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Organisms from temperate zones are exposed to seasonal changes and must be able to cope with a wide range of climatic conditions. Especially ectotherms, including insects, are at risk to desiccate under dry and warm conditions, the more so given the changing climate. To adjust to current conditions, organisms acclimate through changes in physiology, morphology and/or behaviour. Insects protect themselves against desiccation through a layer of cuticular hydrocarbons (CHC) on their body surface. Hence, acclimation may also affect the CHC profile, changing their waterproofing capacity under different climatic conditions. Here, we investigated the acclimation response of two Temnothorax ant species to different climatic conditions. We analysed CHC profiles of queens, nurses and foragers that were acclimated to different humidity × temperature regimes, and tested the beneficial acclimation hypothesis by measuring survival of workers under desiccation‐heat stress. Both ants possessed a species‐specific CHC profile. Nevertheless, they showed similar acclimation responses concerning changes of certain CHC classes, and finally similar survival rates under desiccation‐heat stress. Warm‐acclimated individuals generally showed longer n‐alkanes, fewer dimethyl alkanes, and more (workers) or less (queens) monomethyl alkanes. In contrast, dry conditions resulted in more n‐alkanes and fewer mono‐ and dimethyl alkanes, but these acclimatory changes were only observed in workers and not in queens. Warm‐ and dry‐acclimated workers survived desiccation‐heat stress better, but we found no species differences. Our results indicate that both ant species can plastically adjust their cuticular hydrocarbon profile, allowing them to acclimate to different climatic conditions. Although their CHC composition differs in a species‐specific manner, they showed similar chemical adjustments and concomitant changes in survival rate. Hence, chemical plasticity may be critical to determine a species’ climatic range and its survival under changing climatic conditions. A plain language summary is available for this article. Plain Language Summary

The origin of complex worker-caste systems in ants perplexed Darwin 1 and has remained an enduring problem for evolutionary and developmental biology 2 – 6 . Ants originated approximately 150 million years ago, and produce colonies with winged queen and male castes as well as a wingless worker caste 7 . In the hyperdiverse genus Pheidole , the wingless worker caste has evolved into two morphologically distinct subcastes—small-headed minor workers and large-headed soldiers 8 . The wings of queens and males develop from populations of cells in larvae that are called wing imaginal discs 7 . Although minor workers and soldiers are wingless, vestiges or rudiments of wing imaginal discs appear transiently during soldier development 7 , 9 – 11 . Such rudimentary traits are phylogenetically widespread and are primarily used as evidence of common descent, yet their functional importance remains equivocal 1 , 12 – 14 . Here we show that the growth of rudimentary wing discs is necessary for regulating allometry—disproportionate scaling—between head and body size to generate large-headed soldiers in the genus Pheidole . We also show that Pheidole colonies have evolved the capacity to socially regulate the growth of rudimentary wing discs to control worker subcaste determination, which allows these colonies to maintain the ratio of minor workers to soldiers. Finally, we provide comparative and experimental evidence that suggests that rudimentary wing discs have facilitated the parallel evolution of complex worker-caste systems across the ants. More generally, rudimentary organs may unexpectedly acquire novel regulatory functions during development to facilitate adaptive evolution. In the ant genus Pheidole the growth of rudimentary wing discs—which influence developmental allometry to produce castes with distinct morphologies—is socially regulated to determine the worker-to-soldier ratio in Pheidole colonies.

The high diversity in ants has been associated with some genomic divergences, including karyotype, nuclear genome size and DNA sequences. This study investigated if DNA ploidy levels varies among organs of the worker subcastes of the carpenter ant Camponotus aff . balzani. We also examined if DNA ploidy levels variation may contribute to variations in body size and functional specialization of the worker subcastes and its potential reproductive and behavioral differences among castes. Different DNA ploidy level were identified in the brain, post-pharyngeal gland, thoracic muscles, abdominal sternite muscles and midgut of the workers, resulting in similarities and divergences about the endopolyploid index (EI). The major workers presented the highest EI in the post-pharyngeal gland and thoracic muscles compared to median and minor workers, which may be associated with the different tasks that workers perform in the colony. The lowest EI was found in the brain of all workers subcastes. Moreover, gynes exhibited higher EI levels compared to males and worker subcastes. These findings suggest a complex interplay of genetics and caste determination in this ant. Overall, the results provide fundamental contributions for further studies to verify the role of endopolyploidy in the phenotypic plasticity among castes and subcastes of Camponotus aff . balzani , with potential implications for understanding similar processes in other social organisms.

