Do all spiders molt?

Yes, as spiders need to grow, the way to grow is to shed their skin (exoskeleton) 

The hard exoskeleton doesn’t change size and grow as the spider does. Therefore, spiders must shed their old exoskeletons to make room for new ones

The actual process of shedding an exoskeleton – called ecdysis – is very tricky for these spiders.  First they grow the new exoskeleton inside of the old one.  The new exoskeleton is chemically the same stuff as the old one, but it is not hardened yet and indeed is wrinkled like an egg carton to squeeze inside the old one, like having a large t-shirt on underneath a snug sweater. Some stretching will occur after the old skeleton is shed, but some of the new size must be there from the beginning. 

With the new exoskeleton complete, the spider dissolves the living tissue between the old and new skeletons, leaving only nerve connections to the sensory organs – touch-sensitive hairs, eyes, and chemical sensory organs (taste and smell) located on the outside of the body.  One day, the spider will hang from a thread on the orb web or under a leaf, disconnect those sensory organs, and pop the top on its cephalothorax (the body part with all the legs).  The chemical changes involved in hardening off the new exoskeleton start as the old skin is shed. Spiders not infrequently die as their long legs harden and stick before they can be completely withdrawn from the old exoskeleton.

The new exoskeleton is very soft, during this time spider vulnerable to predators and other spiders. Once it hardens, the spider is ready to resume life as an insect-slaying assassin, bigger and more fearsome than ever before.

Source: https://www.uvm.edu/~lehiggin/Growth_by_molting.html
Linden Higgins
Department of Biology, University of Vermont

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It’s called Molting.

In arthropods, such as insects, arachnids and crustaceans, moulting is the shedding of the exoskeleton (which is often called its shell), typically to let the organism grow. This process is called ecdysis.

Molting, known technically as ecdysis, is literally a period of growth for insect.

Exoskeleton is like a shell. It serves as its external backbone is used for protection and support
The old exoskeleton is called an exuviae

While moulting, insects can’t breathe

Molting or Ecdysis is necessary because the exoskeleton cannot grow.

During this process insect more vulnerable to predators.

Three layers of the insect exoskeleton:

• The outermost layer is called the cuticle.
  This layer protect the insect against physical injury and water loss, as well  as provides rigidity for muscle

• The epidermis. ( second layer )
  It is responsible for secreting a new cuticle when it is time to shed the old one.

• The basement membrane.
  This membrane is what separates the insect’s main body from its exoskeleton.
  

The Process of Molting

In molting, the epidermis separates from the outermost cuticle. Then, the epidermis forms a protective layer around itself and secretes chemicals that break down the insides of the old cuticle.
That protective layer becomes part of the new cuticle. When the epidermis has formed the new cuticle, muscular contractions and air intake cause the insect’s body to swell, thus splitting open the remains of the old cuticle.
Finally, the new cuticle hardens. The bug squeezes out from the outgrown exoskeleton.

Cicada Nymphs usually undergo five molts during the several years required to reach maturity

Most grasshoppers molt about five times, each time emerging larger and closer in size to a fully formed adult. In the last stage of molting, the wings are fully grown.

Caterpillar molts 5 times from the first instar to fifth instar and become pupa.
The caterpillar does this by first growing a new skin underneath the outer skin. Then, when it is ready, it “sheds” the old skin, and the newer, larger skin underneath is exposed.
After the caterpillar has molted for the first time, it is referred to as a second instar, and it has some room to grow.
When the caterpillar molts for the fifth and final time, the new skin underneath forms the outer shell of the CHRYSALIS ( generically referred to as a pupa )

Source and Reference:

• https://www.thoughtco.com/how-insects-grow-1968346

• http://www.butterflyschool.org/new/meta.html

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