The Descent of Ant
The gliding ant Cephalotes atratus was the first described
example of an animal capable of sophisticated aerodynamic control in the absence
of obvious morphological adaptations for aerial behavior. In this thesis I
present a series of studies aimed at quantitatively describing the aerodynamics
of stability and control in C. atratus, using a combination of
field studies and modeling experiments.
In Chapter 1, I present a quantitative analysis of the 3-D trajectories followed
by gliding C. atratus ants, measured using multiple-camera
photogrammetric techniques in a natural rainforest environment. These
trajectories represent the first attempt to measure gliding trajectories in a
natural environment for any animal, and provide a data-driven view of variation
in aerodynamic characteristics and trajectory shape across multiple C.
atratus individuals. The 3-D analysis conducted in Chapter 1 shows
that C. atratus reach equilibrium glide speed (μ=4.12,
σ=0.59) within 1s of dropping from the canopy. Lift-to-drag ratios during
equilibrium gliding were higher for ants dropped further from a target tree
trunk, but for a given horizontal distance between drop point and target tree
trunk, lift-to-drag ratios were not observed to vary significantly with size.
In Chapter 2, I present a kinematic analysis of the use of posture by gliding C.
atratus ants. By dropping C. atratus workers into a vertical
wind tunnel and analysing their behaviors using multiple cameras and posture
estimation techniques, I show that gliding C. atratus ants
immediately adopt a parachuting pose with the legs elevated above the body axis
when dropped. Changes in gaster flexion angle as well as the fore-aft angles of
the mid- and hind-legs were observed in all analysed bouts of gliding, but
statistical attempts to find correlations between static postures and body
kinematics were ultimately unsuccessful due to the rapidity with which ants in
the wind tunnel changed posture.
In Chapter 3, I present a quantitative description of stability and control in
gliding ants, using dynamically scaled physical models to show how the postural
changes discovered in Chapter 2 result in changes in the aerodynamic forces
experienced by the ant as well as its stable body orientation. I show that the
standard gliding posture adopted by a falling C. atratus ant is
aerodynamically stable in both pitch and roll, and that subtle changes in
posture and the location of the center of mass result in significant changes in
the magnitude and direction of the aerodynamic force experienced by the ant.
The postures associated with pitching and turning maneuvers, moreover, are also
aerodynamically stable, which means that gliding C. atratus
ants do not jeopardize their stability while conducting aerial maneuvers.
Finally, by testing the aerodynamic performance of a flat, splayed-out posture
for C. atratus, as well as a modified C. atratus with
shortened legs, I show that the elevated-legs posture adopted by C.
atratus is fundamental to its stability, and that leg length is a key
predictor of aerodynamic stability and control.