Semiterrestrial and terrestrial isopods have a unique biphasic moult, in which first the posterior half is moulted and then the anterior half, separated by a day or two depending upon species.  There are several advantages to this.  In comparison, most marine isopods moult their exuviae in one piece, as do all other crustaceans.

NOTE  lit. “that which is taken off” L., a term used in studies of insects, crustaceans, snakes, and other animals which cast off their exoskeletons or skins during growth

A male Ligia pallasii has just moulted its posterior half. Note its
larger size as compared with the anterior unmoulted half. The
posterior part is still soft, but is in the process of being
mineralised. The ventral or sternal side of this individual bears
the calcium stores to be used in this mineralisation (see Research
Study 3 below). A few days later the anterior half will be moulted 1.6X

What are the advantages of a biphasic moult in an oniscid isopod? Consider these answers, then CLICK HERE for explanations.

It is an energy-savings strategy.

It allows for continued motility.

It reduces evaporative water loss.

It permits cycling of calcium for mineralisation.

It ensures that sensory receptors (e.g., eyes, antennae, uropods) and defensive structures (e.g., repugnatorial glands) are always fully functional on one half of the body.

In marine isopods most or all of the calcium for mineralisation comes directly from seawater surrounding the animal.  In comparison, semiterrestrial and terrestrial isopods obtain their calcium from food, or from freshwater or possibly ingested calcareous soils, and calcium is at more of a premium than it is for marine forms. The main nutritional requirement for calcium by isopods (and other crustaceans) is to replace that lost in the cast-off moult and to provide for growth of the new exoskeleton.  In Ligia spp. about 25% of the calcium content of the exoskeleton is lost when it is moulted but, as ligiids normally eat their exuviae, the actual loss by this cause may be small. The photograph at Right shows an anterior moult or exuvia of L. pallasii, soon to be eaten. Carefoot 1984 Comp Biochem Physiol 79A: 655.

NOTE  a few terrestrial species, including Porcellio scaber are known to consume their exuviae after moulting, and it may be a common behaviour throughout the oniscideans.  Whether it is primarily for calcium-saving, or for resorption of general nutrients, is not known

Terrestrial isopod Porcellio scaber, common under flower pots, pieces of
wood, and leaf litter in gardens; also known as woodbug or woodlouse 2X

Prior to the posterior moult, calcium and carbonate ions are resorbed from the old posterior cuticle and transported in the hemolymph across the anterior sternal epithelium into a space, known as the ecdysal gap (a space created as the old cuticle becomes hard and begins to split off), where they precipitate to form calcium-carbonate spherules. After the posterior part is moulted, the sternal calcium spherules are entirely resorbed and used for the mineralisation of the new posterior cuticle. The deposits are seen in the Left-hand photo as white areas in the first four sternites of a moulting Ligia pallasii. 

In Ligia the deposits consist of microscopic spherules (0.5um dia) of amorphous calcium carbonate precipitated within an organic matrix (see photo on Right). The large surface area and the amorphous state of the spherules facilitate their mobilisation and speed mineralisation of the new posterior cuticle.  These calcium deposits are an adaptation to the terrestrial environment and do not occur in marine isopods. Photo of spherules in Ligia italica courtesy Helge Fabritius and Andreas Ziegler, University of Ulm, Germany.

NOTE  lit. “escape or slipping out” G.  Another word for moulting is ecdysis

NOTE  non-crystalline.  Solubility of calcium carbonate in its amorphous form is 10 times greater than in its crystalline form