Abstract
Stenopsyche marmorata larvae spin underwater adhesive silk for constructing nests and capture nets. The silk can be divided into fiber and adhesive regions, according to their function. The silk fiber region has a two-layer structure: a core layer situated at the center of the fiber and S. marmorata fibroin, the major component of the silk. In the anterior part of the anterior silk gland, the morphological characteristics suggest that the silk insolubilization leading to fibrillation occurs by luminal pH neutralization. The adhesive region is composed of three layers: the outermost (OM), B, and C layers. On the B layer, coated with the OM layer, numerous nano-order pillar structures (nanopillar structures) are located at regular intervals. A nanopillar structure is approximately 40 nm in diameter and 125 nm in length. The precursor materials of the nanopillar structure are electron-dense globules of approximately 25 nm in diameter that are located in the A layer of the lumen of the middle silk gland. The precursor globules autonomously connect to one another on the B layer when the liquid silk is transported to the lumen of the bulbous region. The nanopillar structures probably contribute to the strong underwater adhesion of S. marmorata silk.
Highlights
Stenopsyche marmorata larvae spin underwater adhesive silk for constructing nests and capture nets
The silk gland could be divided into four divisions on the basis of the types of silk secretion and the histological features: the posterior silk gland, the middle silk gland, the bulbous region, and the anterior silk gland (Fig. 1)
There is an example of multiple adhesive proteins produced from single-cell glands in barnacles[19], it was unclear whether multiple proteins are secreted from the same cell in S. marmorata middle silk gland (MSG)
Summary
Stenopsyche marmorata larvae spin underwater adhesive silk for constructing nests and capture nets. The precursor materials of the nanopillar structure are electron-dense globules of approximately 25 nm in diameter that are located in the A layer of the lumen of the middle silk gland. Trichoptera larvae construct retreats and cocoons with sand and gravel or plants using underwater adhesive silk. Since Trichoptera silk has been suggested to have strong insolubility[6], investigating the silk composition and the secretion process is instructive to understand the adhesion mechanism and each constituent protein function. Spider silk functioning as an adhesive secreted from pyriform[11,12] and cylindrical gland[13] consists of two component compositions similar to B. mori silk, which are composed of core fiber and an amorphous coating.
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