Tiny Alchemists: The Chemical Foundations of Termites’ Social and Biological Success  

Rosalie Beaudin, Naia Kim, Mattias Sucher, Lucy Wiggers

Abstract

Despite their individual simplicity, termites are considered one of nature’s most sophisticated social organisms. Remarkably, their lifestyle is enabled by the evolutionary co-option of their chemical environment, a design strategy that governs their socio-environmental success. Termites couple their own digestive enzymes with chemicals produced by symbiotic counterparts in their gut microbiome, allowing for effective cellulose degradation, and enabling a highly cellulose-based diet. Further, termites’ phenotypical plasticity (i.e. the caste system) requires strict biochemical regulation to maintain balanced caste ratios; primer pheromones trigger caste-specific morphological development via regulation of neuroendocrine pathways and gene expression. Pheromones are equally used in colony activities such as communication, sex-pairing and mound-building. Trail pheromones recruit individuals for foraging and nest building, however, they may also double as sex-pairing pheromones in dispersing alates. Pheromonal gradients also play a critical role in mound morphology, creating self-regulated feedback loops that dictate building activity. Finally, soldier termites employ a plethora of chemical weaponry in colony defense through various mechanisms, including biting and injection, direct poison application and glue-squirting. Ultimately, termites utilize chemistry in diverse ways, and it defines their evolutionary prowess.  

Introduction

Termites, like all living organisms, consist of cells that play key roles in their structure and function. At the cellular scale, termites are composed of eukaryotic cells with a nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. Like other insects, they also have exoskeletal cells made of chitin that provide structure and protection. Furthermore, termites’ diet is mainly cellulose, which is obtained from wood, grass, leaves, humus, manure of herbivorous animals, and materials of vegetative origin. Termites also require vitamins and nitrogenous foods, which are supplied by fungi normally present in their decayed wood diet. The fungi can break down wood into components that are easily digested by termites. The lifespan of termites varies significantly depending on their caste within the colony. Worker and soldier termites typically live for about two to five years. In contrast, the primary king and queen, especially in higher termite families, can live for 60 to 70 years. The entire colony can persist for many years, particularly in species that replace the primary king and queen with secondary reproductive. Termites secrete pheromones from different glands, and these are crucial for various functions within their colonies. Pheromones are integral to the social structure and functioning of termite colonies, influencing behaviors from foraging to reproduction and maintaining the colony’s organization (Krishna, 2024). 

Termites’ chemistry is complex and developed in many aspects. Indeed, one of the most fascinating aspects is their symbiotic relationship with microorganisms, particularly protists and bacteria, in their gut. Pheromones are also a very important part of termite chemistry. These chemical signals regulate caste development and maintain balance within the colony. Moreover, termites use pheromones to communicate, which are detected by sensory cells on their antennae. These signals help regulate social behavior, such as nest building, foraging, and sexual pairing. Their chemical defense mechanisms to protect the colony are particularly through the soldier caste and uses pheromones too. This essay will focus on termites’ interaction with the chemical world and their ability to use chemical compounds in many ways.   

Symbiosis and Cellulose Digestion in Termites 

Termites are known as significant wood decomposers, digesting cellulose with the help of their obligatory mutualistic gut microbes. They also have other symbionts, including bacteria, fungi, actinomycetes, and other microorganisms. Almost 3,000 species of termites have been identified, but not all the termites cause damage to wooden structures. They could contribute to soil fertility and crop yield and benefit the ecosystem, especially in dry regions (Brune, 2014). There are two types of termites: lower termites, which possess symbiotic protozoa, relying on both their enzymes and symbiotic protozoa to break down cellulose, and higher termites, which lack protozoan symbionts in their gut and make up 80% of all termite species, consisting solely of the family Termitidae (Zhou et al., 2018) (Fig 1.). 

Fig. 1. Major evolutionary innovation of termite species (Chouvenc et al., 2021) 

Symbionts in Termites 

The termite gut is divided into three sections: the foregut, midgut, and hindgut (Fig 2). In their gut, various microorganisms are present including bacteria, archaea, and eukaryotes. Among these, flagellates are the most important symbiotic unicellular eukaryotes because they play a critical role in cellulose digestion. In this section, diverse microorganisms in termite gut are explored. 

Fig. 2. Anatomical structure of the termite digestive system (Brune & Ohkuma, 2010). 

Termites have a slightly different composition of bacterial symbionts according to the types, yet they also share some common type of bacteria (Fig 3). First, spirochetes are bacteria which dominate the termite gut, comprising 42–63% of the intestinal bacteria in species. They produce formate, acetate, and ethanol, which contributes to lignocellulose degradation. These spirochetes are closely related to cellulose-degrading bacteria in lower termites, and they are essential in acetic acid production and hydrogen consumption. Spirochetes in the termite gut can be freely moving or attached to protozoa. When spirochete populations are reduced by antibiotics, the termites’ ability to degrade cellulose drops significantly. Some spirochetes produce monosaccharides, disaccharides, and oligosaccharides through fermentation, but not all can perform reductive acetogenesis (Zhou et al., 2018). 

There are other bacteria as well. Symbiotic actinomycetes are found in the gut, nest, and surrounding soil of both higher and lower termites. They are the key to breaking down lignocellulose, a key plant material, while also maintaining the gut’s micro-environment. Most actinomycetes in termite guts belong to the genus Streptomyces, which are particularly important for their enzyme secretion, especially cellulase, aiding in lignocellulose digestion. These actinomycetes are highly adapted to the termite gut environment, enhancing digestion (Zhou et al., 2018). 

