How Do Tarantulas Molt? Shedding the Whole Exoskeleton

Why do tarantulas need to molt?

Like all arthropods, tarantulas wear their skeleton on the outside. The exoskeleton is made of chitin and protein, hard on the outside and unable to stretch much. To grow larger, a tarantula must make a new, bigger exoskeleton underneath the old one and then shed the old one. This is the only time it can increase in size.

Molting also replaces what has been worn or damaged. With each molt a tarantula gets:

• New hairs, including the urticating hairs it kicks off in defense, which do not grow back between molts.
• New fangs and claws, and new lenses over its eyes.
• New linings of the foregut and hindgut and of the book lungs, because these internal surfaces are also made of cuticle.
• In females, a new lining of the spermathecae, the sperm storage organs, so any stored sperm is lost and she must mate again.
• Regrown legs or pedipalps, if any were lost.

Molting can also rid the spider of some external parasites and fungi.

Before the molt: the premolt stage

The preparation starts weeks before the visible molt. Under the old exoskeleton, the spider’s skin cells separate from the cuticle and build a new, folded one beneath it. In the final stages, the tarantula:

• Stops eating, sometimes for weeks.
• Becomes sluggish and stays in its burrow, often sealing the entrance with silk.
• Shows a darker abdomen, especially where hairs have been kicked off, because the new skin with its dark hairs shows through.

In the wild, a tarantula molts in a chamber of its burrow. Shortly before molting, it spins a silk mat, sometimes called a molting mat, to lie on.

Step by step: how a tarantula sheds its skin

The molt itself follows a set sequence. Times vary with the spider’s size and species.

1. Turning over. The tarantula lies on its back on the silk mat, legs curled upward. People who see this for the first time often think the spider is dying.
2. Raising the pressure. The spider pumps hemolymph (spider blood) into the front body section, the cephalothorax, raising the pressure inside.
3. Opening the shell. The old exoskeleton splits along the sides of the carapace, and the top of the carapace lifts like a lid. The skin of the abdomen tears along its sides.
4. Pulling free. With slow, rhythmic movements, the tarantula pulls its body, fangs, pedipalps and all eight legs out of the old skin, like drawing hands out of tight gloves. This stage typically takes from under an hour to several hours.
5. Stretching. Once free, the spider lies still to recover, then stretches and flexes its soft legs repeatedly while the new cuticle is still pliable, so that the joints set properly.
6. Hardening. The new exoskeleton hardens and darkens over the following days. The fangs start out pale and soft and take one to two weeks to become fully hard and dark.

Just after molting, a tarantula looks larger and more brightly colored, with fresh, velvety hairs. Its leg span may grow noticeably with each molt in young spiders.

How often do tarantulas molt?

Molting frequency falls with age. In the Mexican red-knee tarantula, for example, spiderlings molt about every two weeks for their first months, then less and less often.

A tarantula molts many times before it becomes an adult; a male Mexican red-knee tarantula reaches maturity at around his 20th instar (the stage between molts) at about four years old.

The final molt of a male, the maturing or ultimate molt, gives him mating hooks on his front legs and mating organs on his pedipalps. After that he usually does not molt again, which is one reason males live much shorter lives than females. Females continue molting throughout life, part of the reason they can live for decades. See how long do tarantulas live?

Regrowing lost legs

Tarantulas can shed a leg that is badly injured or caught by a predator, breaking it off at a joint near the body. The wound seals quickly. A new leg then forms, folded up inside the remaining base segment of the leg, and appears at the next molt.

The new leg is usually shorter and thinner than the others at first and may be paler. It reaches normal size after one or two further molts. Adult females can regrow legs in this way because they keep molting; mature males, which no longer molt, cannot.

Why molting is dangerous

Molting is one of the riskiest moments in a tarantula’s life:

• Getting stuck. If the old skin does not split cleanly or the spider is weak or dehydrated, a leg or the whole body can become trapped. A bad molt can cripple or kill the spider.
• Defenseless hours and days. While molting and while the new exoskeleton is soft, a tarantula cannot run well or bite properly. Ants, centipedes and other predators can kill it. This is why wild tarantulas seal their burrows before molting.
• Water loss. The fresh, soft cuticle loses water more easily, so a humid, sheltered burrow matters.

Mature males that attempt another molt often die, because the mating organs on their pedipalps can become stuck in the old skin.

The shed skin: a perfect copy

The cast skin, called an exuvia (plural exuviae), is a remarkably complete, hollow copy of the spider. It includes the fangs, the claws, the hairs on the legs, the lenses of the eyes and even the linings of the book lungs and of the female’s spermathecae. Because of this, arachnologists can examine an exuvia to tell whether a young tarantula is male or female.

Many people find a shed tarantula skin and think it is a dead spider. A dead tarantula usually lies with its legs curled tightly underneath it, while a molt looks hollow and light, with the carapace lid lifted. Spiders are not the only animals that molt like this; insects and crustaceans do too, as explained in spider vs insect.