I was mildly surprised at the reaction to this cool timelapse video of a molting crab — some people didn't understand how arthropods work. The only thing to do, of course, is to explain the molting process of insects and crustaceans, called ecdysis.
Let's go back to the basics first. In the beginning was the epithelium, a continuous sheet of linked cells that envelops multicellular organisms. These are living, dividing, dynamic cells that are flexible, can repair damage to themselves, and represent the boundary between the carefully maintained internal environment of the organism, and the more variable and often hostile external environment. And that's where the problem lies: living cells are relatively fragile and sensitive, and in particular don't cope well with drying out. Cells like it wet, yet if you look at insects and people, we live under horrible conditions for living cells, surrounded by dryness and heat and cold.
Our external epithelia have evolved different solutions to this problem of the basic inhospitability of terrestrial life. In us, our bounding epithelia divide frequently, pushing new cells outward. As these cells move, they commit suicide, producing a fibrous protein called keratin which forms dense, matted tangles inside the cells; these cells also build tight protein connections between their neighbors. It is these dead, protein-packed cells that face the outside world, protecting the delicate interior. These cells are steadily worn away and cast off — dandruff flakes, for instance, are sheets of these dead epithelial cells — and new protective cells produced by cell division and pushed up from the inside out to replace them. It's a good solution that allows for constant growth and flexibility.
Arthropods, on the other hand, start with a similar sheet of living epithelial cells, but do something completely different. Instead of pushing out a continuous column of dying cells, they secrete dense layers of complex chemical compounds that harden into a tough cuticle. The exoskeleton of an insect or crustacean is acellular — the living cells have protected themselves by secreting an initially fluid set of chemicals that harden like epoxy to form a tough protective armor around themselves. We protect ourselves with sheets of leather; arthropods make plates like fiberglass on their outsides.
And there's the rub. The cuticles of insects do not gradually slough away, replaced steadily by the addition of new material from the inside. They're mostly fixed and rigid and static. This does have the advantage of providing a solid protective armor and a rigid framework for muscles, but isn't so great for accommodating growth. Fiberglass isn't stretchy and flexible!
Here's a closer look at the structure of the arthropod cuticle.
In the diagram on the left, the living epithelium is at the bottom, labeled "epidermis". Above it are multiple acellular layers called the cuticle made up of substances like chitin and waxes (notice that it is also perforated by pores containing ducts of the glands that secrete the chemical substances, and also places where hairs called setae can dangle into the exterior.
In order to grow, the animal must discard the old cuticle and build a new one from the inside out. In (b), this process begins by peeling away the living epidermal cells from the dead cuticle, creating a gap called the exuvial space, which is filled with a fluid called molting fluid. The cells then begin secreting a new cuticle from underneath, which is initially flexible.
What is poorly shown in these diagrams is that the new cuticle can be larger than the old. What that means is that epithelium inside the old cuticle is wrinkled and convoluted to have a larger surface area. Again, it is soft, not hard, so it can wrinkle up freely to fit. Also, to make room, the molting fluid in (c) is busily digesting the old cuticle from underneath, and the protein components are absorbed and reused to build the new cuticle.
In (d), the new cuticle is nearly fully formed, the old cuticle has been reduced to a thinner rind, and the two are separated by a thin fluid-filled space. Ecdysis, the actual molt, then occurs, and the old cuticle is discarded. Free of its confining shell, the animal inflates itself to extend the wrinkled new cuticle into larger smoothness, and the process of sclerotization, or hardening of the cuticle, begins from the outside in. Tanning agents, like polyphenols are secreted through ducts onto the surface, where they are oxidized into quinones, which trigger chemical reactions that cross-link the various substances of the cuticle into a rigid structure.
If you've ever eaten soft-shell crabs, you've caught the poor creature just after a molt and before its cuticle has hardened — in large arthropods, it can take several days for the post-molt cuticle to be fully cured. The hardening is also regional. Next time you're eating a crab leg, notice that the shaft of the limb is rigid and strong and a bit brittle, but it grades into softer, less thickly sclerotized material at the joints called arthrodial membranes, which retains the flexibility of the pre-molt cuticle.
Now go watch the video again, and it should make more sense. What you're seeing near the end is the crab pulling soft and rubbery limbs out of the shell of its old legs, and then resting as the new cuticle slowly hardens.