Moulting Woodlice

 

A recent trip to Fordon Banks to turn some longs for invertebrates revealed an Oniscus asellus midway through its moult. Its not unusual to find moulting woodlice whilst looking under debris, although it is relatively easy to dismiss them after identification. This one was looking particularly photogenic so I took a couple of shots. Upon loading it to the computer I noticed a rather obvious feature of just how big the size difference is between each skin (instar). It must be an incredible squeeze for the head end of this individual. Comparing the segments where they meet reveals quite a difference. 

Once the body of a woodlouse becomes too large for its exoskeleton (outer skin) it has to moult. Immediately prior to the moult the woodlouse will cease feeding and the exoskeleton will then split midway. As with insects a thin layer of moult fluid (or plasma) forms between the two layers of skin, which helps separate the two layers. This is formed in glands within the skin (exoskeleton) which are only used for production when moult occurs. The moult process consists of two phases with the rear half falling off first, then one to two days later the front half falls off. It seems an odd way around given the sensory organs are on the front half of the body.