Scorpions achieve growth in two major ways. Increase in body mass over time with ingestion of food brings about a small size increase as flexible membranes stretch. This method of growth has limitations due to the scorpions hardened(non-living) exoskeleton or cuticle. Expansion only occurs between the hardened sections where flexible membranes exist to allow movement and expansion of the main mesosoma(body). The mesosoma must expand to allow for the development of embryo's in females and also to allow expansion as food is ingested. This expansion comes about via the flexible pleural membranes, found between the top and bottom mesosomal plates. The remaining flexible membranes between sections is to allow for the articulated movement of the various body parts.

The second and most interesting method of growth comes about by shedding or ecdysis of the cuticle. To achieve major growth the scorpion must grow a new cuticle under the old, which will expand and help push the old cuticle off the body.
Many thanks to Dave Gaban for the above sequence series of an
Androctonus australis going through ecdysis.
Androctonus australis is considered one of the worlds most medically important species and fortunately isn't found in Australia. An increase in internal body pressure helps to split the old cuticle from the area around the mouthparts and a little down the side of the anterior prosoma(head). With intermittant periods of activity and rest the scorpion manages to manouver the old cuticle rearwards. This is a vulnerable period normally spent within closed quarters to keep out potential predators and also maintain the correct climatic conditions. Some species require a reasonably high humidity to complete ecdysis without problem, whereas others seem to get by in comparatively dry air. If the old cuticle doesn't release properly and becomes stuck the scorpion is doomed. After a scorpion sheds it's cuticle it's recognised as the next instar, representing the next growth stage of a scorpion. Some scorpions such as the Urodacus genera tend to shed their old skin once a year at about the same time, in response to seasonal changes and food availability. Other scorpions such as the Buthids are likely to shed in relation to how much food has been ingested and don't have particular seasonal ecdysis periods. The number of ecdysial phases varies between species, but is typically around 4-6. The growth increment at each molt is quite substantial. You will find a morphometric study of Urodacus yaschenkoi in the features index which clearly shows the various stages of ecdysis and size increments.

The first ecdysis comes about whilst the young are still on mothers back. At this stage their exoskeletons are membrane like and tend not to fluoresce under uv, showing that the cuticle hasn't hardened or tanned. This may be due to the necessary water uptake from the mothers cuticle or possibly even diffusion of oxygen for respiration. There is a decrease in size at the first instar stage as the young consume the internal yolk supply present when born. Normal feeding is not possible at this stage as mouth parts are still unspecialised.

After the first instars molt they remain with the mother for a while until their exoskeleton has completely hardened, at which time their various body parts have become specialised ready for solitary existance. This post first molt time with mother varies between species, but ranges from a few days to 3 weeks. At this stage their hardened cuticle begins to fluoresce and quite strongly too.

People who keep scorpions find ecdysis one of the most difficult stages for survival. Emulation of normal environmental parameters is critical for success. In the picture at right we see a Urodacus elongatus that's undergone a molt in a sealed scrape beneath a rock(removed for observation). The edge of the scrape where it met the rock can be clearly seen. Without conditions like this the scorpion will either try to molt and fail or not molt at all. Note how the scorpions exoskeleton lacks pigmentation at this early post molt stage. In a week or so the cuticle will have hardened and the scorpion will fluoresce under uv light once again.

An adult Urodacus elongatus showing visible green glow present while the scorpion is exposed to uv light. It is not known if this character which is common to all scorpions has any adaptive value or is simply an artefact of the cuticle maturation process.

Scorpions reach maturity at the final molt stage and there is no evidence of post maturity molting. The spermatophore of males used in the passage of sperm between sexes is constructed inside the male in two halves called paraxial organs or hemispermatophores. These organs are located deep within the reproductive tract and are scleratized. All scleratized structures are replaced and shedding of such an organ would be extremely difficult. Shed hemispermatophores have never been observed in shed exuvae. This is contrast to many mygalomorph spiders, which regularly shed after adulthood.

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