Spinnerets are specialized organs that spiders and some other arthropods use to produce silk. This biological feature allows for the creation of a material with remarkable properties. Spiders have structures specifically for making silk, which starts as a liquid in internal glands and hardens into a solid. The silk is then extruded through the spinnerets, which are located on the outside of the abdomen.
Anatomy of a Spinneret
The spinnerets are typically located on the posterior end of a spider’s abdomen. Most spiders have six spinnerets, though some species may have two, four, or eight. These spinnerets can move independently or together, allowing for a high degree of control over silk placement and tension.
Each spinneret is covered in microscopic nozzles called spigots, with each spigot connected to a specific silk gland. This arrangement allows spiders to produce different types of silk by using different spigots. The number and arrangement of these spigots can vary significantly between spider species, which contributes to the diversity of silk types and web structures observed in nature.
The Silk Production Process
Inside the spider, silk is stored as a liquid protein concentrate primarily composed of proteins called spidroin. This liquid is a gel-like substance that is transformed into a solid fiber as it passes through the spider’s silk glands. The process is initiated when the spider begins to pull the silk from the spinnerets.
The transformation from a liquid to a solid is a result of both physical and chemical changes. As the liquid silk is pulled through the narrow ducts leading to the spigots, it is subjected to physical shearing forces. This stretching aligns the spidroin protein molecules. At the same time, cells in the duct walls draw water out of the silk solution, and hydrogen ions are pumped in, creating an acidic environment. This change in pH causes the protein molecules to lock together, solidifying the silk into a strong and elastic fiber.
The spider has precise control over this process, and can vary the thickness and properties of the silk by adjusting the speed at which it is pulled and the specific spigots used. The final product is a material that is, by weight, stronger than steel.
The Different Types of Silk
Spiders can produce as many as seven different types of silk, each with unique properties and functions. The ability to produce multiple types of silk allows spiders to build complex structures and adapt to various environmental challenges.
One of the most well-known types of silk is the strong, non-sticky dragline silk. This is used for the outer frame and radii of a web, as well as for the spider’s lifeline, which it trails behind it as it moves. Another common type is the sticky capture silk, which is used for the spiral parts of the web that trap prey. This silk is often coated with a glue-like substance to increase its adhesive properties.
Spiders also produce fine silk for wrapping their prey, which immobilizes the captured insect and prevents it from escaping. A softer, more delicate silk is used to create egg sacs, which protect the developing spiderlings from predators and the elements.
Spinnerets in Other Organisms
While spiders are the most famous silk-producers, they are not the only creatures to have evolved this ability. Other organisms also produce silk, though the mechanisms they use can be quite different. This highlights the convergent evolution of silk production in different branches of the animal kingdom.
Silkworm larvae, for example, produce silk from modified salivary glands located in their mouths. This silk is used to spin a cocoon, which protects the larva during its metamorphosis into a moth. Unlike spiders, which have abdominal spinnerets, silkworms extrude silk from a single opening on their head.
Other insects, such as webspinners, have silk glands in their front legs. These insects create silken galleries for protection while they forage and breed.