A failed moult is probably the worst thing that can happen to a captive invertebrate, and the hardest part is that by the time you notice it has gone wrong, there is almost nothing you can do. The animal is stuck partway out of its old exoskeleton, or the new exoskeleton has dried and hardened in the wrong shape, and the outcome is usually death or permanent deformity. Prevention is everything here.
How moulting works
All arthropods moult. Their exoskeleton is rigid and does not grow with them, so they periodically shed the old one and form a new, larger one underneath. The process is called ecdysis. In the days before a moult, the animal stops eating, becomes less active, and often hides. The old exoskeleton separates from the new one beneath it, with a thin layer of fluid in between. The animal then splits the old exoskeleton (usually along the back or sides) and slowly works its way out.
The new exoskeleton is soft and pale immediately after moulting. Over the next few hours to days (depending on the species and size), it hardens and darkens. During this period the animal is extremely vulnerable. It cannot move properly, it cannot defend itself, and its body is essentially liquid-filled soft tissue waiting to set.
What causes a bad moult
The single biggest cause is insufficient humidity. Moulting requires the old exoskeleton to remain pliable enough to split cleanly and the new exoskeleton to stay moist while the animal extracts itself. If the air is too dry, the old skin dries and tightens around the animal before it can fully emerge. Legs get trapped. Abdomens get stuck. The animal cannot complete the process and either dies in the attempt or emerges with crumpled, non-functional limbs.
This is true across the board: spiders, isopods, millipedes, mantids, beetles. Every moulting invertebrate needs adequate humidity during the process. The specific level varies by species, but "too dry" is the most common cause of failed moults by a wide margin.
Disturbance
The second biggest cause. A moulting invertebrate should not be touched, moved, poked, or even breathed on heavily. Vibrations from nearby construction work, a slammed door, or a curious keeper lifting the lid to check on the animal can cause the animal to panic, move at the wrong moment, and tear the new soft exoskeleton. A spider that falls during a moult because it was startled will almost certainly die from the resulting injuries.
Jumping spiders and mantids are particularly vulnerable because they moult while hanging. The spider suspends itself from a silk line; the mantid hangs from the mesh ceiling. If they fall, the soft new body crumples under its own weight. This is why enclosure height matters and why a textured ceiling surface is not optional for mantid enclosures.
Calcium deficiency
Specifically relevant for isopods and millipedes. Both groups need calcium to mineralise their new exoskeleton. An isopod that moults without sufficient calcium reserves will have a soft, poorly formed exoskeleton that may not harden properly. Millipedes have the same issue but on a larger scale, and their exoskeleton failure is more immediately obvious and more likely to be fatal.
Provide a constant calcium source: cuttlefish bone, crushed eggshell, or limestone. It should always be available, not just around moulting time. The animals build up calcium reserves over weeks and months, not overnight.
Insufficient space
Mantids need an enclosure at least three times their body length in height to moult safely. They hang upside down and extend their body downward out of the old skin. If they hit the floor before the process is complete, the moult fails. I have seen this happen with mantids kept in containers that looked big enough but were not quite tall enough. A centimetre short can be the difference.
Millipedes need enough substrate depth to create a moulting chamber underground. If the substrate is too shallow, they moult on the surface where they are exposed to fluctuating humidity and disturbance.
Temperature problems
Moulting uses a lot of energy. An animal that is too cold may not have the metabolic resources to complete the process. Conversely, very high temperatures can cause the new exoskeleton to dry and harden too quickly, before the animal has fully expanded into its new size. Keep temperatures within the normal range for the species and avoid sudden fluctuations.
Signs a moult is coming
Knowing when your animal is about to moult helps you prepare. Common pre-moult signs include: refusing food (sometimes for days or weeks in larger species), reduced activity, hiding more than usual, and in some species a visible colour change. Jumping spiders develop a noticeably darker abdomen before moulting. Beetle larvae stop feeding and may start compacting substrate to build a pupal cell, which is a specialised form of moulting. Isopods become paler and may stay on the damp side of the enclosure more consistently.
When you notice these signs, increase humidity slightly (mist the enclosure a bit more than usual, ensure the damp side is properly damp), avoid opening the enclosure unnecessarily, and absolutely do not rearrange decor or disturb the substrate. Leave the animal alone.
Can you help a stuck moult?
This is where it gets bleak. The honest answer is: rarely. Some keepers have had success carefully misting a spider that is stuck partway through a moult, hoping to soften the old exoskeleton enough for the animal to free itself. Occasionally this works. More often the animal is already too damaged or exhausted to complete the process even if the exoskeleton softens.
Manually pulling old exoskeleton off an animal is extremely risky. The new exoskeleton beneath is soft and tears easily. Limbs separate. Haemolymph (invertebrate blood) leaks. Unless you have very steady hands and a very clear idea of where the old exoskeleton ends and the new one begins, you are more likely to make things worse.
For beetle larvae that have had their pupal cell broken, you can try constructing an artificial pupal cell from damp floral foam or compacted substrate, but the success rate is low. Once the pupal cell is breached, the pupation process is usually compromised beyond recovery.
Prevention summary
Keep humidity at the right level for your species, all the time, not just when you think a moult is due. Provide calcium for isopods and millipedes. Make sure mantid and spider enclosures are tall enough. Give millipedes enough substrate depth. Do not disturb animals that are hiding, refusing food, or showing pre-moult signs. And resist the urge to check on them every few hours during the process. Open the enclosure after you are confident the moult is complete and the new exoskeleton has had time to harden. For most species, that is at least 24-48 hours after the moult.
A failed moult is almost always a husbandry problem, not bad luck. If it happens once, look at your setup and figure out what went wrong. If it happens repeatedly, something specific in your conditions needs changing.