Study On The Microstructure And Ultrastructure Of Silk Glands Of Gryllacrididae

Raspy crickets(Orthoptera: Gryllacrididae)are unique among the orthopterans in producing silk,which is used to build shelters.The silk glands of three species of Gryllacrididae,Capnogryllacris(Capnogryllacris)spinosa(Li,Liu & Li,2014),Ocellarnaca emeiensis Li,Fang,Liu & Li,2014 and Bianigryllacris parvospinus(Liu & Yin,2002),were examined and documented using immunofluorescence,PAS-hematoxylin staining,scanning electron microscopy and transmission electron microscopy.The purpose was to investigate the structural characteristics of the silk glands.We also compared the structure of the salivary glands of Gampsocleis gratiosa Brunner von Wattenwyl,1862 with silk glands of Gryllacrididae,and explored the homology of silk glands and salivary glands of non-spinning Ensifera.Studies have shown that the microstructure and ultrastructure of silk glands in different species are consistent.Dissection of Gryllacrididae reveals large,acinar labial glands in the thorax and head.Ducts from the acini on each side of the body merged into a common duct opening in the salivarium below the hypopharyngeal base.The common ducts from both sides of the body join together and empty into the salivarium through a common aperture.The silk glands are divided into two parts: the acini and the silk ducts.By microscopic observation,each acinus is composed of a connective tissue sheath enclosing 5 main types of cell: sheath cells,type I secretory cells,type II secretory cells peripheral cells and canal cells.Type I secretory cells are near the center of the acinus.The outermost cells are peripheral cells,and type ? secretory cells are located between type ?secretory cells and peripheral cells or sheath cells.The canal cells are scattered between the secretory cells.Type ? secretory cells and type ? secretory cells are glandular cells with large,irregular shapes and secretory particles in the cytoplasm.Peripheral cells and canal cells are transport cells.Pericellular cells are triangular in shape.There is a microvilli cavity formed by cell membrane invagination in a peripheral cell,which is supported by microfilaments.Canal cells surround extracellular canals.The silk ducts of the silk glands consist of monolayer cells.Secretory cells are characterized by containing a large number of endoplasmic reticulumand secretory particles,and different secretory cells have different secretory particles through ultrastructure observation.There are a large number of mitochondria in the cytoplasm of the peripheral cells,and the microvilli in the microvilli cavity are neatly arranged.Canal cells are irregular in shape,and the shape of the cells is compatible with extracellular canals.The microvilli cavity is connected to an extracellular canal surrounded by canal cells.Secretory particles accumulated at the junction of the secretory cells and the transport canals.Then they discharge secretions to the extracellular transport canals.Multiple acinar extracellular transport canals are pooled into one and the same branch silk duct.The cell periphery is involved in the organization of a series of deep invaginations of the plasma membrane,forming numerous cytoplasmic compartments.A large number of elongated mitochondria can be observed around the cell plasma membrane invaginations.The apical border of the cell near the inner border of the lumen has continuous small processes that are closely aligned under the cuticle of the duct wall.Occurrence of silk glands in unrelated orders of insects indicates that these glands are of polyphyletic origin.The structure of silk glands of Gryllacrididae is obviously different from other spinning-insects.Gryllacrididae silk glands and the Gampsocleis gratiosa salivary glands are very similar in terms of the position,tissue morphology,and cells microstructure and ultrastructure,except for secretory granules type and structure and endoplasmic reticula type of secretory cells.It is credible that silk glands of Gryllacrididae and salivary glands of katydids are homologous.The secretory material production and secretion process are as follows: secretory particles are produced by different secretory cells,and then gather in the microvilli area where the secretory cells contact the extracellular canals.The secreted materials are discharged to the extracellular canals.The extracellular transport canals are connected to branch ducts.And then the secreted materials pass through the branch ducts to common duct of the silk glands,which it is finally discharged out of the body through the salivarium and forming solid silk.When the secretions are transported outward from the extracellular canal surrounded by the canal cells,the microfilaments in the microvilli cavity of peripheral cells may provide impetus for the excretion of the secretions.In addition,it is speculated that the two secretions secreted by different secretory cells may be different constituent proteins of silk.