Spiders, like most terrestrial organisms, rely on breathing air and can drown if their respiratory systems are compromised by water. However, the process is complex, as these arachnids possess biological and physical defenses that make them highly resistant to submersion. Understanding how a spider interacts with water requires examining the physics of the water’s surface and the spider’s unique external anatomy. These factors allow many common spiders to survive a brief dunking, even though they are not truly aquatic creatures.
The Role of Surface Tension
A spider often appears impervious to water due to surface tension, the physical force at the air-water boundary. Water molecules are strongly cohesive, creating a thin, elastic film on the surface. Because of their light mass and small size, most spiders do not exert enough downward force to break this molecular film.
The spider’s ability to utilize surface tension is enhanced by a specialized covering of minute, dense hairs and a waxy coating on its exoskeleton. This combination creates a highly hydrophobic, or water-repellent, surface that mimics effective non-wetting materials found in nature. When water touches the spider’s body, the cohesion of the water molecules is stronger than their adhesion to the spider’s surface.
This adaptation causes water to bead up and roll off the spider, keeping the body dry and preventing the surface film from being breached. If fully submerged, this hydrophobic layer traps a thin, silvery film of air tightly against the body. This trapped air acts as a temporary physical gill, delaying drowning by maintaining a pocket of breathable gas near the respiratory openings.
How Water Leads to Suffocation
While the external surface repels water, the spider’s internal anatomy confirms it is an air-breathing animal that can suffocate. Spiders exchange gases using book lungs and sometimes tracheal tubes, rather than lungs that actively inflate and deflate like those of mammals. Air enters these structures through tiny external openings located on the abdomen called spiracles.
The book lungs are named for their structure, consisting of stacked, plate-like tissues that resemble the pages of a book. Oxygen diffuses passively from the air into the spider’s hemolymph (blood) across the thin walls of these plates. Drowning occurs when water pressure forces liquid through the spiracles, flooding the air-filled spaces within the book lungs.
Once the internal respiratory surfaces are covered in water, the passive diffusion of oxygen stops, and the spider is unable to absorb gas. Spiders cannot actively seal their spiracles shut. Some research suggests that when deprived of oxygen, their lung slits may open wider to increase gas exchange. This involuntary reaction, while normally helpful in low-oxygen conditions, ironically hastens drowning when submerged in water.
Submerged Survival and Specialized Species
Despite the risk of suffocation, many common spiders can survive submersion for periods ranging from minutes to several hours. This extended survival is possible because spiders have a low metabolic rate, especially when stressed or inactive, meaning they consume oxygen slowly. The small air bubble trapped by their hydrophobic hairs provides a limited oxygen reserve during this time.
In environments prone to flooding, species such as wolf spiders and fishing spiders tolerate submersion for hours. This defense mechanism likely evolved to survive flash floods or heavy rains. However, the most striking exception is the Diving Bell Spider (Argyroneta aquatica), the only spider species to live almost entirely underwater.
This specialized arachnid actively transports air from the surface down to an intricate web structure anchored to aquatic plants. This silk dome, known as a diving bell, acts as a physical gill, extracting dissolved oxygen from the surrounding water to replenish the air supply. Studies show that a spider can remain in its diving bell for over 24 hours without needing to return to the surface.