Arthropod (World 4-10 or Advanced 30) is the 30th and last level in the Advanced Mode and the 40th level in the game.
The stage consists of five rings, with the goal attached at an angle to the final ring. A large robotic insect is standing the structure with cone-shaped feet, with two on the center ring and one foot on all others. As the rings rotate, the insect will "walk" by moving its legs across the rings, moving three legs at a time.
To reach the goal, a player can run off the edge of the starting platform and bounce to the final ring before attempting to stay on until the goal passes. However, as it takes about 20 seconds for the goal to reach the top of the stage, it is instead recommended to move onto the rings in the 45 - 40 second range and rush to the goal quickly. It is possible to reach the goal by falling off the ring edge at a specific point, but this is very difficult without timing the player's movements exactly.
- This is the first stage in Story Mode to have a difficulty of ten.
- The insect in this stage is often erroneously referred to as a spider, but it only has six legs instead of a spider's eight.
- Arthropod is generally seen as an incredibly difficult stage due to strange physics of the feet and intensity of getting into the goal, and some players believe it should belong in the Expert or Master modes.
- The only part of the insect that appears to have collision detection is the legs.
- In the game's files (Stage ID 80), there is an early version of this stage where the five rings are replaced by a large cylinder. Additionally, instead of hanging off the end, the goal is located in the center of the cylinder and moves in a zig-zag path. This version is also a part of Debug Stage 192.