The exoskelton of an arthropod is hard, and thus cannot accomodate growth. So an arthropod must periodically replace it.
exoskeleton, chitin, molting
Arthropods molt (moult) in a process called ecdysis. It is necessary for growth because of the inflexibility of the exoskeleton. The old shell remnants are called exuviae. Note that the term 'anthropod' should not be confused with 'arthropod'; anthropods are humans or humanoids and do not moult in the canonic sense.
Ecdysis is when an arthropod molts its exoskeleton. Molting is necessary. The arthropod exoskeleton is inflexible, so, for it to grow larger, arthropods must molt.
Molting
Molting
an arthropod sheds it old exoskeleton when it has grown to a point where it needs a larger exoskeleton. This process is called ecdysis.
Its new exoskeleton is soft.
Molting
This is how they grow. The old exoskeleton splits, then the body expands rapidly before the new exoskeleton hardens.
During molting, an arthropod secretes enzymes that soften its old exoskeleton, allowing it to break free. The arthropod then absorbs water to expand its body and forms a new exoskeleton underneath the old one. Once the new exoskeleton is fully formed, the arthropod sheds the old one, revealing the new, larger exoskeleton underneath.
When an insect sheds its outer skin, it's called molting.
Most only do so in larval/juvenile stages, when it's time to grow. Some arthropods such as tarantula females continue growing and molting until death. Also, when damage has occurred (torn off leg for example) molting can be induced earlier to develop and grow new appendages. :)