Terrestrial, and especially arboreal, snakes can generally span a greater distance than aquatic snakes, which have a relatively short saccular lung. Thus, we hypothesize that the saccular lung provides structural support when snakes form a cantilever. Without subsequent study, the saccular lung has long been proposed as a support for a snake's body, especially to help span gaps. To test this hypothesis, we determined whether the pressure in the saccular lung was significantly elevated when the snake was spanning a gap. Further, we reduced pressure in the lung to zero to determine whether this manipulation reduced the cantilever ability of the snake.
The pressure in the saccular lung increased significantly as a snake neared its maximal cantilever distance. However, when we prevented the pressure in the saccular lung from rising above atmospheric pressure, the snake's maximal cantilever distance was not affected. Therefore, the pressure increase in the saccular lung as the snake approached its maximal spanning distance was probably caused by the contraction of skeletal muscles needed to support the body. We conclude that the saccular lung does not help snakes span gaps. We discuss whether the saccular lung helps support the body or maintain posterior body form.