Few animals are as devoted to their mates as the male orb-weaving spider. So-called because of the round shape of their webs, these spiders display a significant degree of sexual dimorphism. As this photo of a (tiny) male and female shows, the males are just a fraction of their mates' size.
Following the onset of copulation, the male willingly sacrifices itself, either through programmed death or by allowing itself to be killed, to feed the female. The male dies immediately after transferring its sperm packet to the female with its palp, a segmented appendage that is used in feeding and copulation; the insertion causes the male's heartbeat to stop almost instantly.
This strategy boosts the male's reproductive success by ensuring paternity. The dead male's palp, which remains in an inflated state and is therefore difficult to remove, acts as a mating plug, preventing rivals from copulating with the female.
Female arthropods of some other species--including, most famously, the praying mantis--also eat their partners in flagrante delicto.