Defense in Social Insects: Diversity, Division of Labor, and Evolution

Tool use aids prey-fishing in a specialist predator of stingless bees

Tool use is widely reported across a broad range of the animal kingdom, yet comprehensive empirical tests of its function and evolutionary drivers remain scarce, predominantly focused on a few relatively intelligent vertebrate lineages. In this study, we provide a comprehensive examination of tool use behavior in the assassin bug Pahabengkakia piliceps, a specialist predator of stingless bees that exploits resin droplet from bee nest entrance to facilitate hunting. Field behavioral experiments demonstrated that resin use is critical for hunting success, as the predator uses resin to stimulate the colonial defense of stingless bees, luring attacking bees toward its optimal hunting position. Chemical analysis revealed that resin processing by the assassin bug enhances the emission of volatile compounds, making the resin more attractive and stimulating to guard bees. Through these experiments, we empirically demonstrate how an invertebrate predator adapts to the colony defense of social insects via tool use behavior. We further propose that complex tool use can evolve under selective pressures driven by diet specialization. Our findings offer a new model for studying the adaptive functions and underlying mechanisms of tool use behaviors in animals.

Protein-rich trophallactic contents transferred from reproductives are crucial for termite soldier differentiation in Zootermopsis nevadensis

Polyphenic castes exist for colony success in social animals. A primer pheromone has been suggested as a well-known regulator of caste development, but the basis of chemical communication has remained elusive over the last few decades. In termites, a long-standing hypothesis is that reproductive-secreted juvenile hormone (JH) plays a role as a primer pheromone involved in soldier differentiation. Here, we tested whether exogenous JH is necessary to induce differentiation of soldiers in the dampwood termite Zootermopsis nevadensis. The unique characteristic of this termite is that soldier-destined individuals can be monitored during their ontogeny under natural conditions. Furthermore, oral-anal trophallactic interactions with reproductives are required for soldier differentiation. First, knockdown of JH biosynthetic genes using RNA interference (RNAi) resulted in the reduction of soldier formation but did not affect the frequency of trophallactic interaction from reproductives to RNAi-treated individuals. Next, we compared the effect of parental replacement on soldier differentiation. Old-age reproductives had fewer stimulus effects on the differentiation of soldier-destined individuals compared to young-age reproductives. This difference in stimulus effects was derived from the protein levels within the digestive fluids of the king, but not those of the queen. Consequently, there is little or no possibility that JH itself plays a role as a primer pheromone. Alternatively, we propose that protein-rich nutrients in trophallactic fluids from reproductives are sufficient to induce high JH levels in larvae required for soldier differentiation.

Morphology of the head-associated exocrine glands in Cornitermes cumulans with the description of a novel gland for the worker caste

Exocrine glands are important mediators of communication in eusocial insects and the description of novel glands reflects the complex context in which these animals live. Here we revisit the head-associated glands in workers of the Neotropical termite Cornitermes cumulans through histological analysis and describe a novel gland for this caste, the intramandibular glands. This structure is located underneath the cuticle of the dorsodistal part of each mandible. The glands showed an epithelial arrangement, but the cytological morphology is complex, comprising classes I and III of secretory cells. The present data highlight the importance of the intramandibular glands in the worker caste and demonstrate different morphology of these glands in Isoptera, probably related to the specialized function of the castes. Features of active glandular activity were also observed in the mandibular, labral, and salivary glands of these workers. Despite the intramandibular glands being also found in workers of other social insects, their occurrence in termites was restricted to the soldier caste of Machadotermes. Even so, their cytological structure differs from those of C. cumulans workers. The likely function of the worker intramandibular glands is discussed considering the separate-nest life type present by C. cumulans and tasks performed by workers. The chemical nature of the secretion and the occurrence of the glands in other termite taxa still require further investigation.

Division of labour during honeybee colony defence: poetic and scientific views

Poets, philosophers and politicians have used bees, and often projected an idealized human society into their view of how beehives are organized, from the ancient Greeks to present times. We first review how division of labour in honeybees was perceived by human observers, before presenting our current understanding. We focus specifically on defensive behaviour and show that this model provides an interesting case study for our conceptual understanding of division of labour as a whole. We distinguish three phases of the defensive response: detection of an intruder, recruitment of individuals into collective defence and attack. Individual bees may selectively contribute to one or more of these steps. Guard bees monitor entering conspecifics or attacking mammals, and release an alarm pheromone to recruit stinging soldiers. However, we are still far from understanding why only subsets of bees become guards or soldiers (or even if soldiering can be considered a task per se). We discuss the stimuli associated with each of these steps, how they define the number of bees needed and how they might combine with individual and developmental characteristics such that individuals take on a particular task. We also highlight pending questions and interesting avenues for future research.This article is part of the theme issue 'Division of labour as key driver of social evolution'.

