Explain it Simply
Imagine wearing a suit of armour made of hard plastic that covers your entire body, including your face. Now imagine trying to grow — your body gets bigger, but the plastic doesn't stretch. What happens? You'd be squeezed and eventually suffocated by your own armour.
This is exactly the problem Arthropoda faces.
The Actual Biology
Arthropods have an exoskeleton made of chitin — a rigid, non-stretchy polysaccharide. It is strong and protective but completely inelastic.
When an arthropod grows, its soft tissues expand but the chitin exoskeleton does not. To allow growth, the arthropod must:
- Secrete enzymes that partially digest the old exoskeleton from the inside
- Simultaneously secrete a NEW, larger exoskeleton underneath the old one (but soft and crumpled)
- The old exoskeleton (exuvium) splits along special lines (ecdysial lines)
- The arthropod emerges from the old shell — this is ECDYSIS (moulting)
- The animal rapidly takes in air/water to expand the new soft exoskeleton before it hardens
- The new exoskeleton hardens (sclerotization) — now the arthropod is bigger
The Vulnerability Window
During moulting, the arthropod is completely SOFT and VULNERABLE — no protection. Many predators specifically target arthropods at this stage. This is why moulting animals hide under rocks or in burrows.
Practical Consequence
The hormone ecdysone (a steroid hormone) controls moulting. Insecticides that mimic ecdysone can force premature moulting or prevent moulting — killing the insect.
Bottom line: Arthropods moult because chitin can't grow — they need a new, bigger exoskeleton. The price is temporary vulnerability.