All castes of termites show specific behavioral tendencies. Especially in defensive behavior, there are conspicuous differences between soldiers and other castes. For example, soldiers attack against predators aggressively, while larvae and pseudergates escape into their nest without any attacks. Based on the hypothesis that some soldier-specific neural modifications should give rise to soldier defensive behavior, we compared the central nervous systems of soldiers and pseudergates in the damp-wood termite Hodotermopsis sjostedti. First, the histological and internal morphological characteristics of brain and suboesophageal ganglion (SOG) were examined. The results showed that the SOG of soldiers is larger than that of pseudergates. In addition, the soldier SOG possesses extremely giant neural somata around at the base of mandibular nerves. Because these giant neurons are observed even in presoldiers, it is suggested that the neurons of SOG are enlarged during the process of soldier differentiation.
    The giant neurons show general characteristics of invertebrate motor neurons, and exist around the base of mandibular nerves. Considering that the nerves, as well as mandibles and mandibular muscles, are enlarged in soldiers, the giant neurons were suggested to control these muscles. Therefore, we performed backfill experiments from mandibular closer muscles using fluorescent dextrans, showing that the giant neurons were stained and approximately 1.5 times as large as the corresponding neurons of pseudergates. Thus, it was revealed that soldier-specific neural modification in Hodotermopsis sjostedti is associated with the gigantism of mandibular motor neuron in SOG.
    Finally, in order to discuss the evolutionary aspects, we examined the giant motor neurons in other families of termites. Expectedly, soldier-specific giant neurons are observed in all families examined. Therefore, it can be concluded that the soldier-specific gigantism of mandible motor neuron has been conserved in all termite lineages.