The evolution of division of labour: preconditions and evolutionary feedback

Division of Labour (DoL) among group members reflects the pinnacle of social complexity. The synergistic effects created by task specialization and the sharing of duties benefitting the group raise the efficiency of the acquisition, use, management and defence of resources by a fundamental step above the potential of individual agents. At the same time, it may stabilize societies because of the involved interdependence among collaborators. Here, I review the conditions associated with the emergence of DoL, which include the existence of (i) sizeable groups with enduring membership; (ii) individual specialization improving the efficiency of task performance; and (iii) low conflict of interest among group members owing to correlated payoffs. This results in (iv) a combination of intra-individual consistency with inter-individual variance in carrying out different tasks, which creates (v) some degree of mutual interdependence among group members. DoL typically evolves 'bottom-up' without external regulatory forces, but the latter may gain importance at a later stage of the evolution of social complexity. Owing to the involved feedback processes, cause and effect are often difficult to disentangle in the evolutionary trajectory towards structured societies with well-developed DoL among their members. Nevertheless, the emergence of task specialization and DoL may entail a one-way street towards social complexity, with retrogression getting increasingly difficult the more individual agents depend on each other at progressing stages of social evolution.This article is part of the theme issue 'Division of labour as key driver of social evolution'.

Mutualism and division of labour: a mutual expansion of concepts

Division of labour within social groups and the interspecific relationships within mutualisms have traditionally been treated as separate research areas. In this opinion, we align terminologies and concepts between the two fields, by comparing within-group division of labour to the outsourcing of functions in mutualisms. Division of labour and interspecific outsourcing share fundamental similarities. Both are built from specialization of some individuals within the relationship on tasks or functions required for survival, growth and reproduction. Both also generate variable fitness outcomes. A key difference is that mutualisms generally generate direct fitness gain, while benefits from cooperative sociality often accrue from a mix of direct and indirect fitness. Additionally, the levels of physical and physiological specialization within many mutualisms expand far beyond the levels of differentiation seen in cooperative social groups, with the exception of reproductive division of labour. The consideration of between-species outsourcing in the context of division of labour allows expansion of our understanding of both fields and beyond, to consider general principles as drivers of division of labour, and role differences more broadly across levels of complexity.This article is part of the theme issue 'Division of labour as key driver of social evolution'.

Division of labour in colony defence in a clonal ant

Division of labour (DOL) plays a key role across all scales of biological organization, but how its expression varies across contexts is still poorly understood. Here, we measure DOL in a crucial task, colony defence, in a social insect that affords precise experimental control over individual and colony traits, the clonal raider ant (Ooceraea biroi). We find that DOL in defence behaviour emerges within colonies of near-identical workers, likely reflecting variation in individual response thresholds, and that it increases with colony size. Additionally, colonies with pupae show higher defence levels than those without brood. However, we do not find evidence for a behavioural syndrome linking defence with exploration and activity, as previously reported in other systems. By showing how colony composition and size affect group response to potential threats, our findings highlight the role of the social context in shaping DOL.This article is part of the theme issue 'Division of labour as key driver of social evolution'.

Bimodal alarm signals modulate responses to monomodal alarm signals in Camponotus modoc carpenter ants

Distressed western carpenter ants, Camponotus modoc, produce alarm pheromone and substrate-borne vibrations. The alarm pheromone attracts nestmates but the effects of vibratory signals, or of bimodal pheromonal and vibratory signals, are not known. Worker ants of two Camponotus congeners reportedly stand still ("freeze") or run fast in response to engineered drumming vibrations inputted on plastic, but many responses to ant-produced vibratory signals on wood have not yet been investigated. Generally, orientating toward signalers under vertebrate predator attack seems maladaptive and not beneficial to ant colonies. We tested the hypotheses (1) that vibratory alarm signals cause freezing, rapid running but not attraction of nestmates, and (2) that bimodal alarm signals modulate responses to monomodal alarm signals, thereby possibly reducing predation risk. Laser Doppler vibrometry recordings revealed that the ants' vibratory signals readily propagate through ant nest lamellae, and thus quickly inform nest mates of perceived threats. With a speaker modified to record and deliver vibratory signals, we obtained drumming signals of distressed ants on a Douglas fir veneer, and bioassayed signal effects on ants in an arena with a suspended veneer floor. In response playback of vibratory signals, ants ran rapidly, or froze, but did not approach the vibratory signals. Exposed to alarm pheromone, ants frequently visited the pheromone source. However, concurrently exposed to both alarm pheromone and vibratory signals, ants visited the pheromone source less often but spent more time "frozen." The ants' modulated responses to bimodal signals seem adaptive but the reproductive fitness benefits are still to be quantified.

