It's not true, as many people think, that female praying mantids always devour their mates. Only a few of the 180 mantid species engage in this shocking practice, and not always under natural conditions. Still, sexual cannibalism is common enough among mantids and other creatures (including some spiders, scorpions, crickets, grasshoppers, and beetles) that evolutionary theorists are forced to account for it. After all, it's hard to see what evolutionary advantage this ultimate sacrifice would offer -- especially for the dined-upon males.
One could argue that the reproductive act and the eating of the male might have nothing to do with each other. Some biologists say it's simply hunger: The females, which are much larger, may be unable to resist a male meal that's so tempting and so vulnerable. And sure enough, researchers have shown that well-nourished female mantids refrained from cannibalism, while those deprived of food ate any male in sight, whether or not mating was involved.
Still, evidence does exist that the two acts are linked, and that the male may even benefit: Not in the short term, obviously, but in the next generation, where the offspring may be more likely to carry his genes. For example, the sex act may be protracted when the female gets caught up in chewing her partner, which could increase the odds he will fertilize her eggs. In addition, when the male loses his head -- literally -- it blocks normal inhibitory nerve impulses and he becomes more abandoned sexually. Bizarre as is seems, the male mantid can continue to engage in sex after his head has been devoured.
Sexual cannibalism isn't a must for the mantis to reproduce. Its advantage for the female may be a handy source of nutrition for herself and to feed her offspring.
There's no clear answer about the male's role in this ritual, and scientists debate whether the male is compliant in his own sacrificial death.