Fluid feeding is one of five major feeding modes used by organisms. The others are filter feeding, deposit feeding, bulk feeding, and phagocytosis.
Fluid feeding is defined as getting your nutrients by consuming the fluids of another organism. Animals which practice fluid feeding include hummingbirds, spiders, aphids, vampire bat, ticks, mosquitos, and leeches. Except for hummingbirds, that's veritably a rogue's gallery of reviled organisms.
Many fluid feeders justifiably have a bad reputation. Although the "fluid" in question may be botanical in origin, when it involves animals, the fluid is blood. So many fluid feeders are bloodsuckers. But fluid feeding is obviously a successful mode, and has probably been around since insects first crawled onto land about 428 million years ago, during the Devonian period.
Fluid feeders need some way to pierce the protective wall (skin or plant walls) getting in the way of the fluid, then some proboscis-like appendage for sucking out all the fluids. Aphids are specialized in doing this with plants, which is why they can be found in such numbers on certain plants, especially those that produce an abundance of sweet fluid.
Hummingbirds, the smallest bird with the highest metabolism, are fluid feeders of flowers. Hummingbirds are a remarkable animal: they're the only bird capable of flying backwards, and controlling vertical and lateral movement with such precision. Their long beaks are used to drink the nectar out of flowers, an excellent example of fluid feeding. Hummingbirds beat their wings between 15 and 80 times per second, and their metabolism is so fast that they are hours away from starving at any given moment.
The most despised fluid feeders are those that feed on human blood, spreading infections and some of the worst diseases. Pliny the Elder called blood-sucking ticks "the foulest and nastiest creatures that be." They spread various diseases, including Lyme disease, which can be fatal if left untreated. Mosquitos annoy us with their buzzing and spread malaria, which afflicts half a billion people every year and kills between one and three million.
Although fluid feeding is not a particularly common method of feeding, we see that animals that use this feeding mode have a diverse range, from foul to beautiful.