The evolution of termite caste differentiation may result from the alterations of some stages or durations in the normal postembryonic developmental process known in the most hemimetabolous insects. Analyses on the development of caste-specific organs can be the effective methods to elucidate the developmental mechanisms. Therefore, we deeply examined the postembryonic compound eye development in Reticulitermes speratus (Kolbe, 1885) to understand how to regulate the eye development during caste differentiation. In this species, the differentiation into the imaginal (nymph and alate) and apterous (worker and soldier) lines occurs at the second molt. In addition, nymphoids (second-form reproductives) and ergatoids (third-form reproductives) emerge from nymphs and workers, respectively.
First, the outer morphology from various instars and castes were investigated using a scanning electron microscopy, and histological observations were carried out with paraffin sections. The eye primordia were shown to exist even in the larval stage. The numbers of ommatidia increased linearly and the eye area increased abruptly in the course of imaginal development. Nymphoids possessed the relatively developed eye and thick optic nerve, but each ommatidium could not clearly discriminated. Whereas, at the line of apterous workers and soldiers, although the outer rim of eye was observed from the 2nd stage workers, there were little morphological differences among instars including ergatoids. Next, we analyzed the gene expression level, which are related to eye morphogenesis, by the quantitative PCR with beta-actin as a control gene. As the results, the expression levels of analyzed genes (Pax6 and opsin) were quite different among each instar or caste. In worker, soldier and larvae, especially, the expression of both genes were similarly repressed. These results suggest that the eye development in the apterous line could be strongly arrested, and that these different morphogenetic patterns would have an influence on the appearance of various castes in termites.