Structural and compositional properties of silk-like materials produced by the larval salivary glands of Keroplatus testaceus (Diptera)

Silk materials have been extensively studied across several insect groups; however, dipterans remain largely unexplored. In this study, we examined the silk-like secretions of Keroplatus testaceus larvae using histochemistry, transcriptomics, proteomics, mechanical assays, and Fourier-transform infrared (FTIR) spectroscopy. Our results show that a single pair of labial salivary glands produces two distinct secretions, the mucus web and the cocoon, which differ in both composition and function. Neither secretion contains fibroin. Instead, both are composed of a complex mixture of hydrophilic proteins, including collagen-like, mucin-like, novel proline-rich proteins, peroxidases, protease inhibitors, and other proteins with no clear homologs in other species. Structural predictions and FTIR analysis of the cocoon reveal the presence of secondary structures, including β-sheets, α-helices, and polyproline II helices. Although the secretion lacks the characteristic fibrillar core of fibroin-based silks, it is highly hydrated, rich in proline-containing proteins, and exhibits relatively high mechanical strength. These findings confirm that K. testaceus produces a highly specialized silk-like material that is structurally and compositionally adapted to larval life within wood-decomposing fungal fruiting bodies.