Your regular spiders are small-bodied insects. So, its not out of the ordinary to imagine how small their bones would be, if they have any at all. 
In fact some bigger creatures like lobsters, crab, snails, worms and shrimps have no bones.
So, do spiders have bones? Well, spiders don’t have bones – and like other arthropods. However, they have an endoskeleton plus an exoskeleton (external skeleton) that provides primary support for the arachnid’s muscles. Its exoskeleton has cuticles plus joints on their surfaces to enable easy movement.
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Details: Do Spiders Have Bones?? Exoskeleton?
The spider’s cuticle is composed of 4 layers (like other arthropods) unlike insects that have 3 cuticle layers. However, the cuticles are all made off chitin fibers plus protein.
But, don’t be mistaken, the spider still has a small internal skeleton – that’s has, however, no bones in it. The spider’s endoskeleton has collagen and a couple of plates which won’t articulate like bones do for the skeleton in vertebrates.
Compared to humans and other animal species -these ones have an endoskeleton that acts like a support structure that’s created from bones.
The endoskeleton offers support to its organs like lungs, brain, heart and maintain the animals size and shape.
Exoskeletons offer Spiders Body Support
As we’ve noted above, spiders have an exoskeleton as compared to having an internal skeleton created from bones. So, what’s these exoskeletons and what makes it up?
The arthropod’s exoskeleton is made from chitin- which covers its body outer surface to offer support, protection, and structure. Thus, the exoskeleton will be shed (through molting) as the spider grows and develops. So, below is a tarantula shedding her exoskelton:
How Spiders Benefit from the Exoskeletons
Compared to endoskeletons, exoskeletons will provide more protection because it covers the whole spider’s body while the bones would only be located internally.
Also, exoskeletons enable the spider’s muscles to achieve more leverage for movement, lifting, or picking objects – this is better than in the animals with endoskeletons.
Therefore, the exoskeletons will give the spider more power and straight than other animals of the same size but having an endoskeletons.
Related: Do Spiders Have Ears?
How Spiders Move
The segments of the exoskeleton in the arthropod are joined using joints to allow it to move up and down. Therefore, the muscles that are attached to the exoskeleton tend to contract allowing the spider to move its legs inward.
However, the spider lacks muscles that would otherwise extend its legs back (outward) to their default position. Therefore, the creature use its blood and other bodily fluids to push the legs outward.
Whenever the spider loses excessive bodily fluid, it’ll lack the required pressure (hydraulic) for moving its legs outwards. You’ll notice the spider lying on its back and its legs in a curled up position.
How Spiders Molt
Spiders will move from one point to another through muscle contraction – these will be attached to the exoskeleton on the outer part of the arthropod.
The spider’s exoskeleton is composed of different cuticle layers – that are made from chitin (polysaccharide) and proteins. This allows the cuticle structure to be layered like a plywood grain.
Therefore, the cuticle structure is very strong for protection while also preventing the spider from losing fluids excessively and drying out. This would in turn prevent effective movement in the arthropod.
But the cuticle won’t grow like the human tissue or bones (endoskeleton) and thus it must be shed off as the spider continues to develop and grow. But spider molting mainly happen when the spider is in its younger years – but other may molt all its life.
1. When time is ready, the spider’s body (directed by its hormones) will absorb a part of its cuticle layer (particularly the lower layer) in its exoskeleton.
2. In addition, the spider’s body will also start generating a new cuticle layer to create another exoskeleton. So, the emerging exoskeleton will be folded slightly to allow it to expand when the spider sheds off the old exoskeleton.
3. The spider will inject a molting fluid at the areas separating the new and old exoskeleton. The fluid allows easy separation of the two exoskeletons by create a gap between them.
4. Next, the spider bust out to shed its old exoskeleton. To achive this, the animals tend to raise the heart rate and thus pumping more blood (hemolymph) into its cephalothorax from their abdomen.
4. Therfeore the hemolymph’s pressure forced the older exoskeleton to crack while the spider continues to flex their muscle to make its old exoskeleton to molt.
