|A firebelly toad waiting for a cricket|
I found a nice explanation in an essay by J.B.S. Haldane (1892-1964) ‘On Being the Right Size’:
There is a force which is as formidable to an insect as gravitation to a mammal. This is surface tension. A man coming out of the bath carries with him a film of water of about one-fiftieth of an inch in thickness. This weighs roughly a pound. A wet mouse has to carry about its own weight in water. A wet fly has to lift many times its own weight and, as everyone knows, a fly once wetted by water or any other liquid is in a very serious position indeed. An insect going for a drink is in as great danger as a man leaning out over a precipice in search of food. If it once falls into the grip of the surface tension of the water - that is to say, gets wet - it is likely to remain so until it drowns.
I often swim at lunch. When I’m done I haul myself out of the water and head to the showers without a second thought for the amount of pool water I’m carrying around with me. It’s lucky for me that my size makes me immune to the same surface tension that drowns the unfortunate crickets.
As a tangent: I found an online copy of Haldane's essay here - it's worth reading and not too long.