Tarantulas, belonging to the family Theraphosidae, are prodigious silk producers, but their use of the material differs significantly from the aerial traps spun by orb-weaving species. Unlike the sophisticated, sticky capture webs used by many common spiders, tarantula silk is rarely deployed to trap flying prey mid-air. Instead, these spiders employ their silk for a wide array of structural, reproductive, and safety applications that anchor them securely to their homes and environment.
The Biological Mechanism of Silk Production
All spiders, including tarantulas, produce silk as a liquid protein solution within specialized abdominal glands. This material is stored in a highly concentrated form before being extruded. When the spider is ready to spin, the liquid protein is channeled through narrow ducts that lead to the spinnerets located at the posterior end of the abdomen.
As the silk travels through these ducts, a change in environment—specifically a drop in pH and the mechanical shear forces of the extrusion process—causes the protein molecules to reorient. This rapid change triggers the liquid protein to solidify into a continuous, strong thread upon exiting the spider’s body. Tarantulas, being a more ancient lineage, typically possess two to four short spinnerets, which are often less prominent than those found on modern web-spinning spiders.
Structural and Safety Applications of Tarantula Silk
Tarantula silk is primarily a utility material used for engineering their environment and ensuring physical safety. Many terrestrial tarantulas are fossorial, meaning they construct burrows in the earth, and they use silk to reinforce the tunnel walls. This lining prevents the collapse of the burrow structure, stabilizes the surrounding soil, and helps deter parasites and soil-dwelling predators from entering their retreat.
The silk is deployed as a sensory tool, with threads radiating outward from the burrow entrance across the ground surface. These subtle tripwires allow the tarantula to detect vibrations from passing prey or potential threats, acting as an early warning system. A single strand of silk, known as a dragline, is a universal safety feature, extruded as a lifeline whenever the spider moves; if it falls, the dragline prevents a potentially fatal impact.
For heavier species, which risk serious injury from a fall due to their large abdomens, silk production has been observed on their tarsi, or feet. When a tarantula walks on an extremely smooth vertical surface, it can secrete tiny silk fibers from specialized spigots on its feet for extra adhesion. This “foot silk” acts as a temporary adhesive to prevent slipping, offering improved traction and stability.
Reproductive Functions of Silk
Reproduction in tarantulas relies heavily on specialized silk construction for both males and females. Before mating, the male must weave a compact, sheet-like structure known as a sperm web or mating mat. He then deposits semen onto this silken platform from his genital opening on the underside of his abdomen.
After depositing the semen, the male draws it up into specialized organs on his pedipalps, the short, leg-like appendages near his mouth. This transfer is necessary before he can seek out a female to copulate. The female utilizes a different, more substantial form of silk to protect her developing offspring.
Once eggs are fertilized, the female constructs a thick, dense, spherical egg sac, or ootheca, composed of multiple layers of silk. This structure safeguards the eggs from temperature fluctuations, humidity changes, and environmental threats. The strength and density of this specialized silk provide a robust, insulated casing that the female guards diligently, sometimes rotating the sac to ensure optimal development conditions.
Habitat-Specific Silk Strategies
A tarantula’s habitat dictates the volume and form of silk it uses, leading to two distinct construction strategies. Terrestrial and burrowing species spend most of their time on or under the ground, focusing on interior design and external signaling. Their silk is deployed primarily for lining the insides of their earthen burrows and creating subtle tripwires around the entrance.
In contrast, arboreal, or tree-dwelling, tarantulas use silk in a far more extensive and visible manner. These species rely on silk to create elaborate, tube-shaped retreats or funnel-like nests high above the ground. These silken homes are essential for shelter, molting, and protection from predators, involving significant amounts of webbing that visually resembles a traditional spider “web.” This aerial use of silk for shelter is the closest a tarantula comes to spinning a true web, though the primary function remains structural and protective rather than for active prey capture.