Vestigial spermatheca morphology in honeybee workers, Apis cerana and Apis mellifera, from Japan

Reduction of reproductive organs in workers is one of the most important traits for caste specialization in social insects. In this study, we investigated the morphology of the sperm storage organ, the spermatheca, in workers of the honeybees Apis cerana and Apis mellifera from Japan. All examined workers of the two species retain a small spermatheca. Ultrastructural observations indicate that the spermatheca of honeybee workers consists of conspicuous muscular layers which correspond with the sperm pump and that the epithelium surrounded by these muscular layers represents the spermatheca duct. As this epithelium shows intact cellular morphology, we conclude that the spermatheca duct and the sperm pump do not degenerate, whereas the spermatheca reservoir has completely degenerated. All workers of A. cerana and most workers of A. mellifera lack the spermatheca gland completely. A few workers of A. mellifera possess the opening of the spermatheca gland, but its structure seems to be degenerated. The spermathecal width including the muscular layers showed a weak, but significant positive correlation with ovariole numbers in A. cerana, but not in A. mellifera. The degree of spermatheca gland degeneration seems to be unrelated to ovariole number. The morphological features of degeneration in honeybee workers are different from those of the vestigial spermatheca in ant workers, indicating that morphological worker virginity has evolved in a different way.