This work presents a short review of a social insect taxon, the termites. The review will show the complex behaviours present in termites, ranging from agriculture to architecture, to the use of medicines to protect the colony against disease.
Termites (Isoptera) are the only hemimetabolous insects that show eusocial behaviour (Korb, 2008a). Hemimetabolous insects are those insects that have only three stages of development: egg, nymph and the adult stage (imago), with no pupal stage (Korb, 2008a). Hemimetabolous insects include the isoptera, the hemiptera, the orthoptera, the dermaptera and the odonata (Korb, 2008a). Eusociality is the term used to describe the situation in which two generations of con-specific animals live together (i.e., form a colony) and cooperate with each other to the extent that a reproductive skew occurs because only a few individuals within the colony lay viable eggs (Abe et al., 2000).
Termites live in large communal nests that are large enough to house a whole colony, with each nest having one King and one Queen, plus thousands of worker or soldier termites serving the nest. Workers provide labour, including such things as foraging and nursing and soldiers defend the colony and the Queen (Abe et al., 2000). As shown in Figure 1, the immature individuals (i.e., the workers and soldiers) are usually pale in colour and look like ants (hence the term ‘white ant’ which is frequently used to describe termites) and have small, compound, eyes with a large cylindrical head and beaded antennae (Abe et al., 2000). Immature termites have chewing mouthparts, often with large mandibles. As shown in Figure 2, the adults (i.e., the King or the Queen) are usually darker in colour than the immature individuals, with a well-developed head, chewing mouthparts, beaded antennae and compound eyes (Abe et al., 2000). In contrast to the immature individuals, adult termites have two pairs of wings, which are shed once the adult has mated (Abe et al., 2000).
As argued by Korb (2008a), termites are one of the most abundant animals on Earth, with four families and forty-four species currently recognised by termite taxonomists. The four main families of termites are the Rhimotermitidae or subterranean termites; the Hodotermitidae or rottenwood termites; the Kalotermitidae or drywood termites and the Termitidae. Food supplies for the termite colony are collected by the workers who feed themselves and then feed the dependent castes, with termites being primary consumers, eating vegetation – whether this be living, dead or decomposing – and other food sources such as fungi and other incidental foods such as other termites of the same colony and the organic portions of the termite colony (Brian, 1978). Abe et al. (2000) groups termites in to three main life types, depending on their mode of resource utilisation: one-piece types that nest in wood and consume only that wood; intermediate types that nest in wood and consume that wood as food and also other wood types; and separate types that nest in various places (wood, soil etc.) and that consume various types of dead plant material, or other materials, such as nitrogen-rich fungal biomass. The phylogeny of termites closely follows this functional separation of the termites (Abe et al., 2000).
Termites are known to forage during the night, early morning and evening, with corridors being constructed to protect workers up until the point they exit these corridors to forage for food. Foraging trails are laid to direct other workers to the richest sources of food. Experiments have shown that termites have distinct food preferences in terms of the type and amount of food they eat (Brian, 1978). They are also known to store food in the Queen’s nest, and in the colony, with some species of termite creating galleries within the colony within which stores of food are placed. There is evidence that Macrotermes cover stored food in saliva and then eat the stored food once this saliva has dried; it is hypothesised that the saliva contains some sort of antibiotic to prevent disease developing (Brian, 1978).
Termites are usually the most abundant animals in any given tropical habitat, with colonies of termites hosting up to 200,000 individuals, giving an estimated 1,000,000 individuals per square metre (Meyer et al., 2000). The biomass of these individuals exceeds the biomass of all vertebrate species living in the same area (Meyer et al., 2000). There is a positive relationship between the number of individuals present in the termite colony and the height of mounds, or nests, with the proportion of soldiers to workers changing as colonies grow larger, with a higher proportion of soldier termites being present in larger colonies (Meyer et al., 2000). Termite colonies, which are very complex structures (see Figure 3), are thought to be built through a stigmergic process, of spontaneous, indirect, coordination between individuals in the colony, where the trace left by one action promotes another action either by the same, or a different, individual (Wilson, 2000). It is thought that this process of stigmergy is propagated through the termite colony by the laying down of pheromone signals, which individuals then follow using a simple, decentralised, set of rules (Wilson, 2000).
The ecological success of termites is attributed to their social life, which allows them to live socially and to allocate tasks to each of the different types of termites that live in the colony. Termites live in highly organised, complex, societies, characterised by a reproductive division of labour, where a few individuals in the colony (the King and the Queen) reproduce whilst the vast majority of the colony forgoes reproduction in order to perform other tasks that are important for the colony as a whole (Korb, 2008a).
