Abstract
Silks and silk-producing organs of the theraphosid species Avicularia metallica were studied using scanning electron microscopy. The spinning apparatus is made up of two pairs of spinnerets located at the end of the ventral side of the opisthosoma. Both pairs of spinnerets are equipped with spigots (modified setae), i.e. external outlets of silk-producing glands which, in the case of posterior lateral spinnerets, are present on all three segments.The secreted silk, which hardens when exposed to air, is processed by movements of spinnerets and the opisthosoma. An investigation of spinning activity revealed two different manners in which silk is affixed to the ground: (1) by smearing silk secretion directly onto the surface of the substratum; or (2) by attaching silken fibers onto a layer of adhesive silk of attachment fields. The fibers connecting the walls of tubular shelters to the silk of attachment fields are essentially bundles of parallel nanofibrils. The connection between multifibrillar connecting fibers and the adhesive silk of attachment fields is, in essence, “silk-to-silk” connection. Nanoglobules (spherical subunits) are the basic microstructural blocks in the studied silk materials irrespective of whether the fibrils are a part of the tube, connecting fibers, or attachment fields. SEM images showed that the liquid silk, running through spigot ducts, has two components, which do not mix as they leave the spigots. The peripheral component of the solidified protein mixture surrounds the central component, which has a granular appearance
Highlights
Evolution of the order Araneae most likely occurred along three major evolutionary lines, currently represented by the infraorders Mesothelae, Mygalomorphae, and Araneomorphae (Coddington & Levi 1991)
Two pairs of spinnerets are a characteristic feature of all theraphosids studied to date, spiders of the infraorder Mygalomorphae include families which have, like the Araneomorphae, developed three pairs of spinnerets
A single functional pair of posterior lateral spinnerets is characteristic of spiders of the family Nemesiidae, whose small, monosegmental posterior median spinnerets have no spigots at all (Glatz 1972)
Summary
Evolution of the order Araneae most likely occurred along three major evolutionary lines, currently represented by the infraorders Mesothelae, Mygalomorphae, and Araneomorphae (Coddington & Levi 1991). The infraorders exhibit differences in the structure of some important organs, the most remarkable of which is the silk-producing organ. The silk-spinning apparatus of spiders consists of spinnerets and silk spinning glands located in the opisthosoma. These glands originate from ectodermal invaginations on the embryonic spinneret limb buds, in relation to the morphogenesis of these buds (Hilbrant & Damen 2015). The saclike silk glands of a spider are lined by a secretory epithelium that is supported by a basal lamina and that secretes silk secretion into the glandular lumen (Kovoor 1987). The epithelial cells are capable of producing different types of silk secretions which consist of liquid fibroin proteins.
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