Sooner or later, every invertebrate keeper has the same small heart attack: you walk over to the enclosure and your pet is lying motionless on its back, legs curled, looking very much like it has died. Don’t panic. What you’re almost certainly watching is a molt — one of the strangest and most wonderful things invertebrates do. This guide covers what molting actually is, how to read the warning signs, and exactly what to do (and what to never do) while your little one is at its most vulnerable.
Quick answer: Molting (ecdysis) is how invertebrates grow — they shed a too-small exoskeleton to reveal a larger, soft new one underneath. For days or weeks beforehand, most pets stop eating, hide away, and may darken or just look “off.” During the molt itself, the single most important rule is don’t interfere: no handling, no feeding, no rearranging. Keep humidity up, pull out any live prey, and wait. Afterward, your pet needs anywhere from a few hours to a couple of weeks to harden — and you shouldn’t offer food until its new fangs or mouthparts have darkened and hardened.
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What Molting Actually Is
Invertebrates wear their skeleton on the outside. That exoskeleton — built largely from a tough material called chitin — is brilliant armor, but it has one flaw: it can’t grow. So to get bigger, your pet has to ditch the whole suit and build a new one. Underneath the old shell, a soft new exoskeleton forms. When it’s ready, the old one splits (usually along the carapace, or cephalothorax, in spiders), and the animal carefully pulls itself free, pumps fluid and air into the new body to expand it, and then waits for everything to harden through a process called sclerotization.
The younger the animal, the more often it molts. Spiderlings and baby isopods may molt every few weeks; full-grown adults often molt only once or twice a year. Many male spiders stop molting entirely once they reach their final “mature” molt. So if your pet is shedding constantly, that’s usually just a healthy youngster growing fast — not a problem.
Reading the Pre-Molt Signs
Invertebrates don’t announce a molt, but they do drop hints. Across the hobby, keepers watch for the same handful of signals:
- Food refusal. The most consistent sign of all. A pet that normally pounces on a cricket suddenly ignores it. In tarantulas this often starts two to four weeks before the molt.
- A darkening abdomen. In tarantulas, the abdomen turns noticeably dark — almost black — as the new exoskeleton forms beneath the old. On New World species with a “bald spot,” that patch darkens too.
- Hiding and lethargy. Your pet seals itself into a burrow, seals the entrance with webbing, or goes still for days.
- A plump, shiny look and dull colors. Bright patterns fade just before a molt, then come back even brighter afterward.
- Building a molt mat or retreat. Spiders lay down a sheet of silk to molt on; jumping spiders reinforce their silk hammock.
No single sign is a guarantee, but two or three together almost always means a molt is on the way. Keeper communities like Arachnoboards are full of “is my T in premolt?” threads, and the consensus is the same: when in doubt, stop feeding and leave them be.
Molting, Species by Species
Tarantulas & True Spiders
Most tarantulas molt lying on their back, legs in the air. This looks alarming and is completely normal — it’s how they work themselves out of the old skin. The cardinal rule: a tarantula on its back is molting, not dying. A genuinely dying tarantula does the opposite — it curls its legs underneath itself in a “death curl” while staying upright. A molt can take anywhere from twenty minutes to several hours, so resist the urge to check every five minutes. Afterward the new fangs are pale, white-and-red, and gradually darken to black as they harden over roughly 7–14 days in adults (faster in spiderlings).
Jumping Spiders
Jumpers do things more privately. They molt sealed inside a thick silk retreat — often tucked high in a corner — and you simply won’t see it happen. They’ll refuse food and stay hidden for days, sometimes a week or more. The biggest thing you can do is nudge humidity up into roughly the 60–70% range during this window (lightly dampen one side of the enclosure; don’t blast the spider with mist), which helps the old skin separate cleanly. Do not open or poke the retreat to “check.” For a species-specific walkthrough, see our jumping spider molting guide.
Isopods
Isopods (roly-polies, springtail-tank cleanup crews, and the fancy designer morphs) do something genuinely weird and wonderful: they molt in two halves. The back half sheds first, and the front half follows anywhere from a few hours to several days later. That’s why you’ll sometimes spot a two-toned isopod that looks pale on one end — it’s mid-molt, not sick. Even cooler, they often eat the shed skin to reclaim the calcium locked inside it, which is why a calcium source like cuttlebone or limestone is a staple in a good isopod bin. New to keeping them? Start with our isopod setup guide and our bioactive terrarium isopods walkthrough.
Mantises, Millipedes & Others
Praying mantises molt hanging upside down, slowly easing out of the old skin while gravity does the work. They need vertical clearance of at least two to three times their body length above a secure perch — if a mantis can’t hang freely, or it falls mid-molt, the result is often a fatal deformity. Millipedes, beetle grubs, and scorpions typically burrow down and molt underground, so don’t dig them up if they vanish for a while. Even moth and butterfly caterpillars molt several times (each stage is called an instar) on their way to pupating — more on that in our moth rearing guide.