Symbiotic fungi are found in some termite species as well. The symbiotic fungus Termitomyces is found in the subfamily Macrotermitinae, where workers cultivate fungus on plant litter. Workers mix plant litter with fungal spores during digestion, and their faeces are used to grow fungus, which provides fungal biomass and preprocessed plant fiber for termites to digest (Brune, 2014). The fungus enhances cellulose degradation, with the help of various enzymes in them, five of which are cellulases (Zhou et al., 2018). 

Termites are one of the few insects that emit methane (CH₄) with the help of symbiotic microorganisms, specifically methanogens in their gut. They produce methane only with hydrogen and carbon dioxide without using methanol, acetic acid, or ethanol (Zhou et al., 2018). This is unique because many methanogens in other organisms degrade those carbon sources to produce methane (Schlegel & Müller, 2011). 

Fig. 3 Classification of bacterial genes in the hindgut of selected termites from different feeding groups and a closely related omnivorous cockroach. The bacterial communities in those species are very diverse. Spirochaetes are common in wood-feeding termites but are absent in cockroaches. In C. formosanus (Lower termites), Bacteroidetes, which colonize the cytoplasm of flagellate are dominated. Higher termites have more varied bacteria based on their diets. Fungus-feeding termites have a microbiota similar to omnivorous cockroaches, while wood-feeding termites have many Fibrobacteres and TG3 phylum bacteria, and humus-feeding termites have an abundance of Firmicutes. (Brune, 2014) 

Cellulose Digestion Mechanism 

Lignocellulose is the predominant component of woody plants and dead plant material. They consist of cellulose, hemicellulose, and lignin, typically in a 4:3:3 ratio (Fig. 4). This ratio could differ depending on hardwood, softwood, and herbs (Chen, 2014). Utilization of lignocellulose is tricky because its degradation is often inefficient. However, termites are among the most important insects that efficiently decompose lignocellulose with microbial symbionts. Termites remarkably degrade 74–99% of the cellulose and 65–87% of the hemicellulose they consume. Due to their digestive ability and huge abundance, termites have a tremendous ecological impact on the bio-recycling of lignocellulose (Schmidt et al., 2021). 

Fig. 4. Major components of woods and their chemical structures. (A) Cellulose (Zabel & Morrell, 2020), (B) Hemicellulose (Medjekal & Ghadbane, 2021), (C) Lignin (Zabel & Morrell, 2020) 

All termite species produce their own cellulase enzymes, particularly endoglucanases, which initiate cellulose hydrolysis. In lower termites, endoglucanases are secreted by the salivary glands, while symbiotic flagellates in the hindgut assist with cellulose breakdown by expressing glycosyl hydrolase genes like cellulases and xylanases. Flagellates are present in large numbers in the gut (10³–10⁷ cfu/ml) and can occupy more than 90% of its volume (Zhou et al., 2018). Therefore, flagellate-dependent termites, like lower termites, are highly sensitive to metronidazole, an antibiotic used for the treatment of bacterial infections. In the study mentioned in Zhou’s article, metronidazole treatment killed flagellates and led to the death of termites within two weeks. In contrast, higher termites secrete endoglucanases from midgut epithelial cells and have few flagellates, relying more on their own enzymes and microorganisms in their gut. Bacteria, such as Actinomycetes in the hindgut of higher termites contribute to lignocellulose digestion by producing enzymes such as 1,4-β-xylanase and 1,3-β-glucan (Zhou et al., 2018). 

Within the termite gut, the initial breakdown of cellulose begins with endoglucanases, but it only breaks down amorphous substrates and struggles with crystalline cellulose. To process crystalline cellulose, termites grind wood into small particles of size about 10–20μm, using their mandibles and gizzard, increasing surface area for digestion. Once endoglucanases break down cellulose, high enzyme concentrations in the midgut quickly degrade the amorphous parts of cellulose fibers. β-glucosidases convert oligosaccharides into glucose, preventing product inhibition during digestion (Brune, 2014). 

Termite gut flagellates belong to two separate unicellular eukaryotes, parabasalids and oxymonadids, and present mainly in the hindguts of lower termites and of the closely related wood-feeding cockroaches, the sister group of the termite (Brune & Ohkuma, 2010).  Flagellates produce enzymes like cellobiohydrolases, endoglucanases, β-glucosidases, which fully digest cellulose, including crystalline cellulose. Flagellates also secrete other glycoside hydrolases to digest hemicellulose. Wood particles are engulfed by flagellates in the hindgut, allowing efficient digestion (Fig. 5). In lower termites, bacteria play a minor role in fiber digestion since flagellates take up most wood particles (Brune, 2014). 

Fig. 5. Major microbial processes in the hindgut of lower termites. (Brune & Ohkuma, 2010) 

Microbes in the termite hindgut convert wood fibers to short-chain fatty acids, which are absorbed by the termite. Flagellates in lower termites’ guts handle most fermentation. They convert cellulose to acetate, hydrogen, and carbon dioxide (Eq. 1). Not all flagellates may produce the same fermentation products, especially those processing hemicelluloses. Flagellates produce lactate, which bacteria quickly convert to acetate in an oxygen-dependent process. Formate is produced in the hindgut of many termites. It can accumulate, be oxidized to CO₂, or reduced to acetate by bacteria, depending on the termite species.  

$$C_6H_{12}O_6 + 2H_2O \rightarrow 2CH_3COOH + 2CO_2 + 4H_2$$

Eq. 1. General fermentation reaction in termites (Breznak & Switzer, 1986). 

