You do not need the plastic cover on the undercarriage of the Beetle necessarily. It is designed to protect the undercarriage from road debris but it is not necessary to the function of the car.

They are know as the undercarriage.

undercarriage

Sequence valves are used in aircraft having a retractable undercarriage because there is a certain sequence to the way things must be done. For example, before the landing gear can come down, the door on the undercarriage must be opened for it.

In the undercarriage