The Golden Rules During a Molt
- Don’t handle. A freshly molted invertebrate is soft as wet paper. Wait until it has fully hardened.
- Pull out live prey immediately. A loose cricket or roach can chew on a defenseless molting animal — this is one of the most preventable tragedies in the hobby.
- Don’t rearrange the enclosure. Your pet chose its molting spot on purpose. Leave hides, branches, and webbing exactly where they are.
- Keep humidity up, gently. For most species, overflow the water dish or dampen the substrate rather than spraying the animal directly. Moisture in the environment helps; a soaked, stressed pet does not.
- Be patient. Molting takes the time it takes. Checking constantly only stresses you out.
Keep an eye on humidity: the easiest way to avoid most molting problems is simply knowing what your enclosure’s humidity and temperature actually are. A small digital thermometer-hygrometer takes the guesswork out of it.
What to Avoid (the Mistakes That Actually Hurt)
- Never peel the old skin off yourself. It almost always tears something underneath. Patience and moisture, not tweezers.
- Don’t drown the enclosure in spray. Heavy misting onto a molting animal raises stress without reliably helping. Aim for damp substrate, not a swamp.
- Don’t feed too soon. Offering prey before the fangs or mouthparts harden can injure your pet or just stress it.
- Don’t flip a “dead-looking” tarantula back over. On its back means it’s working — leave it.
- Skip the photo session. We know it’s tempting. Wait until it’s done and hardened.
After the Molt: Recovery & the First Meal
Once your pet has freed itself, the waiting game continues a little longer. The new exoskeleton hardens over roughly 24–48 hours in small species and up to a couple of weeks in large adult tarantulas. During this time, keep things calm, dim, and humid, with fresh water available.
- Leave the shed skin (exuvia) alone at first. Isopods will eat theirs for calcium. For spiders, you can remove it once the animal has moved off — and it’s perfect for confirming whether you have a male or female.
- Wait to feed. Offer food only once the fangs or mouthparts have darkened and hardened — a few days for slings and jumping spiders, 1–2 weeks for adult tarantulas. When in doubt, wait another day.
- Expect a glow-up. Your pet will be visibly bigger and its colors brighter. This is the payoff that makes keepers fall in love with molting.
Substrate that holds humidity: a moisture-retaining coco-fiber substrate makes it far easier to keep the gentle, steady humidity a clean molt needs — without soaking the whole enclosure.
When a Molt Goes Wrong
Most molts go fine on their own. Occasionally, though, an animal gets stuck — what keepers call a “bad molt” or dysecdysis. The classic warning sign is an asymmetric molt: legs coming free on one side but not the other, or the front half emerging while the back stays trapped. If you see that, it’s time to act — carefully.
- Start with the least-invasive fix: moisture. Gently raising humidity (overflowing the water dish onto the substrate) is usually the first thing experienced keepers reach for, because it lubricates the stuck skin.
- Act while it’s fresh. A soft, just-molting exoskeleton is far easier to work with than one that has started to harden in place. If you must assist, a damp soft brush — never tweezers and tugging — is the gentlest tool.
- Know the cause. Stuck molts very often trace back to low humidity or a dehydrated animal, so prevention is mostly about correct moisture and hydration going forward.
When care advice varies between sources, the keeper-community consensus is consistent on this: humidity first, hands almost never, and when a molt is genuinely going wrong, the dedicated Arachnoboards stuck-molt guide is the go-to resource the hobby points people toward.
Give them something to anchor on: a piece of natural cork bark gives spiders and other climbers a secure surface to grip and molt against — and it holds up to humidity without rotting.
Frequently Asked Questions
How long does molting take?
The molt itself ranges from about twenty minutes in a small spiderling to several hours in a large tarantula. Full recovery — hardening up enough to eat and be handled — takes another day or two for small species and up to two weeks for big adults.
How often do invertebrates molt?
It depends almost entirely on age. Young, fast-growing animals may molt every few weeks; mature adults often molt just once or twice a year, and some male spiders stop molting after reaching maturity.
My tarantula is on its back — is it dead?
Almost certainly not. Lying on its back is the normal molting position. A dying tarantula curls its legs underneath itself while staying upright (the “death curl”). Leave a back-lying tarantula completely alone.
Should I help my pet molt?
Only if it’s clearly stuck (an asymmetric, stalled molt). Even then, start with raised humidity and intervene by hand as a last resort. A normal molt needs zero help from you.
Why won’t my pet eat after molting?
Its new fangs or mouthparts are still soft. They harden and darken over the following days. Once they’re firm and dark, your pet will start hunting again — usually with a big appetite.
Do isopods really eat their own shed skin?
They do, and it’s not gross — it’s smart. The old exoskeleton is rich in calcium they need to build the new one, so recycling it is efficient. Just make sure a backup calcium source is always available.
Explore Related Guides
- The Invertebrates Care Hub — start here if you’re new to keeping inverts
- Jumping Spider Molting, Step by Step
- Isopod Setup Guide
- Raising Moths: A Beginner’s Guide
Be good. Do the research. Love your weirdo.