We came in to work to find Pinki the tarantula on her back in her desert aquarium.
Like many people who see a tarantula in this position, our first thought was "Is she dead?" She wasn't moving. Her legs were wide apart. She was on her back. She certainly appeared dead.
However, even naturalists have things to learn, and some Internet research showed that Pinki was not dead at all. She was just getting ready to molt --- shed her exoskeleton.
All spiders molt, and a sign that a tarantula is getting ready to molt is that it will spin some webbing and flip to its back to help it ease the escape from its old exoskeleton.
Why spiders molt
Unlike humans, spiders don't have bones in their body. Instead, they have an exoskeleton, which is a hard outer covering that protects the inner organs and also is used to move. Spiders contract muscles attached to their exoskeleton to help them move, just like humans do, but opposite. The spider skeleton and spider joints are all on the outside instead of the inside.
However, spiders' exoskeletons can't grow with them. Made out of cuticle, the exoskeleton is extremely strong and rigid and does not expand like human bone and tissue. To increase in size, a spider must create a new exoskeleton inside the smaller, old exoskeleton and then crawl out of the old one.
Molting occurs more often when a spider is young and less often as it ages. Pinki molted in June 2017 and then again in October 2018.
How molting works
The molting process begins before we can see it, before the spider exits the old exoskeleton.
Pinki would have begun molting when her hormones told her to by secreting cuticle material inside her old exoskeleton and creating a new exoskeleton layer inside. The new layer is larger than the old, so it begins slightly softer and folded.
She would then secrete a molting fluid between the two exoskeletons, dissolving the tissue between the two layers and creating a gap.
Tarantulas then contract the abdomen to push fluids into the cephalothorax --- the fused head and upper body on the diagram below. This pressure helps the old exoskeleton lift from the new one and crack open.
This is the point that Pinki turned over onto her back. She flexed her muscles and made the old exoskeleton fall away, sliding from it in her new, flexible exoskeleton.
Right after, Pinki grew into her new "skin." She then needs to be left alone for about a week for her new exoskeleton to fully harden.
Many creatures molt, whether it be losing an exoskeleton or shedding skin. Crustaceans, caterpillars, lizards, snakes, frogs --- all of these molt to reveal a new exoskeleton or new skin as they grow.