It is thought that this altruistic behaviour arose as a result of kin-selection, which describes how genes can be propagated through close relatives (Hamilton, 1964; Maynard Smith, 1964). Termites are diploid, with both sexes developing from fertilized diploid eggs (Korb, 2008a). This process results in symmetrical relatedness associations within monogamous termite colonies, with the relatedness among full siblings and between parents and their offspring being identical and always 0.5 (Korb, 2008a). It is thought that, in the termites, this form of sex determination evolved via the formation of an altruistic mutant that evolved within a group of coexisting relatives that had the effect of increasing group productivity (Korb, 2008a; Korb, 2008b). This altruistic mutant was thus propagated because of the many benefits it gave to the colony as a whole (Korb, 2008b).
Termites participate in symbiotic behaviours with various other species, including Archaea, Eubacteria and Eucarya. Termites, for example, are unable to digest the fibre in wood and so harbour bacteria in their digestive tracts that can break down this fibre; the termites feed on the by-product of this bacterial digestion. In addition, some termite species, such as Macrotermes subhyalinus, farm fungus. This is one of only two origins of fungus farming in social insects, the other being in new-world ants, which originated about 50 million years ago (Aanen and Boomsma, 2000) and only one of three occurrences of agriculture in insects, the third appearance being in ambrosia beetles (Mueller et al., 2005). Aanen et al. (2002) looked at the evolution of fungus-growing termites and their mutualistic fungal symbionts, showing that the symbiosis has a single African origin, and that secondary domestication of other fungi to a free-living state has not occurred, even though host switching has been frequent and nests of termites can have different species of fungal symbionts.
In fungus farming termites, the termites forage for plant material to provide food for the gardens of fungus that they maintain. The carbon-rich plant material is converted to nitrogen-rich fungal biomass, providing food for the termites (Aanen and Boomsma, 2000). This mutualistic symbiosis is thought to benefit both species, on an evolutionary scale, although the direct mechanisms by which unrelated individuals of different species come to be symbiotic in this manner is not well understood (Aanen and Boomsma, 2000; Forster and Wenseleers, 2006). It is interesting to note that these fungus farms are the subject of attack by parasites, and that the termites are resistant to the disease produced by these parasites, thanks to a bacteria they carry with them which produces an antibiotic against the disease (Aanen and Boomsma, 2000; Currie et al., 1999). Mueller et al. (2005) suggest that fungus-farming insects have evolved a combination of strategies to combat diseases: to bring the raw materials for their gardens from the outside environment which is thought to control pathogens, minimising the chance of disease outbreak; to constantly monitor gardens controlling for the development of pathogens; to access genetically variable reserves of cultivars, in order to minimise the chances of disease affecting the whole farming process; and to manage auxiliary bacteria which harbour compounds that are effective in controlling potential diseases that might spread through the farm (Mueller et al., 2005). Fungus farming termites thus have a wide array of strategies available in order to minimise the chance of disease and to control disease, should an outbreak occur within the farm. This is surprisingly complex behaviour for an insect.
In addition to the eusociality of termites and their symbioses, termites show surprisingly complex behaviours. Affolter and Leuthold (2000) showed that Macrotermes subhyalinus, a species of burrowing termite (Macrotermes), uses trail pheromones in both quantitative and qualitative ways in order to transmit specific information about the destination of a trail, depending on the motivational context of the receivers. As Weir (1973) explains, the design and build of termite mounds regulates the internal temperature of the mounds, providing a constant temperature inside the mounds to protect the colony and to allow all of the life processes of the colony to occur. Dangerfield et al. (1998) show how the mound-building termite Macrotermes michaelseni is an ecosystem engineer, creating resource flows that affect the composition and spatial arrangement of both current, and future, organismal diversity. Dangerfield et al. (1998) argue that Macrotermes michaelseni acts as an ecosystem engineer across a range of spatial scales, from the alteration of local infiltration rates to the creation of landscape mosaics, many of which incorporate feedback processes which lead to the accrual of beneficial effects for the colony.
In addition to their eusociality and complex behaviours, termites also exhibit some striking defence mechanisms. Kaib et al. (2004) describe the defence mechanisms, and aggression, of Macrotermes subhyalinus, showing that cuticular hydrocarbons are the most likely candidates for nestmate recognition in termites. An analysis of the cuticular hydrocarbons present in four termite colonies showed that each of the four colonies had a different cuticular hydrocarbon phenotype, with a clear correlation between difference in cuticular hydrocarbon phenotype and aggression between colonies, a relationship that holds for genetic similarity (Kaib et al., 2004). It is thus hypothesised that these cuticular hydrocarbon phenotypes play a key role in colony recognition (Kaib et al., 2004). As for all social insects, it is important for individual termites to be able to distinguish nestmates and non-nestmates (Kaib et al., 2004). The form of this recognition is thought to occur through the use of a label/template system: the individual learns a certain label and if the individual they encounter does not match the template they carry for this label, then that individual cannot be a nestmate (Kaib et al., 2004). As Kaib et al. (2004) explain, this is highly complex behaviour, as it requires the acquisition, expression or transfer of a label and the learning of a template.
In conclusion, then, termites are eusocial, abundant in any habitat in which they appear, and show many highly complex behaviours including stigmergy, agriculture, chemical defence, complex intelligence and the use of medicinal products to protect against disease.
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