Sometimes, the history of people's misconceptions about organisms are nearly as interesting as the organisms themselves - not so much because of what it says about the organism itself, but what it says about us as people. One common pattern is the "pendulum swing" in conceptions about certain organisms. An originally negative over-simplification about a given organism ("wolves are savage human-killers that should be exterminated before they exterminate us"/"whales and dolphins are just fish and can be hunted as such") is replaced by a reactionary viewpoint which is more positive, but arguably just as erroneous an oversimplification ("wolves are completely harmless to humans, and would never attack somebody"/"whales and dolphins are super-intelligent, and, like, just filled with spiritual wisdom"). It is only as our attitudes mature, and the question becomes less politically charged, that opinions settle towards the generally more accurate but usually more complex middle ground.
Sexual cannibalism is one behaviour that has fallen victim to the human tendency to mythologise. Many people are aware of the idea that female spiders, mantids and other such carnivorous arthropods have a habit of eating the male during mating. Any feminist readers I have out there might be interested to consider how the popularity of this concept reflects our own attitudes on the relationship between the sexes (the female eating the male seems scandalous because, of course, we live in a society that tells us it should be the other way around). However, if you open a textbook you will probably be told that this story is generally not true. Bug Girl has recently corrected Isabella Rossellini on just this point*. For the most part, Bug Girl is right - sexual cannibalism is a fairly rare occurrence that usually only happens when things go wrong (for instance, if the male makes his move before properly placating the female). However, there is at least one species in which sexual cannibalism is an integral part of the mating process.
*Actually, speaking of societal attitudes, perhaps the most remarkable thing about the video Bug Girl has posted - other, of course, than the fact that Isabella Rossellini is making videos about arthropods in the first place - is the obvious cultural differences when it comes to talking about sex. The American interviewer with her giggling prudishness does not compare well to Rossellini's far more relaxed attitude.
That species is Latrodectus hasselti, the redback spider. The image at the top of the page (from here) shows two redbacks. The larger individual is a mature female, while the smaller white individual is a male. The genus Latrodectus has a wide distribution around the world, including the black widow (L. mactans) of North America (which does not regularly engage in sexual cannibalism), with most species having a well-deserved reputation for toxicity. The redback had done particularly well out of human civilisation - it is unclear where exactly in Australia it originated*, but it has since been spread throughout the continent, as well as establishing populations in other countries such as New Zealand and Japan. Latrodectus hasselti specialises in constructing its webs between hot, dry, facing surfaces, and humans are very good at building hot, dry, facing surfaces.
*Some people have suggested that the redback is not a native to Australia because of the absence of early records of this species, and its close association in most areas with humans. Conflicting with this is the absence of redbacks in any other part of the world, apart from areas where it has obviously been imported in Australia. It is far more likely that the redback had a much more restricted distribution in Australia prior to European settlement (probably somewhere west of the Great Dividing Range) and has since been spread to the remainder of the country.
In other Latrodectus species, the small male first climbs onto the female's web and approaches her cautiously with regular stops to vibrate the web. The female will usually chase him away a few times, but eventually she calms down and the male is able to climb onto the underside of the female as she hangs upside-down on the web, as shown above in Latrodectus hesperus (image from here). The female remains largely immobile while the male mates with her, and he is able to make his escape quite easily. In the redback, the male approaches and mounts the female as in other Latrodectus but once he has inserted his pedipalp into her he performs a back-flip that brings his abdomen alongside her mouthparts. The female, seemingly unable to resist the temptation, bites into the male and begins chewing. After a while, the male pulls away from the female and rips his badly damaged abdomen out of her grasp. Nevertheless, after a period of grooming the male returns to the female, and inserts and repositions himself as before. This time, the female does not allow him to escape - the male does not survive the second mating.
Why does the male submit himself to this fatal attack? There are a few possibilities that have been suggested. The nutrients the female receives from eating the male may aid in the development of the eggs he has fertilised. Also, the female remains mating with the male for longer while feeding on him than she would have otherwise. Not only does this allow the male more time to fertilise her eggs, it also denies other males the chance to mate with her in this time. A thesis abstract available here indicates that some males try to escape the role of victim, attempting to sneak in and mate with the female without offering themselves. However, females react even more aggressively to such cheaters, who were much more likely to be cannibalised before mating was successfully completed.
Forster, R., & L. Forster. 1999. Spiders of New Zealand and their Worldwide Kin. University of Otago Press: Dunedin, in association with Otago Museum.