It’s true that female praying mantises are known to sometimes bite the heads off their mates, often during copulation.
The act can continue on, sans head, by the way.
Praying mantises are also notorious for eating their sisters, mothers, children, or the other members of the band.
Their eyesight is poor, and they usually prey on anything that moves.
The reason for sexual cannibalism has been widely debated, and one theory is that this behaviour appeared to be an artifact of intrusive laboratory observation.