However, cellulose digestion is different in higher termites. All members of this family, which are considered evolutionarily advanced and make up most termite species today, have lost the gut flagellates characteristic of lower termites and developed a prokaryotic microbial community in their hindguts (Brune & Ohkuma, 2010). Higher termites expanded their diet to dry grass, plant litter, herbivore dung, and humified organic matter, unlike lower termites, which mainly feed on wood. Changes in gut microbiota composition suggest that it plays a new role in the digestive process. Dietary diversification in these termites involved anatomical changes, such as a more complex hindgut with multiple compartments and increased alkalinity in some parts. One innovation was the symbiosis with the fungus Termitomyces, explained in the previous section. However, the roles of termites, fungi, and gut bacteria in digestion are still not fully understood. (Brune, 2014). Fermentation products in higher termites are like those in lower termites, and hydrogen is still a key metabolite. Since they lack gut flagellates, fermentation is derived by bacteria.  For example, Clostridium termitidis is a bacterium isolated from a higher termite species, and they possess hydrogen-producing gene (Brune & Ohkuma, 2010). Hydrogen accumulates strongly in the hindgut of both lower and higher termites because of fermentation, though its emission rates rarely exceed those of methane. In wood-feeding termites, hydrogen is largely consumed through reductive acetogenesis, where H₂ and CO₂ react to form acetate. This process serves as a major hydrogen sink, directing about 25% of the electron flow. Spirochaetes catalyze this process in termites and use the Wood–Ljungdahl pathway to produce acetate (Brune, 2014). The Wood–Ljungdahl pathway is the largest carbon fixation pathway in anaerobic conditions. This pathway differs from other carbon fixation cycles by its noncyclic carbonic fixation that forms acetyl-CoA from CO₂ (De Souza & Rosado, 2018). Genes related to this pathway are found in both lower and higher termites. 

Lignocellulose digestion also requires a mechanism that overcomes the lignin barrier. In termites, this is accomplished by a dual system that combines the activities of both the host and its intestinal symbionts (Brune, 2014). The mechanism is not fully known yet, but the studies show that higher termites can degrade up to 37% of lignin and use about half of the polysaccharides found in wheat straw and lower termites degrade very little or no lignin but are highly efficient in digesting cellulose (Xue et al., 2024). 

Chemical regulation of termite caste differentiation  

Termite caste system 

Termites exhibit a complex social organization governed by polyphenism: individuals exhibit the same genotype but display varying phenotypes, leading to a distinct caste system. This is regulated by internal and external influences on gene expression during postembryonic development (Korb, 2015). Termite colonies distinguish four primary castes: workers, alates, neotenic reproductives and soldiers (Fig. 6). Workers consist of larvae (immature termites without external wing buds) and nymphs (immature termites with distinct wing buds) of at least the third developmental stage, or instar. These sexually immature individuals perform foraging and nursing duties; however, their developmental path is not terminal—they may further differentiate, undergo stationary (worker-to-worker) molts, or regress (Tarver et al., 2010; Oguchi & Miura, 2024). The next three castes are terminal and cannot undergo regressive molts. Alates are true reproductive adults that differentiate from nymphs, developing full wings, compound eyes and reproductive organs. This allows them to leave and found new colonies as kings and queens (primary reproductives) (Oguchi et al., 2021). Secondary reproductives develop from immature workers to supplement reproduction in the death or absence of primary reproductives. They exhibit a mixture of immature and mature characteristics, developing gonads for reproduction but not wings or compound eyes; they are often called neotenic reproductives. Finally, soldier termites differentiate from workers through a presoldier stage, developing defense artillery such as mandibles for colony protection (Oguchi & Miura, 2024).  

Fig. 6. Morphological characteristics of individual termites across the caste differentiation system in a common lower termite, R. aculabialis. (A) Larvae. (B) Larval workers lack wing buds. (C) Nymphal workers with distinct wing buds (WB). (D) Soldiers with large heads and powerful mandibles (Ma). (E) Alates with distinct wings and darkened pigmentation. (F) Neotenic reproductive differentiated from a worker lacking wings but showing darker pigmentation. Scale bar = 0.5 mm [Adapted from (Ye et al., 2021)].  

Two caste differentiation pathways exist across termite species. In the linear pathway, alate, neotenic reproductive and soldier lines differentiate from the same larval instar. In the bifurcated pathway, differentiation between nymph and worker occurs early on in the first instar (Fig. 7) ((Oguchi & Miura, 2024). The focus of this section will be the linear pathway, which allows for high phenotypic plasticity (Korb, 2015). The nature of termite caste differentiation is remarkable, but still widely debated in the scientific community. Factors affecting caste regulation are numerous, often existing in complex feedback loops that enable colonies to maintain specific caste ratios. Research shows strong ties between caste-specific development and endocrine regulatory systems, which dictate gene expression (Oguchi & Miura, 2024). Hormonal activity is triggered by socio-environmental factors, such as pheromones, caste interactions, food quality and temperature (Korb, 2015). These intrinsic and extrinsic factors in caste regulation are explored in the following sections.  

Fig. 7. Caste developmental pathways in the Hodotermopsis sjostedti (Hodotermopsidae) and Nasutitermes takasagoensis (Termitidae). (a) Linear pathway where differentiation into alate, neotenic reproductive and soldier lines occurs in the late (third) larval instars. (b) Bifurcated pathway where nymph and worker lines and separated at the first instar larval molt (Oguchi & Miura, 2024).  

