Spiders Use Hydraulics to Move Their Legs

 

St. Andrews Cross Spider. Photo Credit: Ken Slade

From their unusal and multitudinous eyes to their eight robot-like legs, spiders have a history of receiving a bad rap. While these traits may seem like anatomical abnormalities, there’s a simple physiological explanation for spiders’ odd movements: their legs rely on a combination of hydraulics and skeletal muscles to move.

Spiders are arthropods, which means “joint-footed” creatures. In all arthropods, traditional bones and an internal skeleton have been replaced by a strong, inflexible (but light) exoskeleton (outer shell) made of chitin (the same material as shrimp shells). Since spiders don’t have internal bones, they lack the same movements as vertebrates, like us. Instead, arachnid locomotion is defined by a series of flexions and extenions. To flex their spindly legs inward, spiders use muscles. To extend their legs, however, spiders manipulate the fluid (hemolymph) inside their extermities.

Spiders contain eight legs divided into four locomotory pairs. Each pair of legs performs a specialized task during movement. The spider’s center of mass attaches to each of the extremities, with the forward two pairs in front of the center of mass, and the back two behind. To walk forwards, the front pairs flex inward, pulling the rear of the spider; the third pair pivots to assist with turning; and the fourth pair pushes forward.

Spider Close-up. Photo Credit: The Journal of Controlled Mechanical Power in Southern Africa

Inside, the spider houses an extensive fluid network. Similar to our blood, arachnids contain hemolymph, a circulatory fluid responsible for moving oxygen, nutrients, and waste throughout the body. Too little fluid, and circulation will be ineffective (low blood pressure). Too much, and the body’s fluid processing systems will be overwhelmed (high blood pressure). For humans, blood pressure extremes can be dangerous, but spiders use these pressure differences to their advantage. When hemolymph enters the legs, and pressure increases, the legs extend. As hemolymph leaves the legs, pressure decreases, and legs retract.

Spider Leg Diagram. Photo Credit: Eky.edu

Within each leg, there are seven tubular sections and three functional regions. The hip joint attaches to the circular body of the spider, allowing multidirectional movement (left, right, up, down). With both extension muscles (to push out) and flex muscles (to pull in the legs), it is the most complex soft tissue arrangement. Below it, the femur-patella and (underneath) tibia-metatarsus facilitate vertical movement. These sections only have flexor muscles; to extend the legs, hemolymph is pumped from the spider’s body to the lower ends of the femur-patella and tibia-metatarsus joints. When full, fluid pressure forces leg extension. As a liquid, the hemolymph fills in the gaps between muscle fibers for efficient extension, while simultaneously stiffening the leg. Not only is this energy efficient, but with only flexor muscles attached to the body, the spider can also maximize muscle volume and stiffness for an ultimately stronger grip on prey.

 Lawn Wolf Spider in Web. Photo Credit: Person Huang

To jump, spiders fill their legs with hemolymph, extending them as pressure increases.  Simultaneously, they flex their muscles, preparing to jump. Upon relaxation, the pressurized legs extend to initiate the jump. Spiders can further influence their in-flight trajectory by manipulating the timing, force, and extension of each leg. Compact atomical positions lead to shorter flights and faster speeds, while spread out limbs behave oppositely. During any jump, a silk line allows the spider to stop descent at any point.

Small spiders (under 3 grams) use liquid (hemolymph) excanges to catapult themselves from one area to the next, facilitating jumping movements and predation. Larger spiders however, require additional muscle forces to move and still their masses, otherwise, they succumb to uncontrollable bouncing with high hydraulic pressure. By combining the hydraulic catapualt strategy of smaller spiders with muscle-based contractions, larger spiders manuever with ease.

Legs aren’t the only hydraulic-powered extremities. In some spiders (Entelegynae), muscular genitalia movements have been replaced with expanding fluid membranes, allowing for increased rotation and higher movement complexity. Why complex genital movements are advantageous is difficult to say. Perhaps hydraulics provide a mechanical advantage for copulation or maybe complex movements improve the lock and sperm transfer. Although the rationale is unclear, the sheer dependence on the hydralics system (for both movement and reproduction) poses a key vulnerability: If a spider’s cephalothorax (center) is damaged, movement will be severely limited, as the hemolyph pressure drops precipitously down to zero.

So the next time you see a spider, stop to consider the wonderous, hydraulic physiology hidden beneath its unusual, jointed exoskeleton. Afterall, it’s hard to squish what you understand.

 

What’s you favorite part of the spider’s hydraulics system? Leave your comments in the comments section below.

 

© 2020 Sabrina L. Groves. Creative Commons Attribution-Noncommercial 4.0 International License.