How Do Jumping Spiders Jump? Hydraulic Legs and Silk Lines

How does a jumping spider jump?

A jumping spider prepares carefully. It turns to face the target, crouches, and often raises its front legs. It attaches a silk dragline to the surface with its spinnerets, then pushes off by straightening its rear legs in a fraction of a second. It flies through the air trailing silk and lands with its front legs reaching out to grab.

Unlike grasshoppers or fleas, jumping spiders do not have big jumping legs or a spring-loaded catapult. Their legs look ordinary. What sets them apart is how accurately they can aim. They jump to catch prey, to escape danger and to cross gaps between leaves or stones, and they adjust each jump to the distance.

Hydraulic legs: how spiders straighten their legs

Spiders have a strange leg design. They have muscles to bend their legs, but no extensor muscles at two of the main leg joints. Instead, they straighten those joints by raising the pressure of their blood, called hemolymph, which is pumped into the legs from the front part of the body. In effect, spider legs are partly hydraulic.

A classic 1959 study by D. A. Parry and R. H. J. Brown examined this in jumping spiders. They concluded that a quick rise in blood pressure could straighten the hind legs fast enough to power the jump. During a leap, the leg spines stand up, a sign of the sudden pressure rise.

This also explains a familiar sight: dead spiders lie with their legs curled tightly underneath them. Without blood pressure to straighten them, the flexor muscles pull the legs in.

Muscles or hydraulics?

Newer work suggests the story is more mixed. In a 2018 study, researchers at the University of Manchester trained a regal jumping spider (Phidippus regius) to jump between platforms and filmed it with high-speed cameras. The spider jumped horizontal gaps of two to five body lengths and up and down by up to two body lengths.

The team calculated that the power needed for these jumps could be supplied by the spider’s leg muscles alone. Jumping spiders, they concluded, use direct muscle power rather than a stored-energy catapult. Hydraulic pressure may still help, but it does not appear to be essential.

They also found that the spider changed its technique. For short jumps, it used low, flat trajectories that kept the flight time short, which is useful when catching prey. For longer jumps, it took off at steeper angles, closer to the most energy-efficient path.

Silk safety lines

Before nearly every jump, a jumping spider attaches a dragline. This line has several uses:

• Safety: if the spider misses or is knocked off, it hangs on the line and climbs back.
• Braking and stability: the line helps the spider control its flight and landing.
• Swinging: some species move a short way toward the prey, then jump in an arc at the end of the line to reach targets on vertical or overhanging surfaces.

For more on how spiders produce these threads, see how spiders make silk.

How far and how fast can they jump?

Jump distances vary widely between species and with the purpose of the jump.

The Colonus record comes from a single measured spider, so it shows what is possible rather than what is typical. Most everyday jumps are much shorter, and many are just quick hops of a body length or two. The zebra jumping spider of European and North American walls makes its jump almost entirely by straightening its fourth pair of legs.

How do jumping spiders aim?

Accuracy depends on eyesight. The large front eyes of a jumping spider have layered retinas, and the spider seems to judge distance by comparing a sharp image on one layer with a blurred image on another. Experiments in which the color of the light was changed, altering the blur, made spiders misjudge their jumps, falling short in one kind of light.

The spider also lines up its whole body with the target before jumping, and it may pause for some time to inspect the prey. Read more in how well jumping spiders see.

Myths and facts

Myth: jumping spiders can leap huge distances at people. Most jumps are short, and a spider that lands on a person is usually escaping or exploring, not attacking.

Myth: spiders jump with spring-loaded legs like fleas. Jumping spiders do not store energy in a catapult. Their jumps are powered directly by muscles, helped by blood pressure.

Fact: they can plan jumps. Some jumping spiders take detours to reach prey and pick routes before jumping, showing more planning than expected for such tiny animals.