Parasitism by Entomopathogenic Fungi and Insect Host Defense Strategies

Entomopathogenic fungi, a group of insect pathogens, are characterized by high insecticidal efficacy and minimal environmental impact. They are commonly used as biopesticides for pest control due to their significant practical value. We here classify entomopathogenic fungi according to the process of fungal infection in hosts, changes in host behavior during infection, and mechanisms of spore transmission, and review the strategies employed by insects to resist infection, including physical barrier defenses, immune system defenses, and behavioral avoidance of fungal pathogens. This review also discusses the pathogenic mechanisms of fungi on insects and the closely linked co-evolution between fungal pathogens and insect defenses. In conclusion, a perspective on future research is provided, emphasizing the impact of insect population density and spore concentration in the environment on disease outbreaks.

Bee fear responses are mediated by dopamine and influence cognition

Predatory threats, even when they do not involve direct consumption (non-consumptive effects, NCEs), can profoundly influence the physiology and behaviour of prey. For example, honeybees that encounter hornet predators show responses similar to fear. However, the physiological mechanisms that are connected with this fear-like response and their effects on bee cognition and olfaction remain largely unknown. We show that bees decreased time spent near the hornet, demonstrated fearful behaviour and moved with greater velocity to escape. After a prolonged 24-h exposure, bees adopted defensive clustering, displayed greater predator avoidance, and experienced a decline in brain dopamine levels. Prolonged predator exposure also diminished bee olfactory sensitivity to odours and their mechanical sensitivity to air currents, contributing to impaired olfactory learning. However, boosting brain dopamine reversed one fear-like behaviour (average bee velocity in the presence of a hornet) and rescued olfactory sensitivity and learning. We therefore provide evidence linking dopamine to sensory and cognitive declines associated with fear in an insect.

How Insect Exocrine Glands Work

Exocrine glands release a secretion to the body surface or into a lumen and are likely to be found in all insect taxa. Their secretions are diverse, serving many physiological, behavioral, and defensive functions. Much research has characterized gland structure and secretion identity and function, but little research has attempted to understand how these glands work to release secretion amounts in a timescale appropriate to function: How are some (e.g., physiological) secretions released in small amounts over long times, while others (e.g., defense) are released in large amounts infrequently? We describe a qualitative model, comprising intracellular, extracellular, and external compartments for secretion storage; rates of movement of secretion from one compartment to the next; physicochemical properties of secretions; and controlling behaviors, which may explain the release dynamics of secretions from these glands. It provides a template for quantitative dynamic studies investigating the operation, control, release, and biomimetics of exocrine glands.

Streptomyces odontomachi sp. nov., a novel actinobacterium with antimicrobial potential isolated from ants (Odontomachus simillimus Smith, 1858)

A new actinomycete strain, ODS25T, exhibited antimicrobial activity against Bacillus subtilis, Kocuria rhizophila, Staphylococcus aureus, Staphylococcus epidermidis, Candida albicans, Candida tropicalis, was isolated from the ants, Odontomachus simillimus, collected from National Science Museum Thailand, Pathum Thani, Thailand. A polyphasic technique was used to characterize the taxonomic position. The morphological and chemotaxonomic properties of the strain are typical of members of the genus Streptomyces. Strain ODS25T contained ll-diaminopimelic and glucose in the whole-cell hydrolysate. The major cellular fatty acids were iso-C16:0, iso-C15:0, and anteiso-C15:0. The polar lipids were phosphatidylethanolamine, phosphatidylinositol mannosides, phosphatidylinositol, diphosphatidylglycerol, phosphatidylglycerol, three unidentified phospholipids, three unidentified amino lipids and two unidentified lipids. The menaquinones were MK-9(H6), MK-9(H8), and MK-9(H4). The G + C content of the genomic DNA was 71.3%. The 16 S rRNA gene sequence analysis demonstrated that the strain had the highest similarity to Streptomyces lusitanus NBRC 13464T (98.07%) but shared the phylogenetic neighbour with Streptomyces sulfonofaciens JCM 5069T. Both digital DNA-DNA hybridization and average nucleotide identity values among strain ODS25T and its associated Streptomyces type strains fell within the values lower than the threshold for differentiate the strain to the same species. Based on the phenotypic characteristics and genotypic distinctiveness, strain ODS25T is considered a novel species within the genus Streptomyces, for which the name Streptomyces odontomachi sp. nov. is proposed. The type strain is ODS25T (=TBRC 16204T=NBRC 115862T).

