The societies of Isoptera and Hymenoptera differ in two general features. 1. The termite societies are bisexual. From a primitive symmetry between both sexes, many asymmetries appeared during the course of their evolution. These asymmetries are related either to a sexual dimorphism, or a biased sex ratio, or both, and are differently expressed in the separate castes of a given species. 2. The hemimetabolous development allows a termite to take part in the social tasks before the end of its postembryonic development, and even to reproduce at a larval stage (neoteny). Thus Isoptera exhibit a polymorphism of larvae, unlike Hymenoptera where a polymorphism of imagoes is observed. Moreover, an individual can change its physical caste: this temporal polymorphism never exists in Hymenoptera. These facts have important consequences for the caste system and its evolution. The differences between the social structures of Isoptera and Hymenoptera are related to the differences between their non-social ancestors.

Social structure in termite societies

The caste developmental pathway for the production of soldiers and nymphs in termite colonies is variable among species and limited numbers of studies have properly categorized such pathways. In this study, the developmental pathway for the production of soldiers and nymphs in Coptotermes formosanus Shiraki has been re-analyzed using the data from previous studies in light of new observations. Five distinct colony life stages were determined based on the caste developmental pathway used by the colony at each stage, which we here redefined as: incipient colony, immature colony, juvenile colony, mature colony, and senescent colony. We determined that in incipient colonies, the first soldier (nanitic) emerges from a second instar larva. We suggest that the production of a nanitic soldier is cost-effective as it allows the colony to focus on the production of workers from the first cohort of individuals. However, in an immature colony, the origin for the production of soldiers shifts to first and second instar workers. In juvenile colonies, soldiers are essentially produced from second instar workers. In mature colonies (>5 years), soldiers are mostly produced from second instar workers, occasionally from third instar workers, and we suggest that nymphs are primarily produced from second instar larvae. Finally, in senescent colonies, the absence of brood leads to the accumulation of old individuals. We concluded that descriptions from previous studies may have overlooked the group dynamics within a colony and that the caste developmental pathway in C. formosanus is dependent on the age of the colony.

Colony age-dependent pathway in caste development of Coptotermes formosanus Shiraki

Bartz's (1979) inbreeding hypothesis on termite social evolution predicted that (1) all termite colonies should have supplementary reproductives, (2) the winged alates should be the offspring of inbreeding supplementaries, and (3) alates should outbreed. These predictions are examined here for each family and subfamily of the Isoptera. Prediction 1 is at least superficially upheld for most termites that do produce either neotenic or adultoid reproductives. The rate of supplementary and replacement reproduction, however, varies greatly between different taxa. Prediction 2 is not upheld for the majority of termites because primary reproductives tend to be long lived and primary-headed colonies do generate alates. It may be upheld for Mastotermes, many of the Rhinotermitidae, and some Termitidae. Prediction 3 appears to be true for most termines, although support for it is inferential only. The breeding pedigrees of termites vary greatly with their ecological specializations and it is unlikely that Bartz's m...

Termite Eusocial Evolution: A Re-Examination of Bartz's Hypothesis and Assumptions

Since almost all termite species possess a soldier caste, there must be ubiquitous mechanisms of soldier differentiation throughout isopteran species. In order to reveal the caste differentiation mechanisms, observations during the soldier morphogenesis and identification of soldier specific gene expression are thought to be important. In this article, I summarize research approaches for analyzing caste differentiation in termites, and introduce two of our studies in Hospitalitermes medioflavus (Termitidae) and Hodotermopsis japonica (Termopsidae).¶Colonies of the nasute termite H. medioflavus have soldiers with a frontal projection (nasus) on the head, from which defensive substances are secreted. During soldier differentiation from male minor worker to presoldier, the most dynamic morphogenesis occurs. In the presumptive nasus epithelium of minor workers, a disc-like structure termed "soldier-nasus disc" rapidly develops to form the nasus of presoldiers. This rapid growth is associated with two folding layers of cuticle and epithelium.¶To identify genes specifically expressed in soldiers of the damp wood termite Hodotermopsis japonica, a differential display using RT-PCR was tried, comparing mRNA from the heads of soldiers and pseudergates. An identified gene candidate termed SOL1 was expressed specifically in terminally differentiated mature soldiers, and the product of the gene was suggested to encode a novel protein with a putative signal peptide at the N-terminus. This gene was shown to be expressed in the mandibular glands which actually develop during the soldier differentiation. Thus, these molecular techniques are applicable to reveal the proximate mechanisms of caste determination in termites and other social insects.

Morphogenesis and gene expression in the soldier-caste differentiation of termites

http://www.chikyu.ac.jp/yamamura-pro/yamamura/PDF/02Soldier%20Production.pdf

Soldier production strategy in lower termites: from young instars or old instars?

Publisher Summary The study of post-embryonic development in Schedorhinotermes lamanianus reveals numerous special characteristics. The neuters are all female right from the time of hatching onwards, and presumably from the time the eggs are laid. The determination of the neuter or reproductive line of development is made before hatching and the extent of growth of their gonads is determined from the first larval instar. The neuters are also formed during the period of reproductive formation. Larvae go through three instars and depend on workers for food since they do not participate in any work. Workers moult from the third instar larvae, and then acquire a symbiotic intestinal fauna. The soldiers are divided into small-, medium- and large-sized soldiers. All these soldiers develop through a white soldier intermediate stage and exert an inhibitory action on the formation of new soldiers. S. lamanianus is classified from the anatomical and physiological points of view as a lower termite; its polymorphism and social organization are closer to those of the higher termites, which indicate a differential rate of evolution for different characteristics. This demonstrates the difficulty of adding new data to a general conception and theory of termite evolution.

Dynamic Study of Polymorphism in Schedorhinotermes lamanianus (Rhinotermitidae)

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Dynamic Study of Polymorphism in Schedorhinotermes lamanianus (Rhinotermitidae)