meliphagidae). We found that both climate and foraging behaviour were significantly correlated with the beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a nearly equal effect on size. We also found that evolutionary changes in beak morphology had significant consequences for vocal performance: species with elongate-shaped beaks sang at higher frequencies, while species with large beaks sang at a slower pace. The evolution of the avian beak exemplifies how morphological traits can be an evolutionary compromise among functions, and suggests that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.

A test of trophic and functional island biogeography theory with the avifauna of a continental archipelago

The classical MacArthur-Wilson theory of island biogeography (TIB) emphasizes the role of island area and isolation in determining island biotas, but is neutral with respect to species differences that could affect community assembly and persistence. Recent extensions of island biogeography theory address how functional differences among species may lead to non-random community assembly processes and different diversity-area scaling patterns. First, the trophic TIB considers how diversity scaling varies across trophic position in a community, with species at higher trophic levels being most strongly influenced by island area. Second, further extensions have predicted how trait distributions, and hence functional diversity, should scale with area. Trait-based theory predicts richness-corrected functional diversity should be low on small islands but converge to null on larger islands. Conversely, competitive assembly predicts high diversity on small islands converging to null with increasing size. However, despite mounting interest in diversity-area relationships across different dimensions of diversity, these predictions derived from theory have not been extensively tested across taxa and island systems. Here, we develop and test predictions of the trophic TIB and extensions to functional traits, by examining the diversity-area relationship across multiple trophic ranks and dimensions of avian biodiversity in the Ryūkyū archipelago of Japan. We find evidence for a positive species- and phylogenetic diversity-area relationship, but functional diversity was not strongly affected by island area. Counter to the trophic TIB, we found no differences in the slopes of species-area relationships among trophic ranks, although slopes varied among trophic guilds at the same rank. We revealed differential assembly of trophic ranks, with evidence of trait-based assembly of intermediate predators but otherwise neutral community assembly. Our results suggest that niche space differs among trophic guilds of birds, but that differences are mostly not predicted by current extensions of island biogeography theory. While predicted patterns do not fit the empirical data well in this case, the development of such theory provides a useful framework to analyse island patterns from new perspectives. The application of empirical datasets such as ours should help provide a basis for developing further iterations of island biogeography theory.

The cephalic anatomy of workers of the ant species Wasmannia affinis (Formicidae, Hymenoptera, Insecta) and its evolutionary implications

Despite the ecological significance of ants and the intensive research attention they have received, thorough treatments of the anatomy and functional morphology are still scarce. In this study we document the head morphology of workers of the myrmicine Wasmannia affinis with optical microscopy, μ-computed tomography, scanning electron microscopy, and 3D reconstruction, providing the first complete anatomical treatment of an ant head with a broad array of modern techniques. We discuss the potential of the applied methods to generate detailed and well-documented morphological data sets with increased efficiency. We also address homology problems, particularly in the context of the cephalic digestive tract. According to our analyses the "pharynx" of previous ant studies is homologous to the prepharynx of other insects. We also discuss the phylogenetic potential and functional significance of the observed characters, with internal features such as tentorium and musculature discussed for the first time. Our investigation underlines that detailed anatomical data for Formicidae are still very fragmentary, which in turn limits our understanding of the major design elements underlying the ant bauplan. We attempt to provide a template for further anatomical studies, which will help to understand the evolution of this fascinating group on the phenotypic level.

The Roles of Introgression and Climate Change in the Rise to Dominance of Acropora Corals

Reef-building corals provide the structural basis for one of Earth's most spectacular and diverse-but increasingly threatened-ecosystems. Modern Indo-Pacific reefs are dominated by species of the staghorn coral genus Acropora, but the evolutionary and ecological factors associated with their diversification and rise to dominance are unclear. Recent work on evolutionary radiations has demonstrated the importance of introgression and ecological opportunity in promoting diversification and ecological success. Here, we analyze the genomes of five staghorn coral species to examine the roles of introgression and ecological opportunity in the rise to dominance of Acropora. We found evidence for a history marked by a major introgression event as well as recurrent gene flow across species. In addition, we found that genes with topologies mismatching the species tree are evolving faster, which is suggestive of a role for introgression in spreading adaptive genetic variation. Demographic analysis showed that Acropora lineages profited from climate-driven mass extinctions in the Plio-Pleistocene, indicating that Acropora exploited ecological opportunity opened by a new climatic regime favoring species that could cope with rapid sea-level changes. Collectively, the genomes of reef-building corals have recorded an evolutionary history shaped by introgression and climate change, suggesting that Acropora-among most vulnerable corals to stressors-may be critical for understanding how reefs track the impending rapid sea-level changes of the Anthropocene.

Skeletomuscular adaptations of head and legs of Melissotarsus ants for tunnelling through living wood

Background

While thousands of ant species are arboreal, very few are able to chew and tunnel through living wood. Ants of the genus Melissotarsus (subfamily Myrmicinae) inhabit tunnel systems excavated under the bark of living trees, where they keep large numbers of symbiotic armoured scale insects (family Diaspididae). Construction of these tunnels by chewing through healthy wood requires tremendous power, but the adaptations that give Melissotarsus these abilities are unclear. Here, we investigate the morphology of the musculoskeletal system of Melissotarsus using histology, scanning electron microscopy, X-ray spectrometry, X-ray microcomputed tomography (micro-CT), and 3D modelling.

Results

Both the head and legs of Melissotarsus workers contain novel skeletomuscular adaptations to increase their ability to tunnel through living wood. The head is greatly enlarged dorsoventrally, with large mandibular closer muscles occupying most of the dorsal half of the head cavity, while ventrally-located opener muscles are also exceptionally large. This differs from the strong closing: opening asymmetry typical of most mandibulated animals, where closing the mandibles requires more force than opening. Furthermore, the mandibles are short and cone-shaped with a wide articulatory base that concentrates the force generated by the muscles towards the tips. The increased distance between the axis of mandibular rotation and the points of muscle insertion provides a mechanical advantage that amplifies the force from the closer and opener muscles. We suggest that the uncommonly strong opening action is required to move away crushed plant tissues during tunnelling and allow a steady forward motion. X-ray spectrometry showed that the tip of the mandibles is reinforced with zinc. Workers in this genus have aberrant legs, including mid- and hindlegs with hypertrophied coxae and stout basitarsi equipped with peg-like setae, and midleg femura pointed upward and close to the body. This unusual design famously prevents them from standing and walking on a normal two-dimensional surface. We reinterpret these unique traits as modifications to brace the body during tunnelling rather than locomotion per se.

Conclusions

Melissotarsus represents an extraordinary case study of how the adaptation to – and indeed engineering of – a novel ecological niche can lead to the evolutionary redesign of core biomechanical systems.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0277-6) contains supplementary material, which is available to authorized users.