So bugs have pneumatic limbs. But what moves liquids inside those to make them work ?
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They have *hydraulic* limbs, pneumatic would mean they use pressurized air to move. And their limbs have a neat system, they have connective tissue that’s always trying to force contractions, while they can use the pressure of their blood + lymph substitute (haemolymph) to pressurize compartments which cause limbs to extend against the force of the connective tissue.
More ELI5: Take a spider, it has “cables” in its legs that try to close them into the shape you see when they’re dead. The whole body is full of fluid that can be pumped using pressure from the heart and other muscles to extend those legs against that “closing” force of the “cables.” Basically it’s a series of tubes, doors, and cables.
As someone else has mentioned, the limbs of arthropods, including but not limited to bugs, are hydraulic rather than pneumatic. They operate through moving liquid around, not compressed air.
The answer to the question I think you’re asking is that arthropods have hearts, just like humans do. And just like humans, those hearts are what provide the force and the energy to keep their “blood” moving. (The substance that serves a role very similar to blood in arthropods is called hemolymph, not blood.) And just like we do, they have nervous systems, which control the opening and closing of various valves, which accomplishes movement.
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