Invertebrate Molting 101: What's Normal Across Tarantulas, Hermit Crabs, and Jumping Spiders

New invertebrate keepers tend to panic at the exact same moment: their pet goes still, hides, stops eating, and looks like it might be dying. In the overwhelming majority of cases, it's molting — shedding the rigid exoskeleton to grow — and it's one of the most vulnerable, and most misunderstood, events in an invertebrate's life.

Why Molting Happens at All

Exoskeletons don't stretch. Unlike a vertebrate's skin, an invertebrate's outer shell is rigid, which means the only way to grow is to shed the entire old exoskeleton — including, in some species, the lining of the stomach, book lungs, or fangs — and hardening a new, larger one underneath before it's fully exposed.

How the Signs and Timeline Differ by Species

The duration differences are the most common source of unnecessary panic. A tarantula owner who's used to a molt wrapping up in an afternoon can genuinely believe something is wrong when a hermit crab stays buried for three weeks — but for that species, three weeks is completely ordinary. Our species-specific guides for the Tarantula, Hermit Crab, and Jumping Spider cover the rest of each animal's housing and diet needs in more detail.

Molting Frequency: How Often Should It Happen?

Frequency drops sharply with age across every species covered here. Juveniles, growing quickly, may molt every few weeks to a couple of months. A mature adult tarantula might molt just once a year, and very old females of some species can go multiple years between molts. Hermit crabs follow a similar pattern — small, fast-growing juveniles molt more often than large, established adults. There's no fixed universal schedule, which is exactly why watching for behavioral pre-molt signs matters more than trying to predict molting by a calendar.

The One Rule That Applies to All Three

Never disturb a molting invertebrate. This means no digging up a buried hermit crab to check on it, no attempting to "help" a tarantula that appears stuck, and no handling a jumping spider tucked into its silk retreat. The new exoskeleton is soft and vulnerable for hours to days after emerging, and physical disturbance during or immediately after a molt is one of the most common preventable causes of death across all three species.

Feeding Around a Molt

Live prey left in an enclosure with a molting or freshly molted animal is a genuine hazard, not just a wasted feeding. Crickets and roaches will attack a molting tarantula or jumping spider that can't yet defend itself, and prey should always be removed well before signs of an approaching molt appear. Resume feeding only after the new exoskeleton has visibly hardened — typically 7 to 14 days after a tarantula molt, and a shorter window for smaller species like jumping spiders.

When a Molt Actually Goes Wrong

A failed molt is a real risk, though far less common than a normal one. Warning signs include an animal that appears stuck half in and half out of the old exoskeleton for an extended period, visible struggling that doesn't progress, or limbs left twisted at the end of the process. Failed molts are usually linked to insufficient humidity during premolt, physical disturbance, or an underlying health issue — species-specific humidity and substrate depth requirements exist specifically to reduce this risk. If a molt does appear to fail, this is one of the few situations where experienced keeper or exotic vet intervention (never an untrained attempt to physically assist) may be necessary.

The Bottom Line

A still, hidden, food-refusing invertebrate is unsettling to watch, but across tarantulas, hermit crabs, and jumping spiders alike, it's overwhelmingly the animal doing exactly what it's supposed to do. The best response in almost every case is also the hardest one for a worried keeper: leave it alone and wait.

For natural history and wild behavior, see the Tarantula encyclopedia profile, or browse the full Invertebrates care guide category for more species.

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Sources & Further Reading