Sexual cannibalism is where the female of the species attacks, kills and eats a male of the same species before, during or after mating. It's common in many families of spider, insect and amphipod (shrimp-like crustaceans). In most species in which it occurs, sexual cannibalism is related sexual dimorphism, where the female is much larger than the male.
There are competing theories on how the practice evolved. Some biologists suggest that sexual cannibalism is just the result of a voracious female appetite. But experiments have also suggested that females use it as a technique to select the best fathers for their offspring. Some sexual cannibalism might occur because the female regards her suitor as more acceptable as a meal than as a mate.
As an adaptive female strategy, sexual cannibalism is easy to understand. There are two clear advantages to sexual cannibalism:
Natural Selection - Any male who succumbs to the female's attempts to devour him before mating occurs fails to pass on his DNA.
Nutrition - Any male eaten by a female, whether he has mated with her or not, provides the female with nutritional benefits which should increase the quantity or quality of her offspring.
For males, sexual cannibalism seems a less than satisfactory strategy. For some it seems to have evolved as an extreme form of paternal investment. In other species, the males are less happy with the arrangement, and use diverse strategies to decrease their chances of being cannibalized. Some, particularly Nursery Web spiders, bring the female a diversionary meal.
The Black Widow Spider
The female Black Widow Spider is known for her distinctive black and red markings, which are designed to warn potential predators away. Only the female black widow bites humans, and she bites only when disturbed, especially while protecting her eggs.
Her bite is highly venomous, which is effective for disabling and killing her prey of insects and occasionally other spiders. The toxin affects the nervous system - muscle and chest pain or tightness are some of the most common reactions to the black widow bite. Fatalities in humans have occurred and are most commonly due to respiratory paralysis.
On occasion the female will eat the male after mating, generally the smaller the male is, the higher the likelihood that she will devour him. As a method of self preservation, male black widows often restrain females in silk prior to mating. And if the male survives copulation he will often fertilise multiple females.
Sources:
Is the female of the species more deadly than the male? - Scienceray
Sexual Cannibalism - Wikipedia
Black Widow Jewellery
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