How is Hilara detecting the water surface ?

 Answering this question will come from a study of Hilara landing upon the water surface. A landing is given in the animation (10X retarded) below.

The height of Hilara above the water surface is given as a function of time in the graph given here.

At time 0.14 Hilara is colliding with the water surface with a velocity of approximately 18 cm/s. The fly does not retard before the contact with the surface. In principle the fly may use the distance with its mirror image to estimate the distance to the surface. The fly does not use this information. The surface tension is large enough to stop the vertical motion.

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