A main challenge in the analysis of division of labor in insect societies characterized by worker polymorphism has been identifying the number of physical castes and determining their relationship to task performance. We addressed this question using the extremely polymorphic leafcutter ant Atta cephalotes as a model by measuring 22 morphological characters and applying multivariate Gaussian mixture modeling to define worker size-class boundaries. Our statistical approach discriminated five physical worker size classes (subcastes) and found continuous variation in most morphological characters. Some worker size classes showed patterns of covariance, modularity, and integration in head, mandible, and leg traits, suggesting biomechanical functionality in tasks such as leaf cutting. Task repertoires and act frequencies were recorded to identify how fungal gardening, brood care, leaf-harvesting, and other tasks were distributed across worker size groups. Results showed that small and mid-sized media workers performed more diverse and complex tasks, including leaf harvesting, than workers of other size classes. Minims participated in fungal-gardening and nursing tasks, whereas large medias mainly participated in leafcutting and majors made few direct contributions to fungal cultivation. Results of our integrative analyses demonstrate the existence of an unexpectedly large number of worker size classes that can differ significantly in performance of agricultural tasks. However, overlap in task repertoires of workers at size-class boundaries suggests that not all morphologically distinct groups display discrete behavioral profiles. Morphological groups are instead distinguished by a combination of the tasks that each performs and their relative frequencies.Significance statementThe evolutionary coupling of behavior and morphology is central to understanding division of labor in insect societies. Our work applies robust statistical modelling to offer a novel analysis of worker physical caste categorization based on extensive morphometric sampling. Our method improves upon previous approaches by incorporating measurements of a large number of diverse morphological traits likely involved in task performance to examine worker physical caste evolution. We integrate morphological evolution with behavioral data on relative task performance rates and task repertoire size to describe the morphological and behavioral space of size-variable workers. Our approach provides methods to quantitatively analyze division of labor and behavioral performance, and offers new insights into worker trait modularity and integration and task performance associated with the organization of complex insect societies.

Altruistic caste, including worker and soldier (derived from worker), plays a critical role in the ecological success of social insects. The proportion of soldiers, soldier sex ratios, and the number of workers vary significantly between species, and also within species, depending on colony developmental stage and environmental factors. However, it is unknown whether there are sex-linked effects from parents on controlling the caste fate or not. Here, we compared soldier sex ratios, soldier proportions, and population size among a four mating types of Reticulitermes amamianus (Ra) and R . speratus (Rs) (male × female, mRa × fRa, mRa × fRs, mRs × fRa, mRs × fRs) and demonstrate that the soldier sex ratio and worker population size of hybrid colonies skew to colonies of king’s species, while the soldier proportion skew to queen’s species. The survival rate of offspring resulting from interspecies hybridization was significantly higher for mRa × fRs than for mRs × fRa. The results of this study demonstrate the asymmetric influence of kings and queens on caste determination and colony growth, which can contribute to our better understanding of parental influence on the colony dynamics of social insects.

One of the main aspects associated with the diversity in animal colour is the variation in melanization levels. In ectotherms, melanism can be advantageous in aiding thermoregulation through heat absorption. Darker bodies may also serve as a shield from harmful UV-B radiation. Melanism may also confer protection against parasites and predators through improving immunity responses and camouflage in regions with high precipitation, with complex and shaded vegetations and greater diversity of pathogens and parasites. We studied melanism evolution in the globally distributed ant genus Pheidole under the pressures of temperature, UV-B radiation and precipitation, while considering the effects of body size and nest habit, traits that are commonly overlooked. More importantly, we account for worker caste polymorphism, which is marked by distinct roles and behaviours. We revealed for the first time distinct evolutionary trajectories for each worker subcaste. As expected, major workers from species inhabiting locations with lower temperatures and higher precipitation tend to be more melanised. Curiously, we show a slight trend where minor workers of larger species also tend to have darker bodies when inhabiting regions with higher precipitation. Lastly, we did not find evidence for the effects of UV-B radiation and nest habit in the lightness variation of workers. Our paper explores the evolution of ant melanization considering a marked ant worker polymorphism and a wide range of ecological factors. We discuss our findings under the light of the Thermal Melanism Hypothesis, the Photoprotection Hypothesis and the Gloger’s Rule.

The red imported fire ant Solenopsis invicta is an invasive pest in the USA, eastern Asia, and Australia that causes billions worth of damage where it has been introduced. In the insect colony, workers perform tasks based on their age as well as the subcaste, where the younger workers tend to remain in the nest and tend to the brood while the older workers leave the nest to perform activities such as foraging. In eusocial insects, juvenile hormone has been identified to be a catalyst for behavioral changes among the worker caste, but the involvement of this hormone in S. invicta task allocation has not been investigated. Here, we conducted RNA-seq analyses to identify genes associated with worker division of labor. We compared the expression profiles of foragers and nurses and found 816 differentially expressed genes. We also identified 100 differentially expressed genes between nurses treated with acetone and nurses treated with a juvenile hormone analog. For this study, we focused on the differentially expressed genes between foragers and nurses that were associated with energy metabolism, glycolysis, juvenile hormone synthesis, metabolism, and immunity because these pathways have been identified as differentially expressed between foragers and nurses in different social insects. We also identified changes in gene expression induced by the juvenile hormone analog, some of which were as expected if the juvenile hormone is involved in regulating the shift from nursing to foraging in S. invicta workers. Overall, our results support a potential role of juvenile hormone in S. invicta task transition probably in association with other factors such as insect age or nutritional status which were not controlled in this experiment.