Caste differentiation via endocrine pathways  

The driving force of caste differentiation is the interplay between juvenile hormone (JH) and ecdysone (molting hormone), which inhibit and stimulate molting pathways in termites, respectively. In other words, JH serves to prevent metamorphosis in immature termites, inhibiting active ecdysone until the termite has undergone an appropriate inter-molting period. Specifically, JH III compound is produced by a pair of secretory glands in the brain called the corpora allata and is activated in the hemolymph (Fig. 8). Interestingly, corpora allata sizes differ across termite castes, indicating a strong involvement in caste regulation (Oguchi & Miura, 2024). Scholars propose three JH-sensitive phases during the inter-molt period: the start, mid-phase and second half of the inter-molt, which dictate sexual traits, non-sexual adult traits (eyes, wings) and soldier traits, respectively. Varying JH titres (concentrations) during those periods determine the molting type and, consequently, individual caste pathways, though the specifics are still debated (Fig. 9) (Korb et al., 2021). Generally, throughout the intermolt period, consistently low JH titres are associated with progressive sexual development, leading to alate differentiation, while consistently high JH titres characterize soldier differentiation. Immature workers undergoing stationary and regressive molts to remain workers show a transition from high JH titres to low JH titres, while neotenic reproductives show low to high JH titres (Korb et al., 2021).  

Fig. 8. Chemical structure of juvenile hormone III [Adapted from Morgan, 2010].  

Fig. 9. Proposed model explaining postembryonic developmental pathways in wood-dwelling termites. JH titres during three sensitive phases during the intermolt period determine individual molting types of larvae and nymphs, leading to caste differentiation. Continuously high or low JH titres characterize soldier and alate differentiation, respectively. Stationary and regressive worker molts, as well as neotenic molts have varying titres (Korb et al., 2021).  

This model, adapted by Korb et al., is widely used but still considered incomplete. In fact, JH titres are further complicated by social interaction amongst colony members, particularly in the case of worker-to-soldier and worker-to-neotenic-reproductive differentiation (Korb et al., 2021). Scholars propose that soldier and neotenic differentiation is further regulated by the presence of other soldiers and reproductives, which corresponds to the health and size of the colony (Fig. 10).  When there is a lack of soldiers in the colony, presoldier development is triggered by high JH titres in workers. When there is an abundance of soldiers, workers with high JH titres will preferentially undergo stationary or regressive molts to remain workers (Korb et al., 2021). On the other hand, in the presence of reproductives, neotenic differentiation is inhibited in workers with low JH titres, while it is triggered in the absence of reproductives (Korb et al., 2021). The mechanism of this regulation is primarily pheromonal-based, and it is explored in the sections to come.  

Fig. 10. Updated model for characterizing JH titres in termite caste development including social interaction between workers and other colony members. (a) In the absence of both soldiers and reproductives (indicated by lightning bolts), high JH titres result in soldier differentiation and low JH titres result in neotenic differentiation. (b) In the presence of both soldiers (helmet) and reproductives (crown), high JH titres lead to regressive molts and low JH titres lead to progressive molts (i.e. alate or differentiation) (Korb et al., 2021).  

Pheromonal and genetic regulation of soldier termites 

Soldier proportions must be highly monitored for colony survival. In the absence of soldiers, the colony loses its primary line of defense. However, soldiers are high-maintenance individuals: they are developmentally unable to feed themselves and must require help from workers. Hence, too many soldiers are costly for the colony. As mentioned previously, soldier differentiation is primarily triggered by high JH titres, and these are regulated by socio-environmental interactions. Specifically, developed soldiers and reproductives secrete pheromones and stimulatory substances which dictate neotenic vs soldier differentiation. Primary reproductives stimulate soldier differentiation by feeding JH or other stimulatory substances to developing soldiers via proctodeal trophallaxis (anus-to-mouth feeding), increasing their JH titre (Korb, 2015). In their absence, developing soldier JH titres decrease. In contrast, other soldiers produce substances or pheromones that inhibit JH production. When soldier numbers are too high, worker JH titres remain low and neotenic differentiation is preferred. Of particular interest are the pheromonal components of soldier head extracts (SHE), of which several have been identified as inhibiting substances in soldier differentiation across termite species (Tarver et al., 2011, Mitaka, 2017). In Reticulitermes flavipes, SHE contains two distinct terpene pheromones: γ-cadinene (CAD) and γ-cadinenal (ALD), which act as stimulatory and inhibitory compounds, respectively (Fig. 11). In the presence of other soldiers, worker ALD levels increase significantly (while CAD levels increase minutely), suggesting that ALD and CAD pheromones are transferable from soldier to worker, likely through trophallaxis and grooming. ALD and CAD ratios, likely determined by intrinsic and extrinsic factors in the colony, dictate the inhibitory and stimulatory effect of SHE blends.   

Fig. 11. Candidate soldier primer pheromones in R. flavipes: cadinene (CAD) and its aldehyde (ALD). Transfer of CAD and ALD to worker termites has, respectively, stimulatory and inhibitory effects on soldier differentiation, likely by modulating JH efficacy (Tarver et al., 2011).  

