Sexual dimorphism and ultrastructure of salivary glands in adult Bittacus cirratus Tjeder, 1956 (Mecoptera: Bittacidae)

Sexual dimorphism is ubiquitous in insects and has been extensively studied across various taxa. However, it remains unclear whether the salivary glands exhibit sexual dimorphism in adult Bittacidae. In this study, a comparative study of salivary glands in adult male and female Bittacus cirratus Tjeder, 1956 was performed using light and transmission electron microscopy. The salivary glands are tubular labial glands in both sexes of B. cirratus, consisting of a salivary pump, a common salivary duct, a pair of efferent ducts, and numerous dichotomously branching secretory tubules. Interestingly, the salivary glands in females contain a pair of small salivary reservoirs, which are not present in the males. The male salivary glands have a significantly larger diameter and more secretory tubules than those in females. In contrast, male secretory cells occupy a minimal volume of the secretory tubule, resulting in a larger secretory lumen that nearly fills the entire tubule. This configuration enhances the storage capacity of the male secretory tubules. Male secretory cells exhibit active autophagy, possibly due to their approaching late life stages. Sexual dimorphism in the salivary glands and the possible reasons are briefly discussed in Bittacidae.