It's highly likely that you've never seen one. There's about 600 species, but the larvae don't live long, the adult females never emerge from the host, and the males are small raspberry-eyed freaks whose only purpose in life is to find the tumour-like lump that is a female, and stab her full of sperm.
They earned their name "Twisted-Wing" because of the peculiar way they hold their hindwings when flying - the forward pair of wings has been reduced to a pair of balancing organs very like those used by true flies. On the other hand, their mobile larvae are very like the larvae of parasitic beetles (such as oil-beetles, and the large Feather-Horned Beetle that starred here at wtf_nature some time back ). It remains difficult to tell what they're actually a sister group to, since they've been doing what they do since the Cretaceous ( at least), and have evolved radically to do so.
They parasitise (or 'stylopise' to use the specific term ) wasps, bees, ants, grasshoppers, mantises, cockroaches, flies, true bugs and silverfish (and in one family, the males use ants, whilst the females use grasshoppers), and since they're very well-evolved parasites, usually don't kill the host. The mobile triungulin larvae seek out a host, often the larval or even the egg stage of another insect, and dissolve their way in, staying with the host as it grows to adulthood. The larvae don't have long to find a host before they starve, but they're still doing better than the newly emerged adult male, who has a lifespan of five hours if he's lucky. He does have a few tools to help him, though - for one, his highly unusual eyes. Unlike most insects, in which the eye consists of hundreds if not thousands of individual facets that by themselves are pretty crap, but add up to good 360-degree vision, Strepsipterans have a few dozen giant facets, each of which focuses a complete highly detailed image. They're very like the schizochroal eyes of certain trilobites, although of course there's no evolutionary connection. They just stumbled on the same trick. He also uses his odd branched antennae, and his mouthparts that are no longer useful for feeding but are now used as sensory organs, to track down the pheromones of a virgin female. And then traumatically inseminates her. See this post about bedbugs for an explanation of the term. Be warned it includes the phrase "semen-encrusted steak-knife".
Photo of an adult male emerging from the host here...
... and traumatically inseminating a female he's detected inside the bee, here. One wonders what the bee thinks of this.
The female doesn't have any of that - she's lost almost all her organs and appendages, and all she has to look forward to is hundreds of her own babies swimming in her blood. Oh, and total mind control of her host. Stylopised ants, for example, will climb to the top of grass stems - the better to spread her pheromones. Bees will drag themselves across flowers, the better to leave triungulins for the next bee. And stylopised wasps will sit in the corner of the nest, letting their sisters do all the work, so they can survive winter and be ready to spread their babies anew.