If anyone wants to know, the reason that happens is because spiders move their legs with a hydraulic system and once they are dead the pressure gets released and they revert to their “default” state.
https://asknature.org/strategy/leg-uses-hydraulics-and-muscle-flex/
I love that one too! Though my fave is Avianos. I’d love to see a UFO 50 more where they do some follow ups of some of these. Or open up modding or something.