Fine structure of the silk spinning system in the caddisworm, Hydatophylax nigrovittatus (Trichoptera: Limnephilidae)

Abstract

Silk is produced by a variety of insects, but only silk made by terrestrial arthropods has been examined in detail. To fill the gap, this study was designed to understand the silk spinning system of aquatic insect. The larvae of caddis flies, Hydatophylax nigrovittatus produce silk through a pair of labial silk glands and use raw silk to protect themselves in the aquatic environment. The result of this study clearly shows that although silk fibers are made under aquatic conditions, the cellular silk production system is quite similar to that of terrestrial arthropods. Typically, silk production in caddisworm has been achieved by two independent processes in the silk glands. This includes the synthesis of silk fibroin in the posterior region, the production of adhesive glycoproteins in the anterior region, which are ultimately accumulated into functional silk dope and converted to a silk ribbon coated with gluey substances. At the cellular level, each substance of fibroin and glycoprotein is specifically synthesized at different locations, and then transported from the rough ER to the Golgi apparatus as transport vesicles, respectively. Thereafter, the secretory vesicles gradually increase in size by vesicular fusion, forming larger secretory granules containing specific proteins. It was found that these granules eventually migrate to the apical membrane and are exocytosed into the lumen by a mechanism of merocrine secretion.

Comprehensive analysis of silk proteins and gland compartments in Limnephilus lunatus, a case-making trichopteran

“…This part produces a thin axial thread of unknown composition located in the middle of fibroin filament [ 17 ]. This may be a peculiarity of a small group of species because such axial thread has not been observed in any other species [ 1 , 10 ]. Remarkably, the proposed MSG in S. marmorata seems to produce both fibroin and the adhesive coating [ 17 ].…”

“…Remarkably, the proposed MSG in S. marmorata seems to produce both fibroin and the adhesive coating [ 17 ]. Another study by Kim et al (2020) investigated the morphology of SGs using electron microscopy in the limnephilid caddisfly Hydatophylax nigrovittatus [ 10 ]. They divided the SG into ASG and PSG and found that both parts contain different secretory cells.…”

“…The epithelium of the ASG contains fine granular materials as secretory products in contrast to the large secretory globules found in the rear SG. Thus, Kim et al (2020) hypothesized that the rear SG produces fibroin, while the ASG secretes sticky substances required for gluing together the two silk filaments [ 10 ].…”

“…The connection of the two threads is visible in the form of a seam in the middle of the ribbon. This is typical of species that use silk to bond different natural materials in the construction of their shells, including Hesperophylax consimilis [ 5 , 6 ], Hesperophylax occidentalis [ 7 , 8 ], Neophylax concinnus [ 9 ] Hydatophylax nigrovittatus [ 10 ], Drusus improvisus [ 11 ] , and Limnephilus vittalus [ 11 ]. In contrast, the SG of predatory species such as Plectrocnemia conspersa [ 1 ] , Hydropsyche pellucidula [ 11 ] and Parapsyche elsis [ 12 ] (suborder Annulipalpia) produce more distinct fibers to build self-supporting capture nets and retreats.…”