Primer pheromones, such as CAD and ALD in R. flavipes (Tarver et al., 2011), and more recently, (−)-β-elemene in R. speratus (Mitaka et al., 2017), inhibit the ability and production of JH in soldier differentiation. Workers in the presence of soldiers show distinctly lower JH levels than workers without nearby soldiers, due to pheromonal effects (Fig. 12). 

Fig. 12. Live soldiers limit presoldier differentiation from workers across a range of JH levels. Workers held alone showed consistently high JH concentrations, while those of workers held with soldiers were significantly lower. This suggests an inhibitory effect by soldier termites on JH activity, no matter the surrounding JH levels (Tarver et al., 2011).  

The proposed mechanism of JH interference is the pheromone’s effect on JH-related gene expression (Fig. 13). In the study done by Tarver et al., CAD and ALD blends directly induced gene expression changes in several genes involved in JH regulation and synthesis. For example, JH production and degradation genes, such as cytochrome P450s, were differentially expressed in the presence of SHE blends (Tarver et al., 2010). P450 catalysts involved in JH synthesis are generally upregulated during presoldier molt, leading to high JH levels in workers undergoing differentiation (Miura, 2020). Changing their expression greatly affects JH synthesis, with potential inhibiting effects. Further, hemolymph (blood) proteins such as hexamerins were differentially expressed (Tarver et al., 2010). Hexamerins are storage and transport proteins which bind JH and regulate its activity in the hemolymph. They act as JH inhibitors by reducing JH availability in workers. Hence, upregulating their expression would lead to inhibited soldier differentiation (Korb, 2015). Finally, several developmental genes were shown to have changed expression in response to CAD and ALD. Here, the primer pheromone would directly impact the development of soldier-specific morphology, such as head and mandible development (Tarver et al., 2010). 

Fig. 13. Proposed schematic of the influence of socio-environmental and semiochemical factors (ALD and CAD) on caste differentiation, specifically by modulating gene expression. Gene categories with modified expression after exposure to SHE blends include: chemical production/degradation, hemolymph protein coding, and developmental genes. Dotted lines represent the possible feedback loop created when workers molt into soldiers and consequently further inhibit soldier development (Tarver et al., 2010).  

Pheromonal regulation of neotenic differentiation  

The regulation of neotenic reproductives in the colony is as equally important as soldier regulation. The differentiation of larval or nymphal workers into neotenics is inhibited by the presence of healthy reproductives, such as the queen and king, or other neotenic reproductives (Matsuura et al., 2010). Kings, queens and neotenics secrete sex-specific primer pheromones, which may be volatile or transferred through proctodeal feeding. They are neuroendocrine signals, which interact with endocrine pathways like JH and ecdysone to control molting types. Contrary to soldier pheromones, these primer pheromones seem to directly increase JH titres, disfavouring neotenic production over soldier production. Within the scientific community, only a handful of primer pheromones have been identified despite the consistent inhibitory dynamic between queens and developing neotenics. In Reticulitermes speratus, a lower termite species, the first queen primer pheromone (QPP) was identified in queens and female neotenics to suppress further (female) neotenic differentiation. It consists of an ester/ alcohol volatile blend: n-butyl-n-butyrate (nBnB) and 2-methyl-1-butanol (2M1B) in a two-to-one ratio (Fig. 14) (Matsuura et al., 2010). Importantly, the volatile is airborne, which is much more effective than trophallaxis, given that only a few individuals produce the pheromone (Matsuura, 2012). When exposed to the two-compound blend, neotenic differentiation is significantly reduced in both workers and nymphs (Fig 15). Interestingly, termite eggs also produce the same volatile, likely for use as a recognition signal for workers who protect and groom eggs. Further, the chemical has anti-fungal properties, which suggests that its intended evolutionary use was in anti-fungal defense before being co-opted for pheromonal signalling (Dolejsova et al., 2022).  

Fig. 14. Chemical structures of reproductive primer pheromones in the lower termite species R. speratus. Compounds were identified in queens, female neotenic reproductive and eggs, but not workers or nymphs, by gas chromatography and mass spectrometry [Adapted from Matsurra et al., 2010].  

Fig. 15. Comparison of the inhibitory effect of nBnB and 2M1B compounds individually and in a 2:1 mixture. The 2:1 mixture significantly suppressed the differentiation of new female neotenics (FNs), whereas neither nBnB alone nor 2M1B alone had a significant effect compared with the control (distilled water) [Adapted from Matsurra et al., 2010].  

Recently, QPPs were identified in higher termite species, such as Embiratermes neotenicus. E. neotenicus exhibit a polygynous breeding system in which the queen dies off relatively quickly, but gives rise to several genetically identical neotenic queens through parthenogenesis (a form of asexual reproduction). This is to discourage inbreeding and ensure genetic diversity, as the parthenogenetic daughters are essentially clones of the queen. This asexual queen succession (ASQ) system requires efficient regulation to ensure parthenogenetic offspring do not reach sexual maturity unnecessarily (Dolejosva et al., 2022). An airborne chemical, (3R,6E)-nerolidol (RNERO), signals the presence of a fit queen, suppressing neotenic development in the fourth developmental stage through pheromonal interaction with the neuroendocrine system. When the queen dies or is no longer fit, her parthenogens stop receiving the suppressive signal, and undergo neotenic differentiation until the pheromonal balance is restored (i.e. enough neotenic queens have differentiated to further suppress development). Interestingly, in the absence of a fit queen, neotenic differentiation occurs only in parthenogenic offspring and not nymphs of sexual origin, which preferentially become alates. This indicates a genetic component to the readiness of a female nymph to become a neotenic queen, though the mechanism of this developmental priority is currently unknown (Dolejosva et al., 2022). The proposed neotenic regulation system is shown in Figure 16.  

