  Locomotion emerges from effective interactions of an individual with its
environment. Principles of biological terrestrial locomotion have been
discovered on unconfined vertical and horizontal substrates. However a
diversity of organisms construct, inhabit, and move within confined spaces.
Such animals are faced with locomotor challenges including limited limb range
of motion, crowding, and visual sensory deprivation. Little is known about how
these organisms accomplish their locomotor tasks, and such environments
challenge human-made devices. To gain greater insight into how animals move
within confined spaces we study the confined locomotion of the fire ant {\em
Solenopsis invicta}, which constructs subterranean tunnel networks (nests).
Laboratory experiments reveal that ants construct tunnels with diameter, D,
comparable to bodylength, L=3.5 $\pm$ 0.5 mm. Ants can move rapidly (> 9
bodylengths/sec) within these environments; their tunnels allow for effective
limb, body, and antennae interaction with walls which facilitate rapid
slip-recovery during ascending and descending climbs. To examine the limits of
slip-recovery in artificial tunnels we perform perturbations consisting of
rapid downward accelerations of the tunnels, which induce falls. Below a
critical tunnel diameter, $D_s = 1.31 \pm 0.02$ L, falls are always arrested
through rapid interaction of appendages and antennae with tunnel walls to jam
the falls. $D_s$ is comparable to the size of incipient nest tunnels (D = 1.06
$\pm$ 0.23 L) supporting our hypothesis that fire ants construct environments
which simplify their control task when moving through the nest, likely without
need for rapid nervous system intervention.
