Fairy wasp

[above] The fairy wasp, Megaphragma mymaripenne... pictured next to a Paramecium and an amoeba at the same scale.
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As they get smaller, insects can do away with many of their organs. The feather-winged beetles – twice as big as the fairy wasps, but still impressively tiny – have drastically reduced the size of their genitals, guts and breathing tubes. They have totally lost their hearts: at their size, diffusion is enough to carry liquids around their body without the need for a pump. Their wings, like those of thrips and fairy wasps, are little more than wispy strands, rather than the flat oars of most other insects. That’s all they need to paddle through thick air currents.

From Not Exactly Rocket Science.