Please note that the Scarab is neither a HERC or an infantry unit, it's a mix between the two offering the greatest mobility in the game.
Trajan said it best: The Scarab is a military variant based on the pressure suits used in the Venus arcologies, the SCARAB was designed to fill a void between ground troops and larger vehicles. They have specialized weaponry that only they can carry in addition to being able to mount some small mount vehicle weaponry.
There are three possible modes:
For faster long range travel, treads drop down from the bottom of the feet, the unit assumes a skiing like position, and off it goes. Turning ability is seriously compromised whilst in this mode. Treads can not be used over rough terrain.
Running / Walking:
In the event of rough/sloped terrain, the treads can be retracted and a walking/running mode is entered. This is the most agile mode of the three.
The unit also comes equipped with a jet pack that will allow it to reach areas and vantage points that other vehicles cannot reach, and/or use as an escape option. The jet pack has a long burn rate and a very slow recharge rate, so it should be used sparingly.