If you try to measure a scorpion's growth, you will see an interesting pattern: its length does not change at all for weeks, then increases markedly within a few days. The reason is the exoskeleton. The hard, chitin-based cuticle protects the animal but cannot expand; the scorpion can grow only by shedding it and forming a new one. This process is called moulting (molt in American usage) or ecdysis. The empty shell left behind is called the exuvia.
The structure of the exoskeleton and the body regions were described in the article on body structure. This article looks at how that skeleton is renewed and what the process means for the animal.
Growth stages
Scorpion young undergo their first moult shortly after birth; the fact that this first moult takes place on the mother's back is part of the brood care described in the article on reproduction. After that, the individual grows by passing through stages, each corresponding to the period between two moults. In the literature, each of these periods is called an instar.
The number of moults before adulthood varies by species; for most species, a figure between five and seven is reported. An individual that reaches sexual maturity usually does not moult again, so the size of an adult scorpion is fixed. The time to reach adulthood can range from months to several years, depending on species, temperature and diet. The interval between two moults is not fixed either; it lengthens as the individual grows.
Four stages
Moulting is not a single moment. It includes a preparation phase that begins days in advance, an emergence lasting a few hours and a long hardening period that follows.
| Stage | What happens inside | What can be seen from outside |
|---|---|---|
| Preparation | The old cuticle separates from the underlying tissue, and a new, folded cuticle forms beneath it | Feeding decreases, the animal retreats into its shelter, the body looks swollen |
| Emergence | The old exoskeleton splits along its front edge, and the individual slides forward out of it | An exoskeleton gradually emptying beside an animal that appears motionless |
| Hardening | The new cuticle expands, then hardens and darkens through cross-linking of proteins | A pale, soft individual with a glossy appearance |
| Intermoult | Tissue growth and build-up of reserves for the next moult | Normal feeding and movement |
What triggers the process?
In arthropods, the signal that triggers moulting comes from steroid hormones called ecdysteroids. When the levels of these hormones rise, the underlying tissue begins to produce the new cuticle. Hormonal regulation in scorpions has not been studied in as much detail as in insects; however, adequate feeding and a suitable temperature are observed to speed up the process, while unfavourable conditions delay it. It is therefore not surprising that siblings born on the same day progress at different rates after a few stages.
How does emergence take place?
At the end of the preparation stage, the animal usually lies on its back or side and remains motionless. The old exoskeleton splits along the front edge of the prosoma. The individual first pulls its front section out of the old casing, then its pincers and legs, and finally its tail. At this stage, not only the outer surface but also the thin membranes of the leg joints, the sensory hairs and the sting itself are renewed. The new sting is not functional until it has hardened.
Immediately after emergence, the new cuticle is flexible. In this short window, the animal expands the new exoskeleton using the pressure of its body fluid; it is this expansion that determines its next size. Once the exoskeleton has hardened, the size remains fixed until the next moult.
Risks of the process
Moulting is the most vulnerable period in a scorpion's life. For several hours the animal cannot escape, and for the following days its pincers and sting are not effective. The main risks are:
- Getting stuck: If the old exoskeleton does not come away from a limb, the limb may be lost or the animal may be unable to complete the process. An environment that is too dry increases this risk.
- Injury: The soft cuticle is easily damaged by rubbing against a hard surface or by a fall.
- Attack: Live prey left in the enclosure or another scorpion can harm the defenceless individual.
- Insufficient energy: An individual that enters the preparation stage underfed may struggle to complete the process.
The role of humidity and temperature during this period was discussed in the article on temperature and humidity; for an individual approaching a moult, even a small deviation in environmental conditions can affect the outcome.
Care specific to this period in closed breeding
In a closed-cycle facility, moulting is a separate item on the care schedule. Care starts with recognizing the signs of preparation: as a moult approaches, the old cuticle becomes dull, the membranes between the dorsal plates of the mesosoma stretch and stand out as light-coloured bands, and the animal loses interest in prey. When these signs appear, the main steps in practice can be summarized as follows:
- An individual showing signs of preparation is recorded and taken out of routine procedures.
- No live prey is left in the enclosure; the shelter and substrate are left undisturbed.
- When emergence is complete, the exuvia is removed without touching the animal and recorded with the date.
- The individual is not handled, and feeding is postponed, until the new exoskeleton has hardened and darkened.
- If limb loss or an incomplete moult is observed, this is noted under the individual's code.
Exuvia records are used not only for care but also to estimate the individual's age. For an individual whose number of moults is known, its stage can be determined more reliably than by looking at its size. Because the cuticle of freshly moulted individuals fluoresces only weakly under ultraviolet light, this method also helps to tell them apart; the topic was covered in the article on UV fluorescence.
The role of the moulting pattern in colony management also determines the continuity of production: the balance between adults suitable for material collection and juveniles that are still growing depends on tracking these stages correctly. The general principles of colony management are set out in the article on closed-cycle breeding.