Subterranean termites live in underground colonies with a division of labor among castes (i.e., queens and kings, workers, and soldiers). The function of social colonies relies on sophisticated chemical communication. Olfaction, the sense of smell from food, pathogens, and colony members, plays an important role in their social life. Olfactory plasticity in insects can be induced by long- and short-term environmental perturbations, allowing adaptive responses to the chemical environment according to their physiological and behavioral state. However, there is a paucity of information on the molecular basis of olfaction in termites. In this study, we identified an ortholog encoding the odorant receptor co-receptor (Orco) in the Formosan subterranean termite, Coptotermes formosanus , and examined its expression variation across developmental stages and in response to social conditions. We found that C . formosanus Orco showed conserved sequence and structure compared with other insects. Spatial and temporal analyses showed that the Orco gene was primarily expressed in the antennae, and it was expressed in eggs and all postembryonic developmental stages. The antennal expression of Orco was upregulated in alates (winged reproductives) compared with workers and soldiers. Further, the expression of Orco decreased in workers after starvation for seven days, but it was not affected by the absence of soldiers or different group sizes. Our study reveals the molecular characteristics of Orco in a termite, and the results suggest a link between olfactory sensitivity and nutritional status. Further studies are warranted to better understand the role of Orco in olfactory plasticity and behavioral response.

Background The insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins. The thickness and synthesis of the exocuticle depend on different castes that perform various functions in alates, workers and soldiers. However, it is not clear whether the soft endocuticle is involved in the division of labour in termite colonies. To reveal the structural characteristics of the endocuticle in different castes, we investigated the thickness of endocuticle layers in alates, workers and soldiers of the termite Reticulitermes aculabialis , and then we sequenced their transcriptome and detected the endocuticle protein genes. The differential expression levels of the endocuticular protein genes were confirmed in the three castes. Results We found that there was a great difference in the thickness of the endocuticle among the alates, soldiers and workers. The thickest endocuticle layers were found in the heads of the workers 7.88 ± 1.67 μm. The endocuticle layer in the head of the workers was approximately three-fold and nine-fold thicker than that in the heads of soldiers and alates, respectively. The thinnest endocuticle layers occurred in the head, thorax and abdomen of alates, which were 0.86 ± 0.15, 0.76 ± 0.24 and 0.52 ± 0.17 μm thick, respectively, and had no significant differences. A total of 43,531,650 clean sequencing reads was obtained, and 89,475 unigenes were assembled. Of the 70 identified cuticular protein genes, 10 endocuticular genes that belong to the RR-1 family were selected. qRT-PCR analysis of the five endocuticular genes (SgAbd-2, SgAbd-9, Abd-5, SgAbd-2-like and Abd-4-like) revealed that the endocuticle genes were more highly expressed in workers than in soldiers and alates. Conclusion These results suggest that SgAbd and Abd are the key components of the endocuticle. We infer that the thicker endocuticle in workers is helpful for them to perform their functions against environmental stress.

Polyphenisms in social insects arise from flexible developmental mechanisms that may produce behaviorally and morphologically specialized workers. The ant genus Pheidole is typically characterized by small minor worker and large soldier subcastes, but larger supersoldiers have evolved in several species, including the exceptionally polymorphic Pheidole rhea. To examine worker phenotype evolution in this socially complex ant, we tested the hypotheses that behavior, brain structure, and morphology are integrated within worker size classes due to selection for specialized social roles and that traits have been discretized among these groups. Our analyses revealed significant differences in brain structure and body shape between minor workers and the two soldier size classes. Behavioral repertoires of minors, soldiers, and supersoldiers were similar, but minors performed important tasks at higher frequencies than soldiers and supersoldiers. The extensive overlap in behavioral repertoire between soldier and supersoldier groups correlated with shared neuroanatomical and morphological traits. Although head and body size vary allometrically among P. rhea size classes, scaling analyses revealed little allometry in brain substructure. The covariation of behavior and brain organization between soldier size classes suggests that their functionality may be due to differences in task performance rate or efficiency rather than task repertoire. The early branching position of P. rhea in the phylogeny of Pheidole and patterns of worker phenotypic plasticity suggest this species exhibits an ancestral state of sociobiological and neurobiological organization that served as a ground plan for diversification in this ecologically dominant ant genus. Selection for divergent social roles may impact the degree to which behavioral, neuroanatomical, and morphological phenotypes are integrated within individuals and between specialized groups. Behavioral repertoire, neuroanatomy, and worker body shape in the strongly polymorphic P. rhea, a species of the hyperdiverse ant genus Pheidole that branched early in the phylogeny of the clade appear to show a relatively low degree of integration with worker head-body size. The putatively ancestral and developmentally plastic condition in which largerbodied soldier groups are not distinguishable in behavior, brain scaling relationships, and body shape suggests the advantage of worker head-body allometry may derive from variation in quantitative aspects of behavioral performance rather than specialization on qualitatively different tasks. This appears to be significant to social evolution in the genus.