Fig. 16. Proposed scheme of genetic and pheromonal regulation of the AQS breeding system in E. neotenicus [Adapted from Dolejosva et al., 2022].  

Strikingly, RNERO is an olfactory airborne signal, and is detected through antennal response with high stereoselectivity. In other words, chiral olfactory receptors in termite antennae register RNERO preferentially to its (S) enantiomer and other structurally related acyclic sesquiterpenoids (Fig. 17) (Dolejosva et al., 2022). This suggests a highly precise signalling system, with little room for any other pheromonal interference in neotenic differentiation.  

Fig. 17. (a) and (b) show antennal responses (amplitude values) of fourth stage nymphs to air, RNERO, its opposite (S) enantiomer (SNERO) and two other structurally related sesquiterpenoids, (E,E)-α-farnesene and (E,E)-farnesol at doses of 50 ng (a) and 500 ng (b) of stimuli. Clearly, nymphs are more responsive to RNERO, indicating high pheromonal stereoselectivity (c) Response traces of stimulation series using 500 ng of each compound. (d) Chemical structures of tested compounds [Adapted from Dolejosva et al., 2022]. 

Pheromones and Communication 

Trail-Following Pheromones 

The sternal gland is an unpaired structure found in all termite castes, appearing as an epidermal thickening on the sternite (Fig. 18). In the families Kalotermitidae, Rhinotermitidae, Serritermitidae, and Termitidae, it is located beneath the 5^th sternite (Fig. 18). However, in the families Hodotermitidae and Termopsidae, the gland is situated beneath the 4^th sternite (Costa-Leonardo & Haifig, 2010). 

Fig. 18. Sternal gland of Cornitermes cumulans worker in sagittal histological section. sg, sternal gland; IV, 4^th sternite; 5^th sternite (Costa-Leonardo & Haifig, 2010). 

The abdominal sternal gland is the only termite gland responsible for producing the trail pheromone, occurring in all termite species and development stages. The sternal gland is inactive in larval stages and less developed in young workers. Termite trail pheromones consist of a multicomponent blend with one shared component causing movement and other specific compounds making the trails unique to each species. They play a key role in coordinating collective activities in termite societies. In worker termites, the sternal glands produce trail pheromones when pressed against the substrate, guiding foraging by leaving odor trails. These trails recruit termites to the food source, galleries, and nest repair sites, with trail strength increasing as more workers follow. Soldiers are usually first to respond, followed by workers as the trail concentration rises. Henceforth, termites can regulate the amount and composition of trail pheromones, which vary between castes (Costa-Leonardo & Haifig, 2010). 

So far, only seven distinct compounds have been identified as trail-following pheromones in more than 60 termite species studied (Fig. 19). 

Fig. 19. Simplified phylogeny of the main termite families and sub-families with the chemical nature of the trail-following pheromones identified in at least one species belonging to these families or sub-families (Sillam-Dussès et al., 2020). 

A study on trail-following pheromone in six genera and nine species of Syntermitinae has been made to better understand trail-following communication in the “higher” termites such as Termitidae. The chemical and behavioral experiments showed that (3Z,6Z,8E)-dodeca-3,6,8-trien-1-ol is the most widespread communication compound in termites. It was the single component of the pheromone of all the termite species studied, except for Silvestritermes euamignathus who produces both (3Z,6Z)-dodeca-3,6-dien-1-ol and neocembrene (Sillam-Dussès et al., 2020).  

The high chemical parsimony observed in termites is fascinating. Indeed, a specific compound can be used in different contexts for different purposes. This is useful evolutionarily because these multifunctional pheromones can enable more informative communication while allowing termites to reduce the metabolic cost of producing unique compounds. The compounds mentioned before may act as sex pheromones too. Dodecatrienol is identified as the sex pheromone for some Syntermitinae species, such as Embiratermes neotenicus. Dodecadienol is the sex pheromone of Silvestritermes spp., and neocembrene of Nasutitermes spp (Sillam-Dussès et al., 2020). In 10 species, the components of the trail pheromone and the sex-pairing pheromone are either completely or partially identical. For instance, (Z,Z,E)-3,6,8-dodecatrien-1-ol is both the trail and sex-pairing pheromone in species such as Reticulitermes santonensis, Pseudacanthotermes militaris, and Pseudacanthotermes spiniger. However, in Cornitermes cumulans, this compound functions as the trail pheromone on its own, but is combined with (E)-nerolidol as the sex-pairing pheromone (Mitaka, 2021). 

Sex-Pairing Pheromones 

Termite nuptial flight typically take place once a year, during which female and male alates (adult individuals with wings) pair up to form new colonies. Large numbers of alates (Fig. 20) leave their colonies by flying, then land on the ground or in trees. Then they shed their wings, becoming dealates (Fig. 20), and search for mates (Mitaka, 2021).  

Fig. 20. Termite reproduction cycle (HowStuffWorks, 2007) 

In many species, female dealates lift their abdomen and expose the sternal gland that liberates the sex-pairing pheromone to attract male dealates (Costa-Leonardo & Haifig, 2010). The sternal gland is also involved in termite courtship behavior, but unlike the trail-following pheromones, it is not the only gland responsible for producing the sex-pairing pheromones. The tergal glands are also involved in the formation of the sexual pair after swarming in some species. There are particular species, such as Zootermopsis nevadensis and Zootermopsis angusticollis where both female and male dealates emit the sex-pairing pheromone. The range of action of the pheromone extends from a few centimeters to a few meters, depending on the termite species. When a male locates a female, he follows her in a behavior known as tandem running. During this process, short-range or contact chemical stimuli by the sex-pairing pheromones are crucial in keeping the pair together in tandem formation (Mitaka, 2021).  

