How do stick and leaf insects moult? Phasmid moulting explained

Most phasmids moult several times before adulthood, but the exact number varies by species and sex.

Before delving into the moulting process itself, it’s important to understand how phasmids progress through their lifecycle first. Phasmids develop through stages called instars, which are numbered sequentially. When they first hatch, they’re classified as first instar or “L1” nymphs. Each successive moult advances them to the next instar – after their first moult, they become “L2” nymphs, then “L3” nymphs, and so on until reaching adulthood.

Female phasmids generally require one additional moult compared to males to reach maturity, due to their usually larger size. A great example is the phasmid species, Extatasoma tiaratum, where females grow to ~15cm in length and are notably bulkier than the slender, lightweight males, who reach ~10cm. In most species I have kept, females typically complete seven moults in total, while males complete six. This means males mature after “L5” and females after “L6”. It should be noted that the total number of moults will differ species to species. Once reaching adulthood, phasmids cease moulting entirely.

The Phasmid Study Group gives a useful general range of around 4–8 moults, with females often undergoing one more than males.

Now that we understand the lifecycle stages, we can explore how moulting enables phasmids to progress through these instars.

A phasmid preparing to moult will often change its behaviour before anything obvious happens physically. Reduced feeding, less activity and moving to a suitable hanging position are some of the clues I watch for.

It is very much possible to predict when a phasmid is about to moult. The timing itself provides the first clue – most species tend to moult approximately once per month. However, monitoring individual phasmid behaviour offers more specific indicators. 

Before moulting, phasmids become noticeably less active and stop feeding entirely. This fasting period can last for several days and, rather than being a cause for concern, serves as an excellent indicator that moulting is imminent. During this time, the old cuticle (exoskeleton) begins to separate in preparation for the moult, which affects the phasmid’s digestive system and makes it difficult to process food, hence why they stop eating.

Furthermore, the phasmid will often make its way to the top of the enclosure, where it finds a safe and stable spot to moult. This behaviour highlights the importance of providing adequate height in the enclosure. 

In addition to behavioural changes, physical signs may also become apparent. The phasmid’s body may appear enlarged. The area between the front and middle legs often looks swollen, and in species with wings, the wing buds may appear noticeably enlarged or puffed up.

The moulting process can be divided into two key scientific stages. The first, called apolysis, occurs before the visible moult begins, when the old cuticle separates from the underlying epidermis and a new cuticle begins forming beneath it. The second stage, called ecdysis, is the process we actually observe, when the insect sheds its old exoskeleton.

During ecdysis, the phasmid's old skin splits along its back, creating a small opening behind the head. The insect must then pull its entire body, including all legs and antennae, through this relatively small slit. This is both an incredible feat and a critical moment in the phasmid's life. The process begins with the phasmid pulling its head and thorax through first. During this phase, you may observe the phasmid wriggling, twisting or swaying back and forth. These movements aren't necessarily signs of distress, but rather the phasmid's efforts to extract itself from its old exoskeleton.

The phasmid may pause during the process before freeing the remainder of its abdomen. During this time, it typically hangs upside down, supported remarkably by the feet of its old exoskeleton. Short periods of very little movement can therefore be normal during an otherwise progressing moult, so don't immediately assume something is wrong if your phasmid appears motionless. Once ready, it will continue extracting itself until its abdomen is completely free, allowing its newly exposed exoskeleton to expand and gradually harden.

There isn't one fixed moulting time for every phasmid.

The visible process of escaping the old exoskeleton can take anywhere from minutes to considerably longer depending on the species, size of the insect and how smoothly the moult progresses. Pauses during ecdysis are not automatically a problem, particularly if the insect remains securely suspended and continues making progress.

What matters more to me than watching the clock is whether the moult is still progressing.

A phasmid that remains securely hanging, gradually frees different parts of its body and eventually pulls clear is behaving very differently from one that has fallen, become twisted or remained firmly trapped in exactly the same position for a prolonged period.

