  Diversity and specialization of behavior in insects is unmatched. Insects
hop, walk, run, jump, row, swim, glide and fly to propel themselves in a
variety of environments. We have uncovered an unusual mode of propulsion of
aerodynamic flight in two dimensions in Waterlilly Beetles
\emph{(Galerucella)}. The adult beetles, often found in water lilly ponds,
propel themselves strictly in a two-dimensional plane on the surface of water
via flapping wing flight. Here we analyze the aerodynamics of this peculiar
flight mode with respect to forces exerted on the organism during flight. The
complexity of 2-D flight is captured by accounting for additional forces beyond
gravitational, thrust, lift and drag, exerted on the insect body in 3D flight.
Understanding this constrained propulsion mode requires accounting for viscous
drag, surface tension, buoyancy force, and capillary-wave drag. Moreover,
dramatic differences exist in the magnitude of the resultant forces in 2D vs.
3D flight. Here, in this fluid dynamics video, we discuss this unusual 2D
flight mode via kinematic and non-dimensional analysis comparing it to 3D
flight and draw generalized lessons towards understanding surface skimming
based origin of flight in insects.
