But..there’s nothing to cause the shadow! What’s going on here? Biologist Joe Hanson explains:
An insect like a wasp or a water strider can rest atop the water, held up by surface tension. This means that the cohesive force of the water molecules sticking to each other is stronger than the force of the bug being pushed down by gravity. This works because it spreads its weight out over a large surface area (like snowshoes).
That creates a slight indentation in the top of the water, changing the direction that the light coming down is refracted and re-directing it slightly sideways (that’s where the bright halos around the dark areas come from). And what’s the absence of light?
A shadow! You can see Hanson’s illustrative diagrams at the link.