How do phasmids grow?: The process of moulting

One of the most fascinating aspects of phasmid biology is their growth process through moulting, also known as shedding. This crucial process allows phasmids to transform from tiny nymphs into impressive adults, shedding their old skin and growing into a new one – essentially performing their very own full-body makeover! With each moult, phasmids grow larger and may alter their external appearance, developing new features such as wings or different colouration as they progress toward maturity.

For invertebrate keepers moulting is both an exciting and nerve-wracking marvel to witness. It is important that we understand this process to ensure the health and successful development of our phasmids.

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.

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

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, involves the separation of the old cuticle from the underlying epidermal cells, triggered by cell replication. The second stage, called ecdysis, involves the actual emergence from the old skin.

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, convulsing, or swaying back and forth. These movements aren’t signs of distress but rather the phasmid’s efforts to extract itself from the old exoskeleton.

After the initial emergence, the phasmid typically takes a rest to allow its new skin to start hardening while the abdomen remains within the old exoskeleton. During this time, the phasmid hangs upside down, supported remarkably by just the feet of its old skin. This resting period can last for some time, so don’t be alarmed if your phasmid appears motionless, even for several hours. Once it has the strength, the phasmid will then proceed to complete the moulting process by pulling its abdomen free.

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.