Acorn ant exhibits age-dependent induced defence in response to parasitic raids

When risk is unpredictable, organisms may evolve induced defenses, which are activated after an indication of increased risk. In colonies with behavioural specialization, investment in defence may not be uniformly beneficial among group members. Instead, it should depend on the individual's likelihood of participating in defence. The ant Temnothorax longispinosus uses venom to defend against raids by the social parasite Temnothorax americanus. We tested whether T. longispinosus upregulate investment in venom after experiencing a raid, investigating the relationship between venom volume and worker behavioural caste. Overall, raided colonies had more venom per capita than unraided colonies. When divided into behavioural castes, foragers had more venom after experiencing a raid, while nurses did not. These results demonstrate that T. longispinosus have an induced chemical defence against parasitic raids. However, instead of this defence being deployed uniformly among all workers, the induction of the defence depends on the behavioural caste, and therefore age, of the worker, implying that plasticity in venom production increases with age. Since older social insect workers tend to perform riskier tasks, inducibility may align with an increase in expected risk of death, especially if foragers are more likely to defend the colony against parasites than younger workers.

Social aphids: emerging model for studying insect sociality

Sociality is also evolved in parthenogenetic herbivorous hemipteran aphids, encompassing species with complex life history traits and significant social diversity. Owing to their interesting biological characteristics comparing to other social insect groups, social aphids can be a good model for studying insect sociality. Here, we review the species, behavior, and trait diversity of social aphids, and present recent findings on environmental, physiological, and molecular regulations of caste differentiation and behavior in social aphids. We propose the unique value of social aphids in investigating the evolution and mechanisms of insect sociality as well as future research directions using the social aphid model, including social evolution, caste differentiation, behavioral polymorphism, morphological plasticity, physical mechanics, and interspecific interactions.

Sister predatory mites collectively protect their eggs against predators

Group-living animals sometimes cooperatively protect their offspring against predators. This behavior is observed in a wide range of taxa but, to the best of our knowledge, this is the first report of its occurrence in arthropods that are not eusocial. Adult female predatory mites Gynaeseius liturivorus protect their eggs against egg predators, the predatory mite species Neoseiulus californicus. In the field, several adult female G. liturivorus were often found on the same plant structures such as folded leaves. We tested whether these females might protect their eggs cooperatively, focusing on kinship between the females. When two adult female G. liturivorus were kept in the absence of egg predators, their reproduction was not affected by their kinship. The presence of egg predators reduced the number of G. liturivorus eggs. However, reproduction of two G. liturivorus sisters was higher than that of two non-sisters. Together, sisters guarded the oviposition site longer than non-sisters. We further tested if non-sisters increased egg guarding by having developed together from eggs to adults and found no such effect. Although it remains unclear how adult female G. liturivorus recognize conspecifics as kin or sisters, our results suggest that G. liturivorus sisters reduced predation on their offspring by cooperatively guarding their eggs.

Group Size Buffers against Energetic Stress in Honeybee Workers (Apis mellifera)

Individuals of the Western honeybee species, Apis mellifera, live in large groups of thousands of worker bees, a queen, and a few drones. Workers interact frequently with related individuals while performing various tasks. Although it is well understood why and under which conditions sociality can evolve, the consequences for individuals living in permanent groups are less well understood. As individuals of solitary species become stressed when kept at high density, it might be the opposite in obligate social species. Here, I use an experimental laboratory set-up to study the effect of varying group sizes on the magnitude and within-group variance of stress responses towards energetic and heat stress. While only a weak difference was found in the magnitude of an energetic stress response as a function of group size, the within-group variance showed a statistically significant positive relationship with group size for the glucose/trehalose ratio, a marker for energetic stress. The heat stress marker, hsp70AB gene expression, did not show any relationship to group size. Individuals of obligate social species seem to benefit from adaptations to permanent group living, e.g., buffering against stress, especially at a higher density of individuals. The consequences of infections and immune system activation in isolated individuals are discussed.