  A major challenge to understanding locomotion in complex 3-D terrain with
large obstacles is to create tools for controlled, systematic lab experiments.
Existing terrain arenas only allow observations at small spatiotemporal scales
(~10 body length, ~10 stride cycles). Here, we create a terrain treadmill to
enable high-resolution observations of animal locomotion through large
obstacles over large spatiotemporal scales. An animal moves through modular
obstacles on an inner sphere, while a rigidly-attached, concentric, transparent
outer sphere rotated with the opposite velocity via closed-loop feedback to
keep the animal on top. During sustained locomotion, a discoid cockroach moved
through pillar obstacles for 25 minutes ($\approx$2500 strides) over 67 m
($\approx$1500 body lengths), and was contained within a radius of 4 cm (0.9
body length) for 83% of the duration, even at speeds of up to 10 body length/s.
The treadmill enabled observation of diverse locomotor behaviors and
quantification of animal-obstacle interaction.

Biological Physics;Systems and Control;Systems and Control;Quantitative Methods