Sex-pairing pheromones have been identified in 17 termite species belonging to Archotermopsidae, Rhinotermitidae, and Termitidae families. In the Archotermopsidae ancestral family, the female and male dealates use different compounds; (5E)-2,6,10-trimethyl-5,9-undecadienal is used by the female and 4,6-dimethyldodecanal by the male. Nevertheless, in Rhinotermitidae and Termitidae families, only the female dealates emit (Z,Z,E)-3,6,8-dodecatrien-1-ol, which is widespread among many species (Mitaka, 2021). The number of chemical components in termite sex-pairing pheromones varies by species. Adding species-specific minor components might facilitate species recognition, but the development of these pheromones differs widely across species. 

Building Pheromones and Stigmergy 

In the context of termite nest construction, pheromones play a crucial role as part of the stigmergic communication system that termites use. Stigmergy was first introduced in 1959 by Pierre-Paul Grassé, a French biologist, zoologist, and entomologist (Piveteau, 2024). It refers to a phenomenon where individual insects behave in a decentralized way, but collectively build structures that appear centrally organized (Oberst et al., 2020). 

In other words, through the use of pheromones, termites are able to self-organize without direct communication. Pheromones are secreted and deposited onto the substrate during the building process. These chemical signals serve as stimuli for other termites, informing them that construction is happening in a specific location of the mound. They are attracted to the pheromone-marked areas where they join the building process, which becomes more and more coordinated. The pheromones gradually decay over time, preventing overbuilding in one area and allowing termites to focus on the changing needs of the colony by working on regions of the nest that require more work (Heyde et al., 2021). Henceforth, higher concentrations of pheromones signal that the area needs more building activity, while lower concentrations indicate that construction is less necessary. This self-regulated process helps termites to distribute their building efforts efficiently.  

The architecture of the mound influences both the movement of pheromones and termites, shaping their behavior (Heyde et al., 2021). As a result, nest construction can be viewed as part of a feedback loop that connects physical and behavioral dynamics (Fig. 21A). This loop is created because the building actions of individual termites stimulate further actions by others, leading to a cumulative effect and a large and complex structure that characterize termite nests.  

Fig. 21. Biotectonic model predicts floor spacing and ramp emergence in termite nests (Heyde et al., 2021). 

Therefore, pheromones play a major role in influencing the morphology of the nest. As termites respond to pheromone gradient, they organize their construction activities to produce tunnels, chambers, and airways that eventually contribute to the overall architecture and functionality of the nest (Heyde et al., 2021). This ensures that the nest adapts and evolves efficiently as the colony grows. 

Termite chemical defenses 

Termites have evolved a soldier caste within every species whose purpose is to defend the rest of the colony. They can do this physically with the use of their mandibles to kill invaders or phragmosis, blocking entrances to the mound with their strong heads. They can also defend against enemies by applying harmful chemicals to foreign threats. These chemical defenses come in three categories: Biting and Injecting Soldiers, Poison-brushing Soldiers, and Glue-squirting Soldiers. 

Biting and injecting soldiers 

There are three types of termites that use biting and injecting defenses to make their physical defenses more effective with the use of chemicals. Macroterms and Cubiterms are similar as they use their mandibles to inflict wounds on their enemies. Their chemical secretion is not an irritant but has anti-healing properties, causing the enemy to bleed out. Both types of soldiers use their cephalic defensive glands or frontal glands to store waxy secretions. When the mandibular muscles are used, they heat up the waxy secretions causing them to become oily. This makes them free flowing, allowing them to excrete through a pore called the fontella, located on the termite’s rostrum (forehead). Once excreted, the chemical flows onto the termite’s labrum (upper lip) and is spread on the enemy’s wound. They differ only in their mandible types and chemicals secreted. Macroterms have biting mandibles and can secret one or more of quinones, alkanes, and alkenes (Fig. 22). (Prestwich, 1986) 

Fig. 22.  Macroterms possible chemical secretions. Quinones are two carboxyl groups attached to a six-membered ring. Alkenes are a hydrocarbon chain that contains at least 1 double carbon-carbon bond. Alkanes are alkenes without any double carbon-carbon bonds (Prestwich, 1983). 

Macroterms that secrete a combination of alkenes and alkanes interfere physically with their enemy’s wound healing processes, affecting the initiation of coagulation and resclerotization. Cubiterms have slashing-type mandibles and use diterpene secretions (Fig. 23).  

Fig. 23. Diterpene consist of 20 carbon atoms. Most diterpenes are almost entirely unique to termites. They act as a toxic irritant, with anti-healing properties and slow enemies down due to their stickiness (Prestwich, 1983). 

Armiterms are different from both Macroterms and Cubiterms as they are a part of Nasutitermitinae which contains the glue squirting class, and that have nozzle-like rostrum. Armiterms have sharp, pointed, tong-like mandibles that pierce the enemy. After piercing the enemy, the nozzle drops oily secretion into the enemy’s wound. They secrete macrolides composed of macrocyclic lactone rings that act as toxins poisoning the enemy (Fig. 24) (Prestwich, 1986).  

