Fusulus

A fusulus (plural: fusuli) is a microscopic structure, typically conical or spindle-shaped, found in the spinnerets of spiders and some other silk-producing arthropods. These structures are integral to the production of silk fibers, each fusulus serving as a tiny nozzle through which the liquid silk protein, known as fibroin, is extruded. The shape and size of the fusulus, as well as the type of silk gland it is connected to, determine the characteristics of the resulting silk thread, such as its thickness, strength, and adhesive properties. Different types of fusuli are responsible for producing different kinds of silk, allowing spiders to create a wide variety of silks for web construction, prey capture, egg sacs, and other purposes. The number of fusuli present in a spider's spinnerets varies depending on the species and the complexity of the silk it produces. The morphology and arrangement of fusuli are important taxonomic characteristics in spiders and contribute significantly to the understanding of silk evolution and function.