Why Do Daddy Longlegs Lose Their Legs? Escape by Autotomy

How autotomy works in harvestmen

Autotomy is the deliberate shedding of a body part. Lizards that drop their tails and crabs that release a claw are well-known examples. In long-legged harvestmen, the order Opiliones, it is one of the most important ways to survive an attack.

Each harvestman leg has a weak point near its base, between two short segments close to the body. When a predator seizes a leg and pulls, the leg separates cleanly at this joint. The wound closes quickly, which limits the loss of body fluid. The harvestman then flees on its remaining legs, often dropping off a plant or running into cover.

Harvestmen do this most readily when a leg is actually held. Simply touching or brushing one usually makes it run or freeze instead. Some of the most practiced leg-droppers are the long-legged harvestmen of the family Sclerosomatidae, such as the Leiobunum species common in North America and Europe. The common harvestman can also shed legs to escape.

Why the lost leg keeps twitching

A freshly detached harvestman leg does not lie still. It bends and straightens in a steady rhythm, sometimes for a minute and sometimes for up to an hour. This is not a sign of life in the leg itself but the result of how its nerves are built.

Harvestman legs contain small nerve centers, often described as “pacemakers,” near the end of the long thigh-like segment called the femur. These send repeated signals to the leg muscles even after the leg is cut off from the brain. Each signal makes the leg extend, and it relaxes between signals, creating the twitching motion.

Biologists think the movement works as a decoy. A predator such as a spider, ant or bird may focus on the wriggling leg for long enough that the harvestman gets away. Experiments with ants and spiders as predators have shown that leg dropping helps harvestmen survive attacks.

Why daddy longlegs legs do not grow back

Many arthropods can regrow a lost limb, but only while they are still molting. A new limb forms gradually under the old outer skeleton and appears, often smaller at first, after the next molt. Harvestmen stop molting once they become adults, so an adult that loses a leg keeps that gap for the rest of its life.

Young harvestmen are also not known to regrow lost legs the way many young spiders can. In practice, leg loss in harvestmen is treated as permanent. This makes autotomy a costly trade: the animal gives up part of its body for an immediate chance to survive.

Life with missing legs

Harvestmen missing legs are common in the wild. Field surveys of some long-legged species have found that a large share of adults are missing at least one leg. They can still walk, climb, feed and mate, which shows how well the defense works.

The cost depends on which leg is lost. Harvestmen have eight legs, and the second pair is the longest. These legs work mostly as feelers, tapping the ground and vegetation to find food, detect danger and recognize mates. Losing one of them reduces the animal’s ability to sense its surroundings. Losing walking legs, especially several, reduces speed, balance and climbing ability, and studies of Leiobunum species have found that such individuals move more slowly and climb less well.

A harvestman with fewer legs also has fewer spares for the next attack. Because the legs never return, each escape slightly reduces the chance of surviving the next one. Even so, for an animal that usually lives less than a year, trading a leg for survival is often worth it.

Other defenses daddy longlegs use

Dropping a leg is not the only trick. Harvestmen combine several defenses:

• Scent glands: a pair of glands near the front of the body releases a bad-smelling fluid that makes them unpleasant to eat.
• Bobbing: long-legged species can bounce their bodies rapidly up and down, making them hard to target. Groups of harvestmen may all bob at once.
• Fleeing or dropping: they can run quickly or drop from a plant and stay still in the leaf litter.
• Playing dead: some harvestmen, mostly short-legged species, go rigid when caught.

None of these involve venom. Harvestmen have no venom glands and cannot bite people, as explained in are daddy longlegs venomous?. Their main jobs in gardens are eating small insects, mites and dead material, described in what do daddy longlegs eat?.

Do other daddy longlegs drop their legs?

The other animals called daddy longlegs can also lose legs easily. Cellar spiders can shed a trapped leg, and as young spiders they may grow it back over later molts. Crane flies, which are insects, have extremely fragile legs that break off readily when handled or caught in a web. Adult crane flies do not regrow them either.

Because all three animals lose legs so easily, the best way to help a daddy longlegs indoors is not to grab it. Gently guide it onto a sheet of paper or into a cup and release it outside, where it can carry on eating small insects.