Fig. 24. Monoterpenes, which are made from two isoprene units, are large cyclic structures containing one or more ester groups (–COO–). These rings are typically composed of 12 or more atoms, making them “macrocyclic.” (Prestwich, 1983). 

Poison-brushing soldiers 

Poison-brushing soldiers employ a chemical defense strategy that involves applying contact poisons directly onto attackers. These termites have developed an enlarged upper lip or labrum, often referred to as a daubing brush, which is used to spread toxic chemicals on the surface of invading predators. Poison-brushing termites store their toxic secretions in large cephalic and abdominal glands. When a predator approaches, the soldier termite will use its brush-like labrum to apply these toxic substances directly onto the surface of the attacker. These secretions are lipophilic (oil-soluble), meaning they can quickly penetrate the cuticle of the enemy and disrupt its biological functions. The poisons typically found in these soldiers are electrophilic lipids, which react with the biological molecules of the attacker (Prestwich, 1983). These chemicals consist of nitroalkenes, vinyl ketones and beta-ketoaldehydes (Fig. 25). They are highly reactive and cause immediate damage to the predator by forming irreversible bonds with essential biological structures, essentially “poisoning” them on contact. Nitroalkenes are reactive compounds that can interfere with the biological functions of enemies by reacting with nucleophilic sites (e.g., amino or hydroxyl groups) in proteins and enzymes. Vinyl ketones are another class of reactive lipids that can form covalent bonds with biological molecules, leading to cellular dysfunction or death. Beta-ketoaldehydes are highly reactive, attacking proteins and other essential biomolecules in the predator’s body (Prestwich, 1983). 

Fig. 25. Chemicals produced by poison brushing termites (Prestwich, 1983).  

Glue-squirting soldiers 

Glue-squirting soldiers, also called nasute soldiers, belong to a specialized group of termites in the Nasutitermitinae subfamily. These soldiers have evolved an entirely different defense mechanism, using their elongated rostrum, nasus (nose in latin), to squirt sticky and toxic chemical secretions at enemies, immobilizing them. Instead of biting or applying poisons through direct contact, nasute soldiers eject a viscous, sticky secretion through their nasus at predators. These secretions are rich in terpenoid compounds, which are both sticky and toxic, serving two purposes: physically immobilizing the attacker and chemically incapacitating them (Prestwich, 1986). The sticky substances that nasute soldiers secrete are primarily diterpenes and triterpenes, with main components including trinervitanes and tetracyclic kempanes (Fig. 26). Trinervitanes are diterpenoid compounds that are highly sticky, making them effective in entangling and slowing down attackers. Tetracyclic kempanes are complex terpenoids with toxic properties, found in species like Nasutitermes octopilis, and known for their unpalatability to ants (Prestwich, 1983).  

Fig. 26. Trinervitanes and kempanes found in the glue-like secretes of Nasutitermitinae (Prestwich, 1983.) 

The ejected glue-like secretion acts as both a physical barrier and a chemical weapon. When an attacker, such as an ant, is hit with this substance, it becomes trapped and is unable to move effectively. At the same time, the diterpenes in the glue have toxic effects, damaging the predator’s tissues and preventing recovery (Prestwich, 1986).  

Conclusion

Termites utilize chemistry in innovative ways, and this remains characteristic of their prosperity and survival. Termites are dependent on the microorganisms in their gut to digest lignocellulose. In lower termites, flagellates in the hindgut produce enzymes to break down lignocellulose into acetate, but this lacks in higher termites. Higher termites have bacteria or fungi as symbionts instead, which enable them to feed on a wider range of plant materials compared to lower termites. This symbiosis is crucial for the survival of the termite species but also contributes to nutrient recycling in nature. 

Termite’s caste differentiation system is complex, allowing for colonies to maintain strict caste ratios and achieve maximum colony efficiency. Interactions between environmental stimulus, pheromones, endocrine pathways, and gene expression allow for the development of caste-specific morphology in soldier, worker, alate and neotenic reproductive lines. Juvenile hormone titres are heavily influenced by social interactions between caste members, determining stationary, regressive or progressive development through pheromonal influence. In soldier differentiation, soldier primer pheromones inhibit presoldier development in workers by regulating gene expression and directly influencing JH levels. In neotenic reproductives, volatile, sex-specific king and queen primer pheromones suppress neotenic differentiation until the primary reproductives are no longer fit.  

Termites use pheromones to communicate in multiple ways. Their sternal gland is essential because it produces trail and sex-pairing pheromones that facilitate foraging, recruitment, and mating behaviors. This chemical regulation enables termites to adapt to their changing environment effectively. Additionally, pheromones play a significant role in construction activities, illustrating the principle of stigmergy, where individual actions lead to collective outcomes. Overall, chemical signals not only drive social organization but also shape the architecture of termite nests, highlighting the remarkable adaptability of these insects in their ecosystems. 

Soldier termites have evolved diverse chemical defense mechanisms to protect their colonies. They use their mandibles to bite and wound predators, injecting toxic secretions that can incapacitate or poison invaders. Some soldiers secrete compounds that prevent wound healing, enhancing their defensive effectiveness. Poison-brushing soldiers apply reactive chemicals onto attackers’ cuticles using an enlarged labrum, causing immediate damage. Glue-squirting soldiers squirt sticky secretions through an elongated nasus, immobilizing and poisoning invaders. These varied strategies demonstrate the complex chemical adaptations termites use to defend their colonies. 

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