Even once the old skin has been shed, the process is not really finished. The new exoskeleton is initially soft, and the insect needs time to expand and harden before normal activity resumes.

This is why I leave freshly moulted phasmids alone rather than assuming the moult is finished the moment the old skin falls away.

Following a successful moult, the new exoskeleton is initially soft but gradually expands and hardens over time. The appearance of the phasmid can change dramatically between the immediate post-moult period and several days later as this expansion and hardening process occurs. For instance, if the phasmid is moulting to maturity and belongs to a species with wings, the wings will not be fully developed right after the moult but will expand to their full size gradually.

Interestingly, phasmids often consume their old exoskeleton either partially or entirely immediately after moulting, while it remains soft and moist. Although this self-cannibalism might seem a little unusual, it serves multiple purposes. First, it provides key nutrients to support their new stage of life. Second, in the wild, the shed exoskeleton may attract predators, so consuming it helps minimise this risk. When not consumed, the moult typically falls to the enclosure floor. Old, discarded moults appear white to yellowish in colour, dry, and brittle, often with darker areas around the feet. In such cases, you can simply discard it – or, if you also keep isopods as I do, the moult makes an excellent protein-rich snack for them.

Moulting is the riskiest time in a phasmid’s life under your care. Phasmids may lose limbs during emergence, emerge with crumpled bodies or bent abdomens, or, in the worst cases, become trapped within their old exoskeleton. While losing a leg or two isn't typically fatal – the phasmid will continue its normal activities, and younger specimens may even regenerate lost limbs in future moults – becoming completely stuck during moulting often proves fatal. This emphasises the critical importance of maintaining appropriate humidity levels for your specific species, especially during the vulnerable L1-L3 nymph phases.

The moulting process can be stressful for an invertebrate keeper due to the risks involved. So, what is your role during moulting? In short – nothing. The best approach is typically to leave the phasmid undisturbed, as this natural process occurs successfully in the wild without human intervention. Disturbances such as opening the enclosure, moving it, or changing food plants during moulting can prove fatal. Even after a successful moult, the phasmid should remain undisturbed for at least 48 hours while its new exoskeleton hardens.

However, in certain situations, careful intervention may be warranted. If you notice that a leg is clearly stuck in the old exoskeleton, or if remnants of the old skin remain attached to the body for more than a day, a light spritz of water can help loosen it. You can also use a soft paintbrush to gently encourage the phasmid to free itself. As a last resort, tweezers may be used with extreme care, though this can result in the loss of a limb.

Occasionally, phasmids choose less-than-ideal moulting spots, such as areas too close to the ground. In these cases, you may carefully support the old exoskeleton by its feet while the phasmid completes moulting or attempt to gently reposition it higher up in the enclosure.

In this final section, I will discuss some of the steps you can take to maximise the chances of successful moulting.

Humidity: Maintaining proper humidity levels for your specific species is crucial, particularly for nymphs when moulting is most critical. Light spritzing of the enclosure and using substrate such as paper towel at the bottom helps maintain appropriate humidity levels. For times when you'll be away for several days to a week, placing a plastic container of water with mesh covering (to prevent phasmids from accidentally drowning) can ensure adequate humidity – the large surface area allows water to evaporate steadily into the enclosure.

Adequate space: An equally important consideration is to ensure there is adequate space in the enclosure. Overcrowding can lead to fatal disturbances during moulting attempts. This applies not only to the number of individuals but also to food plant density. Food plants should not fill the entire enclosure; there must be ample free space for the phasmid to hang freely and complete their moulting process.

Mesh roof: I strongly recommend providing a mesh or netted roof for your phasmid’s enclosures. This offers better grip and stability for moulting compared to plastic or glass surfaces.

Handling: Finally, if possible, minimise handling to only what is absolutely necessary, such as during enclosure maintenance and food plant changes. Since it's often difficult to determine when a phasmid is preparing to moult, unnecessary handling can cause harmful stress that may interfere with this delicate process.